INTRODUCTION
Minerals are separated into macrominerals and microminerals according to the amounts required by animals [
1]. They are inorganic elements that are essential for the body’s metabolic processes, such as activating enzymatically catalyzed reactions, serving as components of certain transport proteins and hormones, or maintaining water and electrolyte balance. Besides, minerals are involved in the acid-base status, osmotic pressure, and cellular permeability [
2], which play vital roles in the prevention of diseases and disorders. Either excess or deficiency in one element may result in metabolic disorders, and interactions among these minerals in the body could interfere in their availability [
3], which might impair animal’s production, reproduction, immunity and even survival. Thus, the estimation of mineral requirements should be a priority in small ruminant production systems to reduce the costs of production and the excretion of inorganic elements in the environment without adverse effects on animal performance [
4].
The Jinzhong sheep is a native breed in Shanxi province, China with large physique, strong adaptability, and high disease resistance [
5]. Dorper breed is specialized for meat production and is characterized by hardiness, early sexually maturity, rapid growth and excellent meat quality [
6]. To improve growth performance and carcass quality, Dorper sheep have recently been introduced in Shanxi to crossbreed with Jinzhong sheep, and Dorper × Jinzhong crossbreed has become the dominant crossbreed for lamb production in Shanxi [
7]. Nevertheless, the diets formulated for sheep in China are based on information recommended by international committees on animal nutrition. A recent study of Dorper × Jinzhong crossed ram lambs suggested that energy requirements are lower than the requirements recommended by the National Research Council (NRC) and Agricultural and Food Research Council (AFRC) [
7], and therefore, it is expected that mineral requirements will also vary from AFRC recommendations.
Net mineral requirements for maintenance of ruminants can be estimated by various methods including endogenous losses, comparative slaughter, and radioisotopes [
8]. Taking into consideration of accuracy, environmental safety and strong operability [
9,
10], comparative slaughter has recently been accepted as a better method to estimate mineral requirements for sheep and goats [
8,
11–
15]. Previous studies have mostly addressed the trace element requirements of lambs during their initial stages of growth [
12,
15], however, such reference data are sub-optimal for lambs’ diet formulations in their latter fattening periods, as the mineral requirements vary with the gender, breed, final body weight (BW), and developing stages [
1,
16]. Therefore, this work estimates the net requirements of micro minerals (Cu, Fe, Mn, Zn) for the maintenance and growth of Dorper × Jinzhong crossbred female lambs at the latter fattening period.
DISCUSSION
In this study, we found that final BW, ADG, and EBW of lambs receiving different feed intakes increased as intake levels increased, which was similar with the results for Dorper × Hu crossed lambs and Caninde goats reported by Zhang et al [
12] and Souza et al [
11], respectively.
To correctly estimate the mineral requirements of growing sheep, we should have an exact knowledge of mineral deposition in the body, which is effected by gender, body weight and feeding intensity. In the present study, the retention of the trace elements (Cu, Fe, Mn, Zn) in empty body have decreases of approximately 82.76%, 129.37%, 66.67%, and 61.17%, respectively, as feed intake decreased, which agrees with a previous report [
11]. The mineral intake, which was calculated based on the recorded feed intake of lambs, had a trend in accordance with ADMI. Moreover, the ratios of mineral retention to mineral intake of minerals (Cu, Fe, Mn, Zn) were decreased from 10.03% to 3.40%, 0.88% to 0.51%, 0.12% to 0.08%, and 4.25% to 3.25%, respectively, as feed intake increased. Similar results were discovered in data of Zhang et al [
12]. For the reason, there is a higher proportion of unavoidable loss in feed-restricted groups, which would lead to a lower efficiency of mineral deposition.
In current study, the ADG of lambs fed with diets formulated according to NRC [
1] in both I-AL and F-AL groups were lower than expected (300 g/d), which suggested NRC likely underestimates the nutritional requirements of Dorper × Jinzhong crossed ewes. In addition, there were only minor differences in the ADG of ewes in different BW ranges (35 to 43 kg for I-AL group, and 35 to 50 kg for F-AL group), this differed from the results of Dorper × Jinzhong crossed rams that showed the ADG of lambs during early growing stage were greater than the whole growth period [
22]. The ADG in current study were lower than results of Dorper × Hu crossed rams reported by Zhang et al [
12], which showed the growing rate of males was greater than females. In accordance with the feeding plan, the final BW and EBW of lambs increased as ages increased, which may lead to the differences of muscle, fat, bone, blood, skin and wool weight among the three slaughter groups. However, there is a consistent pattern for growth in young animals, that was first for bone, second for muscle, and last for fat [
22]. As the slaughter weight of our lambs increased, the EBW increased about 44%, which was greater than the increase of muscle and bone (33.26% and 17.60%, respectively), but much less than fat (213.79%). Thus, we deduce that the weight gain of lambs at the latter fattening period is mainly from the growth of adipose tissue, in accordance with Gomes et al [
23].
