Protein requirements of early-weaned Dorper crossbred female lambs

Journal of Integrative Agriculture 2017, 16(5): 1138–1144 Available online at www.sciencedirect.com

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RESEARCH ARTICLE

Protein requirements of early-weaned Dorper crossbred female lambs MA Tao1, DENG Kai-dong2, TU Yan1, ZHANG Nai-feng1, SI Bing-wen1, XU Gui-shan3, DIAO Qi-yu1 1

Feed Research Institute/Key Laboratory of Feed Biotechnology, Ministry of Agriculture/Chinese Academy of Agricultural Sciences, Beijing 100081, P.R.China

2

College of Animal Science, Jinling Institute of Technology, Nanjing 211100, P.R.China College of Animal Science, Tarim University, Alar 843300, P.R.China

3

Abstract The net protein (NP) and metabolizable protein (MP) requirements of Dorper crossbred female lambs from 20 to 35 kg body weight (BW) were assessed in a comparative slaughter trial. Thirty-five Dorper×thin-tailed Han crossbred lambs weaned at approximately 50 d of age ((19.1±2.37) kg of BW) were used. Seven randomly selected lambs were slaughtered at the beginning of the trial as baseline group (BL). An intermediate group of seven randomly selected lambs fed ad libitum (AL) intake was slaughtered when the lambs reached an average BW of 28.6 kg. The remaining 21 lambs were allotted randomly to three levels of dry matter intake: AL or restricted to 70 or 40% of the AL intake. All lambs were slaughtered when the sheep fed AL intake reached 35 kg of BW. Total body N and N retention were determined. The results showed that the maintenance requirements for NP and MP were 1.75 and 3.37 g kg–1 metabolic shrunk body weight (SBW0.75), respectively. The partial efficiency of protein use for maintenance was 0.52. The NP requirements for growth ranged from 10.9 to 42.4 g d–1 for the lambs gaining 100 to 350 g d–1 from 20 to 35 kg BW. The partial efficiency of MP for growth was 0.52. In conclusion, the NP and MP requirements for the maintenance and growth of Dorper crossbred female lambs were lower than those reported by AFRC (1993) and NRC (2007) recommendations. Keywords: growth, maintenance, metabolizable protein, net protein, lamb

Scientific and Industrial Research Organisation (CSIRO

1. Introduction At present, several feeding systems, such as Agricultural and Food Research Council (AFRC 1993), Commonwealth

2007), and National Research Council (NRC 2007) have reported protein and other nutrient requirements for sheep, which are widely adopted for diet formulation around the world. In the intensive livestock industry, protein is commonly the most expensive feed component and therefore, it is necessary to have a precise understanding of protein requirements of livestock not only to ensure farm profitability,

Received 2 June, 2016 Accepted 21 July, 2016 MA Tao, E-mail: [email protected]; Correspondence DIAO Qi-yu, Tel: +86-10-82106055, Fax: +86-10-62169105, E-mail: [email protected]

but also to help reduce nitrogen (N) emission to the envi-

© 2017, CAAS. All rights reserved. Published by Elsevier Ltd. doi: 10.1016/S2095-3119(16)61455-7

protein (MP), which is defined as the total digestible true

ronment (Ma et al. 2016). The current nutritional systems for sheep specify protein requirements as metabolizable protein (amino acids) available to the animal for metabolism

MA Tao et al. Journal of Integrative Agriculture 2017, 16(5): 1138–1144

after digestion and absorption of the feed in the animal’s digestive tract (AFRC 1993). China has the largest sheep and goat population (approx. 300 million) in the world and 15 autochthonous sheep breeds (Tu 1989), among which the thin-tailed Han sheep is one of the most famous native breeds. It displays excellent characteristics of high prolificacy, as it carries mutations in both the BMPR-1B and BMP15 genes; therefore, it has a greater litter size (2.61) than those with either mutation alone (Chu et al. 2007). In recent years, the Dorper sheep was imported to improve meat production traits and thus, the Dorper×thin-tailed Han crossbreed has become one of the most important sheep breeds for dual purposes. Our research team conducted a systematic study on the nutrient requirements (energy, protein, and minerals) of fattening Dorper×thin-tailed Han sheep using a comparative slaughter technique (Deng et al. 2012, 2014; Xu et al. 2015; Ma et al. 2016). In this paper, we reported the protein requirements of female lambs after weaning with body weights (BWs) ranging from 20 to 35 kg, with an aim to provide knowledge of the protein requirements of Dorper crossbred lambs.

