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Review

The Impact of Drought, Heat and Elevated Carbon Dioxide Levels on Feed Grain Quality for Poultry Production

by
Harris D. Ledvinka
1,
Mehdi Toghyani
1,2,
Daniel K. Y. Tan
1,
Ali Khoddami
1,
Ian D. Godwin
3 and
Sonia Y. Liu
1,2,*
1
School of Life and Environmental Sciences, Faculty of Science, The University of Sydney, Sydney, NSW 2006, Australia
2
Poultry Research Foundation, The University of Sydney, Sydney, NSW 2570, Australia
3
School of Agriculture and Food Sciences, The University of Queensland, Brisbane, QLD 4072, Australia
*
Author to whom correspondence should be addressed.
Agriculture 2022, 12(11), 1913; https://doi.org/10.3390/agriculture12111913
Submission received: 11 August 2022 / Revised: 4 October 2022 / Accepted: 9 November 2022 / Published: 14 November 2022
(This article belongs to the Section Farm Animal Production)
Versions Notes

Abstract

:
Climate change has wide-reaching consequences for agriculture by altering both the yield and nutritional composition of grains. This poses a significant challenge for the poultry industry which relies on large quantities of high-quality feed grains to support meat and egg production. The existing literature shows that elevated atmospheric carbon dioxide concentrations (eCO2), heat and drought overall reduce grain yield and quality. However, these results are inconsistent, with some studies reporting small or large decreases and others even indicating potential improvements. These variations may occur because many studies only investigate one climate factor at a time, without considering interactions between factors. Additionally, most studies investigate just one grain type, rather than comparing grains and their morphophysiological differences. The present review offers a novel approach by investigating how eCO2, heat and drought interactively affect both the yield and nutritional composition of four key animal feed grains: wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), maize (Zea mays L.) and sorghum (Sorghum bicolour L. Moench). The photosynthetic pathway is a key determinant of a plant’s response to climate factors, so this review compares grains with both C3 photosynthesis (wheat and barley) and C4 photosynthesis (maize and sorghum). The present review found that eCO2 may promote starch synthesis in C3 grains of wheat and barley, thereby improving grain yield but diluting protein, lipid, vitamin and mineral concentrations. This potential yield improvement is further counteracted by heat and drought which limit the photosynthetic rate. Unlike wheat and barley, C4 photosynthesis is not CO2-limited, so neither the yield nor the nutritional quality of maize and sorghum are significantly affected by eCO2. On the other hand, heat stress and drought reduce photosynthesis in maize and sorghum and may offer minimal increases in nutrient concentrations. This review highlights that while eCO2 may increase the yield of wheat and barley grains, this effect (i) dilutes nutrient concentration, (ii) is counteracted by heat and drought, and (iii) does not benefit C4 grains maize and sorghum. An additional novel insight is offered by discussing how the impacts of climate change on animal feed production may be mitigated using alternative crop management practices, plant breeding, feed processing and enzyme supplementation.

1. Introduction

Feed quality plays an important role in animal health, welfare and performance, and it counts for up to 70% of production costs for pigs and poultry [1]. Among the myriad challenges that the animal feed supply chain will face in the future, climate change poses a significant problem by altering both grain quantity and quality [2,3,4,5,6,7,8,9,10]. Worldwide, around 800 million tonnes or one-third of total grains produced are used for animal feed; however, with a growing global demand for animal protein due to population and economic growth, this quantity will need to increase by 38% by 2050 [1]. Hence, understanding the impact of climate variability and extreme weather events on grain quality and quantity is necessary to prepare the poultry industry.
Elevated atmospheric carbon dioxide concentrations (eCO2) have been identified by Mariem et al. [4] as the most important factor, since atmospheric CO2 acts on plants regardless of crop location or management, whereas drought and heat stress may be mitigated by irrigation or relocation. However, the interactions of eCO2, heat and drought may have significant ramifications for plant physiology, functional properties and ultimately, nutritional composition [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30].
The responses of wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), maize (Zea mays L.) and sorghum (Sorghum bicolour L. Moench) to these climate variables depend on several complex and interconnected factors including radiation, soil salinity, soil nutrient availability, fertiliser application, pest and disease prevalence and other variations in precipitation such as waterlogging and flooding [8,9,10,31]. However, photosynthetic pathway type is perhaps the key determining factor. C3 and C4 photosynthesis are two of the main types of photosynthesis and contain key differences in their responses to climate factors [3,11].
The economically important C3 crops include wheat, barley, rice, cotton, soybean and potatoes. C3 photosynthesis uses the Calvin cycle to fix carbon dioxide (CO2) from the atmosphere which takes place in the chloroplast of the mesophyll cells and is catalysed by the enzyme ribulose-1,5-bisphosphate carboxylase (RuBisCO). C3 photosynthesis also contains a metabolic pathway known as photorespiration, which occurs when RuBisCO acts on oxygen (O2) instead of CO2. Therefore, C3 photosynthesis is considered less energy-efficient in comparison to the C4 pathway.
The economically important C4 crops include maize, sorghum, sugarcane and switchgrass. Many C4 plants are tropical grasses [32] which are relatively more tolerant to heat and drought [33]. The C4 pathway owes its name to malic acid, which contains four C atoms and is the first product of C4 photosynthesis. C4 photosynthesis has two major differences in comparison to the C3 pathway: (i) photosynthesis is divided into two different cells—the mesophyll and the bundle sheath cells—and (ii) an additional enzyme, phosphoenolpyruvate carboxylase (PEPC), is used to concentrate CO2 around RuBisCO. Consequently, the C4 pathway can suppress photorespiration and perform photosynthesis at 50% higher efficiency [34].
The effects of eCO2, heat stress and drought on yield have been extensively researched [3,4,5,7,8,9,10,11,12,35,36,37,38,39,40,41], but these climate factors may also significantly impact grain nutritional quality causing inconsistent growth performance in animals [1,42]. Being almost the only source of starch, cereal grains contribute to more than 50% of typical broiler diets, and the four predominant grains for feed industries globally are wheat, barley, maize and sorghum. Climate factors may influence both nutrient content in cereal grains and their utilisation by animals.
This review seeks to understand why existing literature is so inconsistent about the effects of climate change-induced factors on grain yield and nutritional quality. Rather than investigating the effects of eCO2, heat and drought stress individually, the present review examines the impact of all three factors and considers potential interactions between the three factors. Further, instead of investigating just one or two grains with the same photosynthetic pathway, two grains with C3 photosynthesis (wheat and barley) and two with C4 photosynthesis (maize and sorghum) are analysed, allowing for a comprehensive comparison of these four key animal feed grains. The analysis of the effects of climate-induced factors on these grains is divided into two sections, according to the photosynthetic pathway of each grain: wheat and barley (C3), and maize and sorghum (C4). Within each section, the effects of eCO2 are assessed independently of heat stress and drought, since these factors are associated with opposing impacts on plant physiology [4,13].