As expected, the amounts of minerals examined (Cu, Fe, Mn, Zn) increased significantly with higher BWs, similar results were also found for the German Merino Landsheep [
24]. The deposition of Cu and Mn during the fattening period increased, respectively from 4.47 to 4.94 mg/kg EBW and from 0.83 to 1.02 mg/kg EBW. However, a decrease in the concentration of Fe and Zn was observed (from 56.19 to 47.53 mg/kg EBW and from 24.99 to 24.17 mg/kg EBW).
The mineral requirements of growing sheep are directly linked to the mineral deposition in the body, which varies with genotype, gender, developmental stage, and experimental condition. The ewes used in this study were at the latter finishing stage, wherein adipose tissues grow more quickly in relation to general body development, which is also rapid in adults in comparison to the young animals. To date, few studies have estimated the nutritional mineral requirements of lambs in the later stage of growth. Thus, a comparison of existing work in the literature is difficult.
In the NRC [
1], the mineral requirements of copper, iron, manganese and zinc were estimated with the factorial method for sheep. In addition, the estimation of mineral requirements for maintenance in recommendations, such as ARC [
19], NRC [
1], were mostly based on the fecal endogenous losses, and the mineral retention is determined from the difference between the mineral intake and the total excretions. However, it does not necessarily reflect the total mineral required for maintenance, because, in addition to feces and urine, there are other inevitable losses of minerals, for example, through the skin, sweat, and so on [
20]. Therefore, the comparative slaughter method, a direct method to determine the retained mineral content, was used in present study to estimate the mineral requirement of Dorper × Jinzhong crossed ewe lambs.
As a very important element, copper (Cu) plays key roles in the actions of many crucial enzyme systems [
1]. The deficiency is manifested as anemia, enzootic ataxia and osteochondrosis, but excessive copper intake would cause haemolytic, liver tissue damage, even death [
1]. Because the availability of Cu is reduced in the presence of molybdenum, sulfur and iron, the net requirement of Cu is difficult to determine [
2]. In current study, the net requirement of Cu for maintenance was 0.017 mg/kg BW/d, which was greater than those reported by the NRC [
1], which estimated the net Cu requirement for maintenance as 0.004 mg/kg BW/d for the postweaning lambs kept in feedlots. For Dorper × Jinzhong crossed ram lambs, the maintenance requirement of Cu was 0.013 mg/kg BW/d from 35 to 50 kg [
21], slightly lower than those for ewe lambs in present study. Animals in our study were in the later finishing period, whose requirements of Cu for maintenance were lower than those reported by Zhang et al [
12], which have reported the net maintenance requirements of Cu for Dorper × Hu crossed females as 0.0238 mg/kg BW/d from 20 to 35 kg BW. Expressed as mg/d, the maintenance requirements of Cu estimated in our study was (0.595 to 0.850) mg/d, which was lower than the data reported by Ji [
25] as 1.12 mg/d for ewes from 20 to 35 kg BW. These results indicated that maintenance requirement of Cu was reduced for lambs with growth. The net requirements for growth of Cu increased from 0.48 to 0.51 mg/d per 100 grams live weight gain for ewes from 35 to 50 kg. Little difference was found among animals with the same ADG but different BW, maybe this is the reason why the growth requirement recommendations of NRC [
1] are invariable per gram live weight gain for postweaning lambs. In present study, the growth requirement for lambs of 35 kg with ADG 100 g was 0.48 mg/d, which was much less than the values (1.02 mg/d) estimated by Jin et al [
21], but greater than the results (0.106 and 0.17 mg/d) reported by NRC [
1] and Zhang et al [
12], respectively.
Iron (Fe) is the most abundant trance element of the body and mainly found in hemoglobin which is involved in transporting oxygen and carbon dioxide to and from the tissues and the lung [
1]. Besides, iron can activate enzymes, such as succinate dehydrogenase, hydroxylases, peroxidases, to participate in the metabolism. Iron supplies are rarely inadequate for older farm livestock, in which the major problem is presented by excess dietary Fe [
26]. Net requirement of Fe for maintenance was 0.160 mg/kg BW/d, or (5.6 to 8.0) mg/d from 35 to 50 kg in present study, which was close to values (0.131 mg/kg BW/d, and 5.98 mg/d) reported by Zhang et al [
12] for Dorper × Hu crossed ewe lambs from 20 to 35 kg BW, and by Ji [
25] for Dorper × thin-tailed Han crossbred ewes from 20 to 35 kg BW. Compared with recommendation in NRC [
1] for lambs weaned, the value in our study was nearly ten times greater (0.014 mg/kg BW/d). However, the result in this study was approximately 30% lower than the value (0.228 mg/kg BW/d) estimated for rams of the same breed and body weight, which manifested that genders had a great influence on the Fe requirement for maintenance. The growth requirements of Fe decreased from 2.63 to 2.17 mg/d per 100 grams live weight gain for ewes from 35 to 50 kg. For lambs of 35 kg with ADG 100 g, the net requirements for growth of Fe (2.63 mg/d) was less than the value (5.5 mg/d) recommended by NRC [
1], and lower than the growth requirement (6.28 mg/d) for Dorper × Jinzhong crossed rams, but greater than the result (1.76 mg/d) reported by Zhang et al [
12].