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Table 1 Ingredient and chemical compositions of the pelleted mixture diet Items1) Ingredients (DM basis) Milled Chinese wildrye hay (%) Cracked corn grain (%) Soybean meal (%) Dicalcium phosphate (%) Salt (%) Mineral/Vitamin premix (%)2) Chemical composition ME (MJ kg–1 DM) DM (% as fed) CP (% of DM) EE (% of DM) Ash (% of DM) NDF (% of DM) ADF (% of DM) Calcium (% of DM) Phosphorus (% of DM)

Value 55.0 29.4 14.0 0.86 0.50 0.24 8.89 95.5 11.9 2.71 6.32 40.9 15.2 0.68 0.33

1)

DM, dry matter; ME, metabolizable protein; CP, crude protein; EE, ether extract; NDF, neutral detergent fibre; ADF, acid detergent fibre. 2) Manufactured by Precision Animal Nutrition Research Centre, Beijing, China. The premix contained (per kg): 113.7 g FeSO 4·7H 2O, 5.62 g CuSO 4, 27.0 g MnSO4, 66.9 g ZnSO 4, 0.42 g Na2SeO3, 1.66 g Ca(IO3)2, 0.36 g CoCl2·6H2O, 3.2 g vitamin A, 0.8 g vitamin D3, and 0.4 g vitamin E.

2. Materials and methods The research was conducted from March to June 2011 at the Experimental Station of the Chinese Academy of Agricultural Sciences (CAAS), Nankou (40°22´N, 116°1´E), Beijing, China. The mean minimum and maximum room temperatures observed during the experimental period were 6.0 and 20.0°C (average 13.0°C), respectively. The experimental protocol was approved by the CAAS Animal Ethical Committee, and humane animal care and handling procedures were followed throughout the experiment.

2.1. Comparative slaughter trial Thirty-five Dorper×thin-tailed Han crossbred female lambs weaned at approximately 50 d of age with (20.4±2.15) kg of BW were used in a completely randomized design to measure protein requirements for maintenance and growth. The experimental diet with a concentrate-to-forage ratio of 44:56 on a dry matter (DM) basis was formulated according to the NRC (2007). The diet was pelleted to prevent possible selectivity and waste for accurate measurements of feed intake. The ingredient and chemical compositions of the diet are shown in Table 1. The lambs with ad libitum (AL) intake were fed once daily at 0800 h and allowed 10% of orts. The amount of feed provided to the restricted feed intake groups was adjusted daily based on the average DM intake of the AL group from the previous day. Feed and orts

were sampled daily and frozen at –20°C until the analyses. A comparative slaughter trial was conducted, as described by Xu et al. (2015). Briefly, the initial body composition was measured on seven lambs slaughtered at 20 kg BW (baseline group). An intermediate slaughter group with seven randomly selected lambs fed AL were slaughtered when they reached 28.6 kg BW. The remaining 21 lambs were randomly assigned to three levels of DM intake: AL or restricted to either 70 or 40% of the AL intake. Thus, the lambs were pair-fed in seven slaughter groups, with each group consisting of one lamb from each level of intake. When the lambs fed AL of each slaughter group reached 35 kg BW, all three lambs within a slaughter group were fasted and slaughtered. All lambs were slaughtered by exsanguination after stunning by CO2 inhalation. Blood, carcass, head, feet, hide, wool, viscera, and adipose tissue removed from the internal organs were weighed. The empty body weight (EBW) was calculated by subtracting the weight of the digestive tract contents from the shrunk body weight (SBW), which was measured as BW after a 16-h fast of feed and water. Carcasses and heads were split longitudinally into two identical halves and the muscle, bone, and fat were dissected from the right-half carcass, head, and feet, while the whole hide and whole viscera were ground and homogenized separately and frozen at –20°C until the analyses. Wool was clipped with electrical clippers after slaughter, and subsamples were collected and stored at 4°C.