2. Background

2.1. Nutritional Consideration in Cereal Grains

Primarily, cereal grains provide starch and protein for poultry diets. Protein concentration in grain is critical because grain protein may contribute to greater than one-third of the total dietary protein in conventional broiler diets and up to 60% of total protein in reduced crude protein diets [43]. Cereal grains are almost the only source of starch in typical poultry diets and they are the primary source of energy. Protein quality is often expressed as amino acid profile and its digestibility in poultry. Most broiler diets nowadays are formulated to standardise digestible amino acids which considers the variation in endogenous nitrogen flow. The digestion of starch by pancreatic α-amylase and absorption of glucose in broilers is well documented and it is often considered not to be limited [44]. This is certainly the case in maize but starch utilisation in wheat and sorghum is reported to be sub-optimal. Liu et al. [45] reported that the apparent distal ileal starch digestibility for broiler chickens offered diets based on maize, sorghum or wheat was 0.949, 0.885 and 0.877, respectively. The factors affecting starch digestion include the nature of the starch, the starch-protein interaction, soluble cell-wall polysaccharides and other anti-nutritive factors such as phytate. Barsby et al. [46] suggested that amylopectin makes starch more soluble and amorphous and has a larger surface area than amylose; therefore, cereals with higher amylopectin, as opposed to amylose, tend to have better starch digestion. However, Classen [47] showed that there is no significant relationship between starch type, animal growth rate and feed efficiency in waxy and normal dehulled barley, which suggests that the cell-wall polysaccharides may have a more pronounced impact on starch utilisation. In addition to the extent of digestion, which is often expressed as ileal digestibility, rates of starch and protein digestion are also important to growth performance [48]. Selle et al. [49] reported that wheat and barley have numerically higher rates of starch digestion than maize and sorghum.
Globally, maize is the most used cereal grain in poultry diets. The apparent metabolisable energy (AME) of maize ranges from 15.5 to 17.0 MJ/kg and it contains more than 600 g/kg starch and 88 to 122 g/kg protein [50]. Maize contains less soluble non-starch polysaccharides (NSP, ~1 g/kg) than wheat and barley and is generally lower in other anti-nutritive factors including trypsin inhibitors and lectins; potentially, other anti-nutritive factors including resistant starch and phytate may influence energy utilisation in broiler chickens offered maize-based diets [51]. Overall, maize contained less crude protein than wheat (95 versus 147 g/kg); consequently, maize contained less Lys (2.23 versus 3.07 g/kg), Met (1.10 versus 1.47 g/kg) and Thr (2.67 versus 3.30 g/kg) than wheat.
Wheat is the second dominant grain used globally in the animal feed industry and certainly is the most common feed grain in Australia. The AME of Australian wheat ranges from 10.35 to 15.9 MJ/kg for broiler chickens [52,53], and the protein content varies from 11.1 to 20.5% [54]. Soluble NSP is an important anti-nutritive factor in grains including wheat and barley. It influences digestion by increasing the viscosity of digesta and limiting contact between digestive enzymes and substrates [55]. The low-AME wheat phenomenon reported by Mollah et al. [52] was thought to be caused by soluble arabinoxylan and β-glucan [54,56]. Choct et al. [57] showed that exogenous xylanase significantly increased starch digestibility coefficients in the jejunum (0.732 versus 0.788) and ileum (0.949 versus 0.984) in wheat-based broiler diets which was probably a consequence of the reduction in digesta viscosity. Consistently, Liu et al. [45] reported the combination of xylanase, amylase and phytase improved apparent ileal starch digestibility in wheat-based broiler diets by 8% from 0.877 to 0.946, the level which is comparable to maize-based diets. Nowadays, xylanase and phytase are the two enzymes routinely included in poultry diets for most regions of the world. It remains unknown whether climate-induced factors influence phytate and NSP concentrations in cereal grains and whether inclusion rates of exogenous enzymes need to be adjusted.
Alpha-amylase inhibitors are also identified as anti-nutritive factors in cereal grains and they have been found in wheat, rye, triticale and sorghum, but not in rice, barley and maize [58]. Although they are widely found in cereal grains, they do not play a significant role [47] as the inhibitors are heat-labile and susceptible to pepsin inactivation in the gastric region. However, a correlation between increased pancreas weight and α-amylase inhibitors has been found, so this should be considered for non-heat-treated diets where digestion rate of starch may be a limiting factor [59].
