1. Introduction
Swan geese (
Anser cygnoides) are migratory birds that are widely distributed in Mongolia, northeastern China, and southeastern Russia. Compared to intensively produced domestic geese, Swan geese meat is considered healthier due to its high protein but low fat and cholesterol [
1]. Thus, making it one of the most popular and economical wildfowl in several provinces of China. Consequently, there is increasing interest in raising naturalized Swan geese. However, there is no published nutrient requirements for farmed Swan geese, and its feed formulation is usually based on requirements for domestic geese or ducks.
Farmed Swan geese are commonly reared in artificial environments like domestic geese, eating forages and/or formulated diets until they reach a standard market body weight (BW) of 3.0–3.5 kg. There are some common concerns, such as (1) will geese reach market BW on time if fed only forage? (2) Will grazing pasture alone have negative effects on bone development and will it affect fattening performance? (3) If geese are fed only formulated diets, will they be over-fattened and have lower meat quality? These concerns are also relevant for domestic goose production. Domestic geese fed only forages cannot reach standard market BW by 70 days of age [
2]. Furthermore, early nutrition affects growth rate and physical development of birds, especially adult skeletal size [
3,
4]. Excessive fat accumulation, especially abdominal fat, is a major problem facing modern commercial poultry production, leading to reduced growth efficiency and meat quality [
5]. Therefore, determining a feeding regime that optimizes both growth efficiency and meat quality of Swan geese is needed.
Dietary composition and feeding regimes have potential to improve growth performance, promote physical development, and reduce body fat deposition in poultry [
6]. Regulating dietary metabolizable energy (ME) is one of the most common strategies, and its effects on growth performance and carcass characteristics have been studied in domestic ducks and geese [
7,
8,
9], with ME of 10.87–11.29 MJ/kg and 11.29–12.13 MJ/kg recommended for geese of 0–4 and 5–10 weeks of age [
10], respectively. However, relevance of these recommendations for farmed Swan geese are unknown. Moreover, effects of feeding management, e.g., intensive feeding versus free range feeding on growth performance, carcass traits, and meat quality, were studied for domestic geese [
2,
11,
12,
13], but apparently not for Swan geese.
We hypothesized that offering grazing Swan geese with supplemental feed containing suitable dietary ME would promote growth performance and physical development and improve carcass traits and meat quality without adversely affecting health. Therefore, our objective was to investigate effects of offering supplementary feeding diets with various levels of ME on growth performance, physical and bone development, carcass characteristics, and blood biochemistry of grazing naturalized Swan geese (Anser cygnoides).
4. Discussion
Geese can consume green forages and fibrous byproducts of crops to derive a considerable amount of their nutrient requirements [
18]. Dietary fiber is essential for geese to maintain normal performance; consequently, a low-fiber diet had negative effects on nutrient utilization and health, with poor growth and slaughter performance [
19]. Although wild geese can be grown entirely on pasture and domestic geese also can subsist almost entirely on forage, duration of feeding prior to reaching market weight was prolonged by 2–3 weeks [
14]. Domestic geese that consumed forage failed to reach standard market weight (3.0–3.5 kg) on time (usually 120 days of age) due to inadequate CP and energy in forage [
2]. In the current study, Swan geese that grazed on pasture alone for 56 days had a final BW of 1932.1 g, which were well below standard market weight. In contrast, grazed Swan geese that received supplemental feeding had a final BW of ranging from 3008.3 to 3258.3 g, meeting the target for standard market weight. The lower final BW of CON geese was attributed to their lower feed intake as compared to supplemented geese, which was expected, as nutrients derived from pasture may not fulfill nutrient requirements. Current results were consistent with previous studies [
2,
11,
12], in which grazing geese had significantly lower BW than those fed supplemental feeds. Furthermore, all body-size measurements, except shank length, were significantly lower in CON versus supplemented geese, suggesting that bone development was slowed due to inadequate macronutrients. There were significantly lower body-size parameters in grazing geese and grazing geese that received only 1 week of supplemental feeding at the end of fattening compared to those with supplemental feeding throughout the experimental period [
2]. Therefore, we recommended that supplementing grazing Swan geese is a practical method to obtain ideal growth performance by providing well-balanced nutrition. This is not only a feasible method to ensure geese reach standard market weight by 100 days, but also to promote bone development.
