1. Introduction
Rapid urbanization and globalization have led the world to face severe food shortages, including of meat and milk, with the increase in human population. It is estimated that approximately 821 million people in the world will face hunger in the near future [
1]. The livestock population of Pakistan consists of 82.5 million heads of goat, 53.4 million heads of cattle, 43.7 million heads of buffalo and 31.9 million heads of sheep. Livestock is a source of cash income for 8 million families in Pakistan as they are earning about 35% of their income from livestock and agricultural activities [
2]. Despite of this huge livestock population, per capita availability of meat is less due to high cost of production [
3]. Although the goat population reared for meat purposes is high enough in numbers with variety of goat breeds, its average weight gain is very poor due to less availability of balanced and adequate feed, which makes goat farming economically unviable for the producers [
4,
5,
6]. For better productivity, the farmers have to use cereals and meals for the provision of adequate energy and protein to the animals. The inclusion of conventional types of cereals and meal sources sometimes results in a high cost of production due to various reasons including competition for available resources from grains, such as land being used for industrialization, and use of cereals for human consumption, and also restricts the ability of the farmers to operate [
7] on a commercial level [
8]. These conventional feed resources such as wheat, maize, barley, rice, canola, rapeseed, sunflower cakes and meals are not commonly used in goat farming, which are much more expensive, and their prices are also constantly rising [
9]. Additionally, deforestation, which is also another factor in the limited provision of adequate pasture space for growing populations of livestock, has a negative effect on livestock production.
To address these challenges of increasing feed cost, there is an urgent need to explore new dimensions in animal feed such as the use of non-conventional alternate feeding resources or byproducts which may replace the use of conventional feed resources in an animal’s diet [
10]. Among these non-conventional feed resources is citrus pulp that is wasted in large quantities. It is a good energy source to be used in animal rations and has significant health-promoting properties. Citrus pulp contains 7% CP, 14% CF, 21.6% NDF, 15.9% ADF, 2.5% fat, 24.4% total sugars, and 12.1% ME MJ/kg, on a dry matter basis [
11]. The amount of citrus pulp waste produced in Pakistan is not well documented. About 2 million metric tons of citrus fruits are produced annually in Pakistan [
12]. Almost 66% of citrus fruit is left as waste after the extraction of juice [
12]. Management of citrus industry waste is difficult as waste cannot be burnt due to high moisture content [
13]. Citrus waste is a potential source of environmental pollution, as it contaminates air, soil, and water, produces unpleasant odors and harbors insects [
14,
15]. However, it can be used in ruminant feed throughout the year after preservation by different methods without any chemical change. It has great potential to be used in animal rations as it has a nutritive value comparable to any of the conventionally used energy-rich concentrates in animal feeding [
16]. Replacement of cereals with citrus pulp in goat rations up to 61% has no adverse effects on performance and may reduce the cost of production [
17,
18].
The current research was designed with the hypothesis that the inclusion of dried citrus pulp could replace corn grain in goat TMR without any adverse effects on growth performance parameters or on animal health with reduced or equal cost of production.
4. Discussion
In the present study, feed intake in goats showed non-significant differences between all experimental groups with differing quantities of dried citrus pulp (DCP). These findings were compatible with those of Guzmán et al. [
27], who reported no effect on the nutrient intake of steer fed with varying levels of DCP. Likewise, Taniguchi et al. [
28] reported that feed intake remained constant while the duodenal flow of microbial protein was improved by feeding various levels of DCP to Holstein steer. Moreover, another study reported that the addition of carob pulp up to a level of 35% in diet has no effect on feed intake in lambs [
29]. Gilaverte et al. [
30] also observed no effect on feed intake when completely replacing corn grain with dried citrus pulp in the diet of sheep. Contrary to the present study, Sarker et al. [
31] reported an increase in feed intake when early lactating crossbred Holstein Friesian cows were fed non-conventional feed as compared with TMR comprising maize stover and concentrate having a 50:50 proportion. Moreover, another study also revealed an increase in nutrient intake using 18% DCP in male Lacaunelambs’ rations [
32]. Likewise, feed intake was higher in finishing pigs when basal diet on the basis of feed (DM) was replaced with either 15%, 22.5% or 30% dried citrus pulp (DCP) [
33]. The reason for these improvements in feed intake was the specific taste and aroma of dried citrus pulp [
34]. Feed intake also remained unaltered in fattening calves when 60% DCP was used to replace barley [
35].