Sorghum is the world’s fifth major cereal crop with an annual production of 58 million tonnes [60]. Interestingly, sorghum and maize have similar starch and protein structures, but sorghum is often considered ‘sub-optimal’ to maize as a feed grain. Australian sorghum has an AME range from 13.5 to 17.7 MJ/kg and protein concentration varies from 70 to 170 g/kg [54]. Sorghum contains 560–760 g/kg starch [61]; however, its poor starch digestibility and deficiencies in basic amino acids are two major nutritional limitations [60,62]. Sorghum grains are reported to contain around 65 g/kg of enzyme-resistant starch [61]. Kafirin represents almost half of the protein in sorghum and it lacks basic amino acids (Lys, His and Arg) and has poor digestibility; moreover, phytate, tannin and non-tannin phenolic compounds may also negatively impact starch and protein utilisation in sorghum [63]. The NSP in sorghum consists of cellulose and non-cellulosic polysaccharides, mainly located in the pericarp; unlike wheat, sorghum is known as a ‘non-viscous’ grain and may not benefit from the usage of NSP-degrading enzymes [64].
Barley was one of the first domesticated crops and is today widely grown as a highly adaptable winter cereal crop. The world’s barley production is around 140 million tonnes, which places barley as the fourth most-produced cereal grain after maize, wheat and rice [65]. Although barley is generally known for its use in the brewing industry, it is also used for animal feed, particularly for cattle, pigs and poultry [66]. It is generally agreed that barley contains lower and more variable starch and higher fibre content, providing lower energy values for mono-gastric animals than wheat and corn.
On average, barley contains 400–550 g/kg starch which is believed to have a slower digestion rate compared to wheat and maize [67]. The protein content of barley is often higher than maize by 2% or more, but less than wheat. The essential amino acid profile in barley is comparable to both maize and wheat when expressed as a percentage of total crude protein, but its protein and amino acid digestibility coefficients are poorer than both wheat and maize [68]. The fibre and NSP profiles of barley are substantially different from those of wheat, corn and sorghum [69]. Barley contains 33.3% and 55.2% higher crude fibre, and 46.5% and 95.0% higher soluble NSP than wheat and corn, respectively [69,70]. Large variations in the chemical and physical characteristics of barley exist even among similar types of barley. Depending on the cultivar the concentration of soluble β-glucan in barley could be nearly 10 times higher than that of wheat [71]. β-glucan is known as the main anti-nutritive factor in barley and is responsible for increased gut viscosity [72], sticky droppings [73] and impaired nutrient digestibility [74]. Supplementation of high barley diets with multi-carbohydrase enzymes including β-glucanase may increase feed intake, weight gain, flock uniformity, feed efficiency, gut health and nutrient utilisation [65].

2.2. The Impact of Climate-Induced Factors on Grain Yield

2.2.1. Wheat and Barley

Most of the existing literature on the effects of climate change on grain yield has focused on C3 grains such as wheat and barley [8,35,36]. However, the results are highly inconsistent for several reasons. First, variations in data may be attributed to variations in experimental conditions, rooting volume and soil nitrogen [14]. Second, the effect on grain yield depends on the number and combination of climate factors investigated, such as eCO2, heat and drought. Third, outcomes depend on the yield traits tested, such as grain number or size.
However, when considered in isolation, the consensus is that eCO2 increases C3 grain yield [2,3,4,5,6,23,30]. This is because C3 photosynthesis is limited by the availability of CO2, so eCO2 may enhance C3 photosynthesis by providing more substrate for photosynthesis and allowing C3 plants to reach CO2-saturated photosynthesis. This rise in the photosynthetic rate increases the assimilation of carbon (C) into carbohydrates, hence increasing starch content in grains, and ultimately increasing yield [2,3,4,5,6,23,30]. However, while eCO2 may promote starch synthesis, Mariem et al. [4] offer a comparison of different yield traits. They suggested that eCO2 may increase grain number without improving grain weight; hence, instead of producing larger grains, eCO2 may only produce a greater number of nutrient-poorer grains of the same size. It should also be considered that even when eCO2 improves grain yield in wheat and barley, this improvement is conditional on a consistently increasing availability of soil nitrogen (N) [14], because N availability is a determining factor for yields of both wheat [75] and barley [76]. Therefore, to allow wheat and barley to benefit from eCO2-induced yield improvements, crop producers may need to rely on the increased application of N fertilisers which may prove a challenge due to the high cost of fertilisers and the potential environmental impact [77].
Even when soil factors such as N availability are favourable, the potential yield improvements offered by eCO2 are counteracted by heat and drought. These two climate factors reduce C3 grain yields by shortening the grain growth period and reducing the photosynthetic rate during grain-filling [4]. According to Balla et al. [38], wheat yield is reduced by 31% under heat stress and by 57% under drought. Moreover, when combined, heat stress and drought reduced yield by up to 76% and decreased grain weight by 67% [38].