It was noteworthy that BW of geese supplemented with diets with ME ranging from 9.5 to 13.5 MJ/kg were not different throughout the study. Similarly, weight gain of Italian Legarth goslings on starter and grower diets with ME ranging from 11 to 13 MJ/kg did not differ significantly [
16]. In addition, similar results were also reported in ducks with dietary ME ranging from 11.8 to 13.8 MJ/kg [
9]. This was attributed to decreased feed intake as energy concentration increased, thereby achieving consistent energy intake [
9,
16]. Furthermore, excess energy consumption does not further increase body weight gain but makes apparent digestibility of nutrients decline [
20]. Notably, ADG of supplemented geese decreased dramatically from day 29 to 56, accompanied by decreased DMI as compared to days 0 to 28. Similarly, there was a very rapid live weight gain of goslings during the starter period, followed by a gradually reduced live weight gain after 10 weeks of age, with poor feed conversion [
21]. Although geese are fast growing, their efficiency to convert feed to weight gain diminished rapidly with age [
16]. Despite compensatory growth for grazing geese during days 29 to 56, their bone development and growth was greatly reduced due to low nutrient quality and quantity. Offering supplemental feeding to grazing geese at the end of the fattening period for only 1 week (days 64 to 70) did not enable geese to reach standard market weight, as it was too late to support optimal bone development [
2]. Geese have reached complete bone development after 8 weeks, whereas development of breast muscle continues until 9 to 10 weeks [
22]. Thus, it is critical to offer supplemental feeding at the proper time to avoid irreversibly retarded bone development.
Geese have a stronger gizzard than other waterfowl, making them successful grazers, able to break down and digest plant cell walls [
14,
23]. As degradation in the gizzard is largely mechanical, geese that accessed grazing pasture or were supplemented with roughage always had higher relative weights of gizzard and a longer digestive tract. In the current study, there were significantly greater relative weights of gizzard and proventriculus in CON geese on days 28 and 56, as well as a longer jejunoileal segment on day 56. Due to lower nutrient contents and poorer feed efficiency of pasture, grazing geese had to ingest more pasture than supplemented geese to meet nutrient requirements. Greater roughage intake increased bulk of digesta, which triggers increasing contraction frequency and weight of gizzard and proventriculus to promote digestion. Geese fed >20% defatted rice bran had greatly increased proventriculus weights [
24]. Increased jejunoileal length of grazing geese implies increased intestinal surface area for nutrient absorption. It is accepted that geese modify volume and weight of digestive tract and digesta passage rate to adapt to fiber-rich diets [
25]. However, there are also some contradictory reports that fiber-rich diets shortened relative lengths of duodenum, jejunum, and ileum [
26,
27], or had no effects on length and weight of gastrointestinal tract sections [
21]. Apparent discrepancies in relative lengths of the digestive tract were possibly caused by breed and age and by sources and levels of fiber [
26,
27], and perhaps other factors, e.g., physical form of diets [
28,
29]. Similar relative weights of gizzard and proventriculus and length of various sections of intestine of geese that received diets with varying dietary ME, implies similar digestion and absorption ability, consistent with their similar growth performance and body-size measurements.