Weekly weight gain results showed non-significant differences between all experimental groups in this trial. Similarly, nonsignificant results were reported related to the effect of dried citrus pulp on weight gain in lambs [
36,
37]. This suggested that dried citrus can be used in a satisfactory way in animals’ feed as a replacement for corn grain upto 20% without affecting weight gain [
38]. This amount of 20% inclusion of citrus waste is based on the findings of the current study and is not the absolute maximum level as demonstrated in below discussion. Current study results have demonstrated that this level was effective in replacing corn grain. Moreover, another study reported improved weight gain when citrus pulp was included in ruminants’ diets at about 46% inclusion [
39]. Also, improved weight gain was found when cows were fed TMR having 21% dried citrus pulp when compared to 10% inclusion levels [
40]. Moreover, Aregheore [
41] also reported enhanced live weight gain in goat and sheep when up to 36.65% dried citrus pulp was used in their diet. This is due to the high digestibility of dried citrus pulp [
42] with pectin having 87% ruminal digestibility [
43]. However, Faiz et al. [
44] reported that the use of citrus pulp as supplementation in broiler chickens’ diets decreased body weight due to the increased fiber content of citrus pulp [
44] that is a favorable component for ruminants as they can digest fiber effectively that is beneficial for their rumen health [
45].
This study also showed non-significant results for the feed conversion ratio (FCR). This aligns with the findings of Hentges et al. [
46], who used dried citrus meal in Hereford and Aberdeen-Angus bulls in ratios up to 63.2% of the ration mixture and observed no impact on FCR. Furthermore, ref. [
47] also generated results in agreement with this study when 0.5%, 1.0%, 1.5% and 2% dried sweet orange (
Citrus sinensis) pulp (DCSP) residues were used in poultry rations. Likewise, it was also shown that FCR was not affected by using 25%, 50% and 75% dried citrus pulp as are placement fordried brewers’ grains (DBG) in West African dwarf goats’ diets [
48]. On the contrary, it was reported that including citrus pulp in ratios up to 10% improved the feed conversion ratio in broiler chickens [
44]. In addition, Bueno et al. [
39] also suggested that the inclusion of dried citrus pulp in kid diet with a replacement level of 50% positively impacts growth performance [
49].
In our current study, serum biochemical parameters showed non-significant results among all groups. Gobindram et al. [
50] showed similar results, reporting that dried citrus pulp did not affect total protein, triglyceride and cholesterol levels. A similar trend in total protein levels indicates the same quantity of amino acids in the blood that manifest due to iso-nitrogenous characteristics of diet. The normal and equal values of triglycerides and cholesterol in all groups indicate normal liver function. It suggests that dried citrus pulp can be used without negatively impacting animal health. Likewise, it was also observed that 20% dried citrus pulp in dairy cows as a replacement for ground maize and dried beet pulp did not alter total serum protein and triglycerides in all groups regardless of treatment [
51]. However, serum cholesterol showed a different pattern which was observed to be increased.
Moreover, Alzawqari et al. [
52] also reported no effect from dried citrus pulp on blood biochemical parameters, indicating that dried citrus pulp is an excellent substitution for conventional energy sources to be included in animal rations without harming animal performance. On the other hand, it was observed that serum triglyceride levels decreased when using dried citrus pulp in the animals’ diet [
47]. Similarly, dried citrus pulp in a broiler’s diet also displayed no effect on total serum protein but decreased blood cholesterol and triglyceride levels [
53].
Further, hematological parameters showed similar normal values among the groups. Some studies reported elevated values, while others reported no differences [
24,
50]. Replacement of dried brewer’s grains in goats’ diets with dried citrus pulp at 25%, 50% and 75% increased packed cell volume, hemoglobin levels, white blood cell count, red blood cell count and total protein levels [
48]. This might be due to a much higher inclusion level than current study which has only 20% inclusion.
The present study showed non-significant difference in catalase activity among all experimental groups. This is justified in that the inclusion of dried citrus pulp in goats’ diets did not affect the catalase activity of goat blood [
54]. Gladine et al. [
55] stated that the use of citrus waste has been reported to have beneficial health effects and a positive effect on serum antioxidant levels. It was found that supplementation with citrus waste improved antioxidant status without affecting total protein, globulin and blood urea nitrogen levels [
53]. On the contrary, another study by Faiz et al. [
44] reported that serum antioxidant enzymes such as superoxide dismutase, catalase and glutathione peroxidase increased when dried citrus pulp is included in the diet.
Use of dried citrus pulp was found to decrease feeding cost as inclusion level was increased. Previous studies reported that there would be a high price at the minimum inclusion level while a low price would be incurred at a high inclusion level [
23,
56]. Including dried citrus pulp in small ruminants is very economical [
57]. After all, it is industrial waste, cheaper than cereals, resulting in the lowest-cost feed formulation [
58]. Chis, Muste and Vidal [
59] also stated that adding citrus pulp in animals’ rations lowers the cost of production.