2.2.2. Maize and Sorghum

Maize and sorghum are C4 crops and therefore exhibit different responses to eCO2 compared with barley and wheat [3,4,24,33,35]. While C3 photosynthesis is limited by the availability of CO2, C4 photosynthesis is already saturated by CO2 at the current atmospheric concentrations [35]. Existing literature tends to assume that C4 plants will not be influenced by eCO2; therefore, little attention has been paid to the impact of eCO2 on C4 crops including maize and sorghum [8,35,36].
The papers that do investigate the impact of eCO2 on C4 photosynthesis suggest that some improvements may occur but only when eCO2 serves to alleviate the yield reductions caused by heat or drought stress [29,37]. Even though some researchers have found eCO2 improved photosynthesis independently, they indicated that these improvements were caused by inconsistent rooting volume between studies instead of eCO2, where greater rooting volume may confound results by providing plants with more water from a larger soil volume [14].
Similar to C3 grains wheat and barley, drought and heat stress may significantly compromise C4 crop yields. Maize yield was found to decrease by up to 40% under drought conditions [39] and by 10–40% under heat stress [40]. This may be because drought decreases the activity of starch synthases and ultimately starch concentration [16]. Heat may also depress the yield of C4 crops by shortening the grain-filling period and disrupting plant metabolic reactions, especially C assimilation and starch synthesis [17]. Shortening the grain filling duration through heat leads to significantly smaller grains and more variable grain sizes [4,78].

3. Impacts of Elevated [CO2], Heat Stress and Drought on Grain Nutritional Value

3.1. Wheat and Barley

3.1.1. Effects of eCO2

Given that eCO2 allows C3 grains to reach CO2-saturated photosynthesis by providing additional CO2 as a substrate, C3 grains tend to increase in starch concentration under eCO2 [2,3,4,5,6,23,30]. Despite starch being the primary source of energy in typical poultry diets, grains may also play a role in providing other nutrients to support animal health and welfare [1,42]. Most literature agrees that the increased concentration of starch in wheat and barley grains may reduce the concentrations of other key nutrients [4,7,63,64,65]. For example, eCO2 was reported to decrease the concentrations of protein (nitrogen), lipids, vitamins and minerals including iron (Fe), sulphur (S), zinc (Zn), and magnesium (Mg) [19,20]. Indeed, Mariem et al. [4] reported that eCO2 reduced the concentration of protein, particularly gliadin and glutenin. A meta-analysis of the effect of eCO2 on the protein concentrations of major food crops found that increasing CO2 from 315–400 μmol mol−1 to 540–958 μmol mol−1 reduced protein content by 9.8% and 15.3% in wheat and barley, respectively [21].
Finally, in terms of anti-nutritive factors, eCO2 can cause significant reductions in the phytate concentration of wheat grains [23,24,25], with Fernando et al. [23] identifying a phytate concentration reduction of 17% under eCO2. Therefore, although eCO2 may dilute the concentration of nutrients other than starch, this effect may be moderated by the improved bioavailability of these nutrients in animal feed. Moreover, eCO2 was reported to increase the fructan concentration of wheat grains [23,25], which may be considered a prebiotic [79]. To summarise, eCO2 may stimulate photosynthetic rate and increase starch content in wheat and barley grains (Table 1), but this may dilute the concentrations of other key nutrients including protein, lipids, vitamins and minerals (Table 2) and anti-nutritive factors such as phytate.

3.1.2. Effects of Heat Stress and Drought

Whereas eCO2 is expected to stimulate starch synthesis and in some cases improve yield traits in wheat and barley, heat stress and drought may counteract these improvements (Table 1) [2,3,4,38]. Heat stress reduces starch synthesis by shortening both the duration of photosynthetic tissue and the grain growth period and reducing the photosynthetic rate during grain-filling [4,15,41], where the duration and rate of grain-filling determine final grain weight [78]. Drought is expected to have similar effects on starch synthesis by reducing the production of photoassimilates, a source of C skeletons for starch synthesis, and decreasing the activity of enzymes involved in starch synthesis in the endosperm [4,15].
Importantly, adequate soil moisture enables the efficient uptake of nutrients by plant roots [10], so drought conditions may impede plant nutrient uptake. In addition to drought, heat stress may further reduce soil moisture independently of rainfall or irrigation by promoting the evaporative loss of moisture to the environment [84]. Nevertheless, heat and drought may increase the concentration of certain minerals in grains due to a reduction in starch concentration, and heat stress may promote greater remobilisation of protein from vegetative tissues [4]. Indeed, concentrations of Zn and Fe in grains tended to increase under heat stress and drought [4,27,28].
Protein concentration is also expected to increase under heat stress and drought [4,38]. Mariem et al. [4] found that grain protein concentration, which is expressed as a percentage of grain dry mass, increased by 10.4% under heat stress. However, this increase was almost equivalent to the relative decrease in grain starch concentration (−9.9%), reinforcing the findings of Balla, et al. [38] that protein concentration only increases as a consequence of starch content reduction. In contrast, Mariem, et al. [4] suggested drought may also independently increase the protein concentration of grains because drought stress promotes the translocation of senescence-inducing resources such as amino acids from the source to the sink, which may increase N accumulation and therefore total protein [4,28]. Interestingly, heat stress and drought were also found to increase phytic acid which is different from the impact of eCO2 [28].