Carcass traits, reflecting differential deposition of nutrients in various tissues or different parts of the same tissue, are important indicators in evaluating growth performance of meat animals [
24]. In the current study, supplemented geese had significantly greater slaughter weight, semi-eviscerated weight, and eviscerated weight than CON, which was consistent with several previous studies. For example, geese under intensive or semi-intensive feeding had significantly greater carcass weight and edible meat production than those under a pasture system [
12]. Supplemented geese had significantly greater eviscerated carcass yield than grazed geese [
2], whereas intensively reared geese had higher slaughter and carcass part weights than free-range geese [
30]. In the present study, geese that grazed pasture alone had significantly lower thigh muscle yield than supplemented geese, in direct contrast to a previous report [
2], which was perhaps due to differences in animal management that altered animal movements and exercise. In that regard, supplemented geese were grazed during the day in the present study but kept indoors in the previous study [
2]. Perhaps, when geese are fed a well-balanced diet, more exercise may promote thigh yield. Poorer carcass quality of grazing geese was mainly due to a higher dietary fiber intake and lower feed digestibility, plus unbalanced and inadequate available nutrients [
12]. Notwithstanding, based on previous studies [
2,
12,
30], grazing geese have some advantages to intensively fed geese, e.g., less subcutaneous and abdominal fat, lower percentage of skin, and greater meat quality. Similarly, in the present study, there was significantly lower abdominal fat yield in CON versus supplemented geese, which agrees with Liu and Zhou [
31] that pasture intake reduced subcutaneous fat thickness and abdominal fat yield of geese compared to control. As the main organ involved in fat metabolism, relative liver weights were significantly lower in grazing versus supplemented geese on day 28, consistent with El-Hanoun et al. [
12]. However, greater relative weights of liver in grazing geese were probably due to a compensatory response to low dietary fat level, as reported [
7]. Furthermore, breast and thigh muscles of grazing geese had significantly higher protein content and muscle collagen than those of intensively fed geese [
2]. These advantages not only increase consumer demand, but also stimulate producers to seek ways to improve both meat quality and growth rate. Either supplemental feeding of grazing geese or adding an appropriate portion of forage to intensively fed geese were effective to modulate meat quality and growth rate. Liu and Zhou reported improved carcass characteristics, meat quality, and enhanced polyunsaturated fatty acid ratios in geese with ad libitum access to a corn-based ration and an alfalfa-based pasture [
31]. Janicki et al. also reported that semi-intensive feeding resulted in lesser monounsaturated and higher polyunsaturated fatty acids in abdominal fat [
11]. Greater meat
L* and
b* in CON geese was in line with previous studies of higher breast meat
b* and thigh meat
L* values in poultry reared in a free-range system [
23,
30,
32]. Shear force is used for evaluating tenderness of meat (samples with lower shear force are more tender). In the current study, supplemented geese had significantly lower shear force than CON geese on day 56, suggesting that supplemental feeding will increase meat tenderness of grazing Swan geese, presumably due to increased intermuscular fat.
Although slaughter, semi-eviscerated, and eviscerated weights did not differ among geese offered diets with ME ranging from 9.5 to 13.5 MJ/kg, slaughter, semi-eviscerated, and eviscerated yields linearly decreased with increasing dietary ME, as well as a tendency for increased abdominal fat yield. Similarly, in a previous study, dietary ME significant affected eviscerated yield, which peaked at 11.87 MJ of ME/kg of diet [
7]. Current results concerning abdominal fat yield also agreed with previous reports in both geese and ducks that increasing dietary ME was associated with increased abdominal fat percentage [
7,
8,
9], whereas relative abdominal fat weight was reduced significantly by decreasing dietary ME in broiler chickens, ducks, and geese [
5].
Health status can be reflected by serum biochemical parameters. Serum cholesterol, triglyceride, and total protein concentrations were lowered by feeding fibrous diets [
21]. In the current study, serum ALB, GLO, and TP did not differ between CON geese and those supplemented with LMED or MMED diets on day 28, whereas they were significantly lower in grazing Swan geese than all the supplemented geese on day 56. This was likely attributed to an insufficient CP intake of geese that grazed pasture alone as compared to geese supplemented with well-balanced diets. Similarly, replacing a basal diet with 20% grass meal or dried sugar beet pulp meal during the grower period significantly decreased serum TP and ALB concentrations [
21]. Greater BUN and glucose in geese supplementally fed MMED and HMED diets than the grazing geese, suggested better N and energy metabolism status in supplemental fed geese. Numerically greater TC in supplemental fed geese than the grazing geese on day 56 agreed with results of abdominal fat yield. However, although the value fluctuated from day 0 to 56 and among treatments, that all serum biochemical parameters were within a normal range, based on a previous report [
24], suggests that all geese were under good health during the study.