Manganese (Mn) concentration in the body is low, but it is an essential element for ruminants. Manganese deficiency leads to bone abnormalities, interrupted reproductive activity, ataxia in newborns and defective lipid and carbohydrate metabolism. The requirements of Mn for sheep were estimated by the factorial method in NRC [
1], and the endogenous loss (2 μg/kg BW) was used as maintenance requirement. However, the maintenance requirement of Mn estimated by the comparative slaughter method in present study was 0.004 mg/kg BW/d, which was greater than the recommendation reported by NRC [
1] and the value (0.0027 mg/kg BW/d) estimated by Zhang et al [
12], but lower than the result (0.006 mg/kg BW/d) reported by Jin et al [
21]. Expressed as mg/d, the maintenance requirements of Mn estimated in our study was (0.14 to 0.20) mg/d, which was similar to the data reported by Ji [
25] as 0.22 mg/d for ewes from 20 to 35 kg BW. Thus, the maintenance requirement of Mn is greatly influenced by the research methods. The growth requirements of Mn increased from 0.12 to 0.15 mg/d per 100 grams live weight gain for ewes from 35 to 50 kg. For lambs of 35 kg with ADG 100 g, the net requirements for growth of Mn (0.12 mg/d) was lower than the growth requirement (0.22 mg/d) for Dorper × Jinzhong crossed rams, but greater than the values (0.047 and 0.025 mg/d) reported by NRC [
1] and Zhang et al [
12], respectively.
Zinc (Zn) plays a major role in growth and health of all animals and is involved in numerous metalloenzymes, gene expression and appetite. Zn deficiency is a worldwide nutritional problem, the incidence rate of which is ≥25% of the world’s population estimated conservatively [
27,
28]. Lambs having insufficient Zn in the diet may have inhibited development of the spleen and thymus and exhibit symptoms such as anorexia, lose wool and have swollen hocks. Therefore, it is necessary to estimate Zn requirement accurately to prevent Zn disorders. It was considered that daily endogenous loss of Zn was 0.076 mg/kg BW, which was accepted as maintenance requirement of Zn by NRC [
1]. In the current study, the Zn requirement for maintenance was 0.067 mg/kg BW/d, which represented 2.35 to 3.35 mg/d when the ewes grew from 35 to 50 kg BW, which was slightly lower than both the recommendation of NRC [
1] and value (0.0816 mg/kg BW) estimated by Zhang et al [
12], but greater than the result (2.34 mg/d) reported by Ji [
25]. In addition, the estimated value in present study was very close to our previous finding in ram lambs of 0.069 mg/kg BW [
21]. These results suggested that gender has little impact on the maintenance requirement of Zn, which agrees with previous observations [
25]. The growth requirements of Zn decreased from 2.07 to 2.00 mg/d per 100 grams live weight gain for ewes from 35 to 50 kg. For lambs of 35 kg with ADG 100 g, the net requirement for growth of Zn (2.07 mg/d) was close to the values in previous reporters [
1,
12,
21].
The net maintenance requirements of Cu, Fe, Mn, and Zn were 0.017, 0.160, 0.004, and 0.067 mg/kg BW/d, respectively, for Dorper × Jinzhong crossed ewes from 35 to 50 kg BW. The net growth requirements per 100 grams of ADG ranged from 0.48 to 0.51 mg of Cu, 2.63 to 2.17 mg of Fe, 0.12 to 0.15 mg of Mn, and 2.07 to 2.00 mg of Zn as lambs grew from 35 to 50 kg. The requirements for both maintenance and growth of elements were quite different from the recommendations of NRC [
1], except for Zn. In addition, gender wasn’t considered in the NRC [
1] for growing lambs, while our results indicated that gender may have a marked influence on the requirement of Cu, Mn, and Fe. Thus, it is necessary to estimate the requirement for both male and female lambs to provide accurate reference data for sheep production. Because of great difference between the results of Zhang et al [
12] and Ji [
25], we can’t get an accurate conclusion about the trend of requirements with lambs’ growth.