3.2. Maize and Sorghum

3.2.1. Effects of eCO2

Unlike in C3 grains wheat and barley, C4 photosynthesis is already almost CO2-saturated even at current concentrations [35]. Therefore, since eCO2 is unlikely to significantly increase C4 photosynthesis and starch concentration, it follows that eCO2 may not significantly dilute the concentration of other grain nutrients in maize and sorghum. This is supported by most of the existing literature. Indeed, a meta-analysis conducted by Myers et al. [24] on the effects of eCO2 on the growth of major crops found that eCO2 does not have a significant impact on the protein content of C4 crops. Further, De Souza et al. [29] found that eCO2 may increase the protein content of sorghum grains when grains are grown under eCO2 and drought conditions, compared with only drought conditions, which may confirm that eCO2 protects sorghum grains from quality reductions caused by drought stress. Loladze [30] found that eCO2 has no impact on the total mineral element content of C4 plants. Similarly, Myers et al. [24] found that eCO2 did not influence Zn content in maize and reduced Fe in maize but not in sorghum. Interestingly, Bhargava and Mitra [85] identified that sorghum exhibited greater tolerance to eCO2 when compared with maize, both in terms of quantity and quality traits. These authors, therefore, suggested that sorghum may be more “climate-change ready” than maize. There are insufficient data to conclude the effect of eCO2 on the concentration of anti-nutritional factors such as phytate and phenolic compounds in maize and sorghum. This highlights a research area that would benefit from more investigation.

3.2.2. Effects of Heat Stress and Drought

Drought decreases the activity of starch synthases and ultimately starch concentration in C4 plants [12,16,17]. In terms of starch structure, the high temperature had opposite effects on C3 and C4 plants [16,80]. Heat stress decreased amylose and increased amylopectin concentration in maize [80], whereas the opposite effect was observed in wheat and barley [16]. Drought stress may also reduce protein synthesis and degrade existing proteins, reducing the overall protein content in grains (Table 2) [81]. Similarly, Monjardino et al. [82] found that heat stress reduced protein content in maize, including a 53% reduction in zein. They further concluded that heat stress influenced zein content differently at various stages of plant development: at early stages of endosperm development, high temperatures reduced zein synthesis, and later in development, zein accumulation was predominantly reduced by protein degradation. Contrastingly, researchers in the US concluded that dry growing conditions often lead to increased crude protein levels; however, these increases are not accompanied by proportional increases in amino acid content and the commonly used amino acid prediction equations may overestimate the actual amino acid content [86]. Drought was also found to reduce pigment concentrations in maize, including chlorophyll, chlorophyll-a, chlorophyll-b, carotenoids and anthocyanin concentration which may have implications for the yellow-bird market in some regions [83,87].

4. Potential to Minimise the Variation of Grain Quality

4.1. Crop Migration, Sowing Windows and Management Practices

Several strategies may help to manage the effects of climate change on grain yield and nutritional quality. In agricultural systems including animal feed production, climate change adaptation can involve adjusting crop management practices, plant breeding, feed processing and feed enzyme supplementation in order to produce adequate amounts of nutritionally rich animal feed in a changing climate [88].
Crop migration is an example of adjusting crop management practices. It involves moving crops to different geographical regions to adapt to changing climatic factors [88]. This can include abandoning existing growing locations, expanding into new regions and cultivating different crops that are better suited to new climatic conditions [89]. Crop migration helps to mitigate changing growing season temperatures and is already occurring worldwide [88,89,90]. This can be reinforced using supplementary irrigation, which helps to counteract the effects of drought on grain yield and nutritional quality by alleviating the effects of heat and drought stress on grain crops [89,91]. However, as Mariem et al. [4] note, neither crop migration nor irrigation may mitigate the effects of eCO2 which influences crops regardless of latitude.
An alternative cost-effective strategy for adapting to climate change is adjusting sowing windows. This involves timing the sowing of crops so that plants flower at the optimum flowing window when environmental conditions are favourable. Recent literature has found this to be a promising method for alleviating the negative effects of climate change on grain yield and quality [92,93,94,95].
Finally, other crop management practices such as zero tillage and stubble retention may offer short-term and cost-effective strategies for adapting to climate change impacts including drought stress. Zero tillage and stubble retention may increase soil water storage by reducing evapotranspiration and runoff, thereby reducing the impact of drought stress on crops [96,97]. However, the consensus among existing literature is that zero tillage does not function as a one-stop solution. For example, Liu [95] found that zero tillage reduced yield loss under climate change scenarios compared with conventional tillage. However, it could not reverse this effect unless it was combined with the use of adapted crop cultivars. Similarly, Phogat et al. [96] found that zero tillage was most effective at increasing water productivity when it was combined with the control of weeds and the maintenance of adequate crop residue, which helps to suppress evapotranspiration.
These strategies may offer cost-effective solutions to climate change, enabling producers to counter the effects of climate change on grain yield and quality reductions. However, as climate change continues to alter growing conditions, animal feed production may need to rely on more long-term solutions for mitigating climate change. For example, while crop migration may offer a short-term solution for adapting to changing environmental conditions, it (i) does not counter the effects of eCO2 [4], (ii) may be environmentally harmful by reducing wildlife habitats and biodiversity [89] and (iii) disproportionately places vulnerable populations at a higher risk of economic and social insecurity [88,90].

4.2. Plant Breeding

A straightforward option to reduce climate impact on grain quality and yield is through breeding programs. In conventional breeding with genomic selection, a highly diverse set of agronomically adapted plant materials is assembled for phenotyping. This is supported by genomic selection where a reference population is phenotyped for the trait of interest using several DNA markers including Single Nucleotide Polymorphism (SNP) across the whole genome. One example is the selection of heat-tolerant wheat at the Plant Breeding Institute at the University of Sydney using a three-tiered approach [98]. First, multiple genotypes were evaluated in replicated field plots at different times of sowing. This was followed by the evaluation of later sown materials under high-temperature stress and selection based on estimated genomic breeding values. Materials identified as heat tolerant in the field were subsequently evaluated in field heat chambers set at 4 °C above the ambient temperature to induce heat shock during reproductive development and grain filling to confirm heat tolerance. Finally, those lines that demonstrated heat tolerance characteristics in the field heat chambers were screened in temperature-controlled greenhouses to assess pollen viability under heat stress. Materials that survived all three stages of testing were considered highly heat tolerant. Moreover, a common method of investigating the impact of eCO2 on crop plants is free-air CO2 enrichment experiments which are useful for measuring crop physiological and yield responses but are less economical with limited suitable genotype options.
Controversially, there is inconsistent consumer acceptance of genetically modified food and animal feed in Australia and overseas. Emerging gene editing (GE) technologies including CRISPR-Cas9 have shown promise and despite gene-edited crops being regulated as GM crops in Europe and New Zealand, the Australian Office of the Gene Technology Regulator recently announced that gene-edited crops using SDN-1 (site-directed nuclease) techniques will not be classified as GM crops, bringing Australia in line with regulations in Japan, Brazil, Canada and USA [99]. This is because gene editing only cuts DNA in a specific location, and the natural DNA repair process of the cell is allowed to work; consequently, gene editing is considered similar to changes that occur in nature. It has been demonstrated that it is possible to alter nutrient composition in sorghum via GM or GE. Godwin [100] reported the success of developing sorghums with high protein ranging from 135 to 161 g/kg in comparison to 98 g/kg crude protein in Liberty sorghum and 126 g/kg crude protein in Cracka sorghums. Moreover, altering the expression of regulators of grain size has been demonstrated to modify grain size in transgenic sorghums [101].

4.3. Feed Processing

As previously discussed, climate-induced stressors increase variation in grain quality by influencing grain nutrient profile (Table 2). High variability in grain quality and chemical composition will decrease the accuracy and the degree of precision in least-cost diet formulations. Certain techniques and procedures may counteract the impact of variations in grain quality on the accuracy of feed formulations. Feed processing and the use of exogenous enzymes are among the most promising and cost-effective technologies applied to improve the availability of nutrients and destroy anti-nutritive factors present in some ingredients. Feed processing includes processing single feed ingredients or complete feed prior to presentation to the animals. Feed processing may destroy or deactivate some anti-nutritive factors in feed ingredients and enhance the rate and extent of nutrient digestion and their utilisation. For example, the heat applied during oil extraction destroys trypsin inhibitors in soya beans and deactivates the enzyme myrosinase which hydrolyses glucosinolates into toxic metabolites in canola seeds. However, excessive heat processing and grinding increase the solubility of NSP, which in turn increases the viscosity of the digesta [102], negatively impacting animal performance. Processing temperatures above 105 °C may also promote the occurrence of Maillard reactions which denature protein structure and reduce the bioavailability of amino acids and proteins.
A combination of technologies including de-hulling, cracking and physical grinding, in conjunction with hydrothermal processing such as steam-flaking, pelleting, expansion and extrusion, is used to process different feedstuffs and manufacture mono-gastric diets. The extent of particle size modification, processing temperature, pressure and retention time during steaming and conditioning are considered the main processing parameters which determine the physiochemical properties of processed feedstuffs and diets [103,104]. These factors could directly influence the impact of processing on feedstuff anti-nutritive metabolites, availability and the digestion site of nutrients, and thus indirectly influence animal performance and feed cost. Heat treatment of grain prior to diet mixing is very rare for pig and poultry feed. Cracking the pericarp is the simplest and minimum processing required to expose the endosperm and enhance grain digestion by most animals. Particle size reduction via milling is the next most efficient way to increase the surface area for endogenous enzymes to digest starch and protein. However, it is worth noting that particle size should be optimized based on animal species and stage of production. Feeding coarsely ground maize and wheat has been reported to increase gizzard weight and function, decrease gastrointestinal pH in broiler chicks [105] and layer hens [106], and reduce stomach ulcerations in pigs [107].
During steam-pelleting, starch in the feed may be gelatinised by heat and steam during the conditioning process. Starch gelatinisation improves starch digestibility and also acts as a pellet binder to enhance pellet durability and hardness. Other processing techniques such as steam flaking, extrusion, micronisation, microwave and chemical treatments are used to change grain density, break the grain coat and endosperm, release nutrients (starch and protein bodies) and expand the surface area for enzymatic digestion. However, these techniques are not commonly applied or have very limited application to feed ingredients used in mono-gastric diets.

4.4. Feed Enzyme Supplementation

Grain inclusion in mono-gastric diets can account for up to 60–70% of the metabolizable energy and 30–35% of the protein requirements of animals. Therefore, variation in grain quality is expected to have a major impact on performance parameters [108]. Dietary feed enzymes have long been used to improve animal production output, particularly when ingredient variability is perceived to limit the predictability of performance [107,109]. The benefits of using feed enzymes in mono-gastric diets include not only enhanced growth performance and improved feed efficiency but also fewer environmental problems due to reduced output of undigested nutrients in excreta. Increased accuracy and flexibility in least-cost feed formulations and improved well-being of animals are other possible benefits of using feed enzymes. Exogenous feed enzymes have a stronger impact on poor-quality ingredients than on higher-quality ingredients [110]. The mode of action of exogenous enzymes is described by several possible mechanisms: breakdown of antinutritional factors present in feed ingredients, elimination of nutrient encapsulation effect thus increasing availability, breakdown of specific chemical bonds in raw materials not cleaved by endogenous enzymes, the release of more nutrients, and complementation of the enzymes produced by young animals [110]. Therefore, the principal reason to use exogenous feed enzymes is that they improve the digestibility of several nutrients, specific to the enzyme employed, and as such, can be used to counteract the negative impact of climate-induced stressors on the chemical composition of grains.
Feed enzymes are mainly dominated by carbohydrates and phytase [111]. The anticipated improvement in productive traits from exogenous enzyme application has been reported to be closely associated with an improvement in nutrient and energy utilisation [112]. Scott et al. [113] reported increased AME content of different wheat cultivars (10 samples) in response to dietary enzyme supplementation and pelleting in broiler chickens. This improvement was followed by reduced variations in wheat AME values. NSP degrading enzymes are principally designed to act on NSP and spare more energy from the diet energy; however, there is some evidence suggesting that amino acids are also spared by using exogenous carbohydrases, particularly in diets based on viscous grains such as wheat and barley [110]. The most common fibre-degrading enzymes used in pig and poultry diets are β-glucanase, xylanase, pectinase, hemicellulose and cellulose. Different feedstuffs have different amounts and structures of fibre; as a result, the selection of enzymes for each feed ingredient is important for improving the nutritional value of feed and possibly reducing the variations in grain quality [114]. A prime example of this is the use of ß-glucanase in barley-based diets and xylanase in rye- or wheat-based diets. Xylanase and β-glucanase are considered to be effective by decreasing intestinal viscosity, thus increasing the digestibility of nutrients including energy and amino acids [115]. The use of xylanase in both pig and poultry rations leads to a constant improvement in the digestibility of the undigested fraction of fibre and amino acids [116]. Many nutritional and environmental factors may influence the response of animals to enzyme supplementation of the diet including cereal type, breed, growing environment, cereal inclusion rate, the age of the animal, pelleting of diet, age at first exposure to the enzyme and fat type and inclusion rate.

5. Conclusions

Climate change poses a significant challenge to agriculture by altering both grain yield and quality. Poultry production relies on large quantities of high-quality feed grains. Therefore, potential reductions in feed grain quantity and chemical composition may risk the production of meat and egg protein, which is a vital element of food security. While existing literature tended to focus on just one or two climate variables and grain types, the present review offers novel perspectives by investigating the interactions between eCO2, heat and drought and their effect on both C3 grains wheat and barley and C4 grains maize and sorghum. Considering these factors contemporaneously is important because eCO2 has been found to have opposite effects compared to heat and drought. Additionally, comparing both C3 and C4 grains is important since the photosynthetic pathway is a key determinant of a plant’s response to climate factors. The present review found that while eCO2 may promote the C3 photosynthetic rate and therefore increase wheat and barley grain yield, this not only dilutes nutrient concentrations but is also counteracted by heat and drought. For anti-nutritive factors, eCO2 may reduce phytate concentration in wheat, whereas heat stress and drought have been reported to increase phytic acid concentration in wheat grains. In comparison, eCO2 does not offer the same yield benefits to C4 grains maize and sorghum which are already near CO2-saturated photosynthesis at current atmospheric concentrations. Both heat and drought decrease C4 yield and interrupt protein synthesis. More research into the effects of climate change on antinutritional factor concentrations in C4 grains is required. Finally, methods for mitigating these yield and quality reductions include alternative crop management practices, plant breeding, feed processing and enzyme supplementation.

Author Contributions

All co-authors were actively involved in the completion of this paper. H.D.L. conducted the literature review and drafted the original manuscript. M.T. contributed to Section 2, Section 4.2 and Section 4.3. D.K.Y.T. and I.D.G. contributed to Section 3 and Section 4.1, A.K. contributed to Section 1 and Section 2, and S.Y.L. contributed to the editing of the original manuscript and Section 1, Section 2, Section 4.2 and Section 4.3. All authors contributed to the editing and approval of the final submitted manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

We would like to acknowledge the organisations that supported our research. These include AgriFutures Australia (Project 012265 The impact of climate on sorghum utilisation in poultry diet) and the School of Life and Environmental Sciences Seeding Grant (Project 214478 Feeding chickens in the future—The potential of heat-tolerant wheat). Importantly, we would like to acknowledge the Denison Research Scholarship awarded to HDL by the University of Sydney.

Institutional Review Board Statement

Not applicable.

Conflicts of Interest

We declare that we have no financial and personal relationships with other people or organisations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the content of this paper.

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Table
Table 1. Effects of eCO2, heat stress and drought on quantity in C3 cereal grains wheat and barley and C4 cereal grains maize and sorghum. Upwards arrows indicate an increase and downwards arrows indicate a decrease in the corresponding variable.
Table 1. Effects of eCO2, heat stress and drought on quantity in C3 cereal grains wheat and barley and C4 cereal grains maize and sorghum. Upwards arrows indicate an increase and downwards arrows indicate a decrease in the corresponding variable.
Climate VariableEffect on Wheat and BarleyReferencesEffect on Maize and SorghumReferences
eCO2↑ photosynthetic rate
↑ starch synthesis
↑ grain yield		 [2,3,4,5,6,23,30]Insignificant * [23,30,35,37]
Heat stress and drought↓ photosynthetic rate
↓ starch synthesis
↓ grain yield		 [2,3,4,38]↓ photosynthetic rate
↓ starch synthesis
↓ grain yield		 [39,40]
* When assessed independently, eCO2 did not have a significant effect on quantity or yield of maize and sorghum grains; however, when plants are also experiencing drought, eCO2 may counteract this stress and increase photosynthetic rate [37].
Table
Table 2. Effects of eCO2 and heat stress and drought on quality in C3 cereal grains wheat and barley and C4 cereal grains maize and sorghum. Upwards arrows indicate an increase and downwards arrows indicate a decrease in the corresponding variable.
Table 2. Effects of eCO2 and heat stress and drought on quality in C3 cereal grains wheat and barley and C4 cereal grains maize and sorghum. Upwards arrows indicate an increase and downwards arrows indicate a decrease in the corresponding variable.
Climate VariableEffect on Wheat and BarleyReferencesEffect on Maize and SorghumReferences
eCO2↑ [starch] [2,3,4]insignificant effect on [starch] [37]
↓ [protein], including [gliadin] and [glutenin] [4,21]insignificant effect on [protein] 2 [24]
↓ [lipids]
↓ [vitamins]
↓ [minerals] including [Fe], [S], [Zn] and [Mg]		 [4,7,19,20]insignificant effect on [minerals] [24,30]
↓ [phytate]
↑ [fructan]		 [23,24,25]insufficient data to conclude effects
Heat stress and drought↓ [starch]
↑ [amylose]
↓ [amylopectin]		 [4,41]↓ [starch]
↓ [amylose]
↑ [amylopectin]		 [16,80]
↑ [protein] 1 [4,38]↓ [protein], including [zein] in maize [81,82]
↑ [minerals], including [Zn] and [Fe] [4,27,28]↓ [pigments] in maize, including [chlorophyll], [chlorophyll-a], [chlorophyll-b], [carotenoids] and [anthocyanin] [83]
↑ phytic acid [28]insufficient data to conclude effects
1 Mariem et al. [4] found that the increase in grain protein concentration under heat stress was roughly equivalent to the decrease in grain starch concentration. This reinforced the findings of Balla et al. [38] that protein concentration only increases as a result of decreased starch concentration. 2 When assessed independently, eCO2 did not significantly affect the protein concentration of maize and sorghum grains [24]. However, when plants were grown under both eCO2 and drought, eCO2 was found to counteract drought stress by increasing protein concentration [29]. This may indicate that eCO2 helps to protect quality traits in maize and sorghum when plants are also under drought stress [29].
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Ledvinka, H.D.; Toghyani, M.; Tan, D.K.Y.; Khoddami, A.; Godwin, I.D.; Liu, S.Y. The Impact of Drought, Heat and Elevated Carbon Dioxide Levels on Feed Grain Quality for Poultry Production. Agriculture 2022, 12, 1913. https://doi.org/10.3390/agriculture12111913

AMA Style

Ledvinka HD, Toghyani M, Tan DKY, Khoddami A, Godwin ID, Liu SY. The Impact of Drought, Heat and Elevated Carbon Dioxide Levels on Feed Grain Quality for Poultry Production. Agriculture. 2022; 12(11):1913. https://doi.org/10.3390/agriculture12111913

Chicago/Turabian Style

Ledvinka, Harris D., Mehdi Toghyani, Daniel K. Y. Tan, Ali Khoddami, Ian D. Godwin, and Sonia Y. Liu. 2022. "The Impact of Drought, Heat and Elevated Carbon Dioxide Levels on Feed Grain Quality for Poultry Production" Agriculture 12, no. 11: 1913. https://doi.org/10.3390/agriculture12111913

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