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2.2. Chemical analyses The DM content was measured by drying samples (feed and orts) in an air-forced oven at 135°C for 2 h (method 930.15; AOAC 1990). The ash content was measured by placing the samples into a muffle furnace at 600°C for 6 h (method 924.05; AOAC 1990). The organic matter (OM) was calculated by the difference between DM and ash contents. Nitrogen was measured with Kjeldahl digestion, using selenium as a catalyst (Marshall and Walker 1978), and crude protein (CP) was calculated as 6.25×N. The gross energy (GE) was measured with a bomb calorimeter (C200, IKA Works Inc., Staufen, Germany). The ether extract (EE) was determined by DM weight loss after extraction with diethyl ether in a Soxhlet extraction apparatus for 8 h (method 920.85; AOAC 1990). The neutral detergent fiber (NDF) and acid detergent fiber (ADF) were measured according to Van Soest et al. (1991) and Goering and Van Soest (1970), respectively. Calcium (Ca) was analyzed using an atomic absorption spectrophotometer (M9W-700, Perkin-Elmer Corp., Norwalk, CT, USA) (method 968.08; AOAC 1990). The total phosphorus (P) was analyzed by the molybdovanadate colorimetric method (method 965.17; AOAC 1990) using a spectrophotometer (UV-6100, Mapada Instruments Co., Ltd., Shanghai, China).

2.3. Data calculations Metabolizable protein supply The ratio of MP to OM intake reported in a previous in vivo study (i.e., 69.4, 88.9, and 102.4 g MP kg–1 OM intake for AL, 70 and 40% of the AL intake, respectively; Ma et al. 2015) with Dorper×thin-tailed Han crossbred sheep subjected to the same feeding regime as the present study was used to calculate the individual MP intake by the female lambs. Prediction of the initial body N content and N retention Nitrogen retained in the body of the lambs in the comparative slaughter trial was calculated as the difference between the final and initial body N content. The initial body N content of each animal was calculated from its initial EBW using a regression equation developed from the relationship between the body N content and EBW of the BL animals (r2=0.89, root mean square error (RMSE)=0.011, n=6, P<0.001): log10empty body N (kg)=1.788(±0.160)+[0.750(± 0.130)×log10EBW (kg)]. The initial SBW of each animal was computed from its initial BW (r2=0.99, RMSE=0.165, n=7, P<0.001): SBW (kg)=–1.653(±1.019)+[1.009(±0.047)×BW (kg)], and the initial EBW of each animal was computed from its initial SBW (r2=0.95, RMSE=0.423, n=7, P<0.001): EBW (kg)=–1.232(±1.400)+[0.921(±0.118)×SBW (kg)]. An outlier was removed from the database of body N content and N retention.

Protein requirements for maintenance A linear regression of daily retained N on daily N intake was used to calculate the net protein requirement for maintenance (NPm). The intercept of the regression was assumed the endogenous and metabolic losses of N multiplied by the factor 6.25, which is assumed the NPm. The MP required for maintenance (MPm) was estimated by regressing the retained N on MP intake and extrapolating the linear regression to zero N retention. The efficiency of MP use for maintenance (kpm) was computed as NPm/MPm. Protein requirements for growth The net protein requirements for body weight gain (NPg) were calculated as the difference between body protein content at different intervals. For example, the NPg of a lamb with 20 kg of SBW and 250 g of average daily gain (ADG) was computed as the difference between body protein contents at 20.25 and 20 kg of SBW. Body protein contents were predicted from EBW using an allometric equation according to ARC (1980): log10protein (kg)=a+[b×log10EBW (kg)]. To estimate the partial efficiencies of MP use for body weight gain (kpg), a multiple regression model through the origin was used to partition the utilization of MP intake above maintenance for body protein retention as follows: MPIg=b×RP, where MPIg (g kg–1 SBW0.75) is the MP intake above maintenance calculated as the difference between the total MP intake and MPm, RP (g kg–1 SBW0.75) is the daily retention of body protein, and the estimated parameter b is the amount of MP (g) required to retain 1 g of protein, and its inverse was assumed to be kpg.

2.4. Statistical analyses The data were analyzed as a completely randomized design using the SAS statistical software package (ver. 9.1; SAS Institute, Inc., Cary, NC). Intake, body composition, and growth rate were analyzed using a one-way ANOVA. Pairwise comparisons of means were performed by Tukey’s multiple range tests once the significance of the treatment effect was declared at P<0.05. Linear regressions were conducted with a GLM, and observations with a studentized residual >2.5 or <−2.5 were considered outliers. The assumptions of the models, in terms of homoscedasticity, independency, and normality of errors, were examined by plotting residuals against the predicted values.

3. Results 3.1. Nutrient intake and N retention The intake of OM (P<0.001), N (P<0.001), and MP (P=0.015) increased with an increasing feeding level (Table 2). The lambs fed 40% of the AL had a lower N retention than the

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Table 2 Daily protein intake of Dorper×thin-tailed Han crossbred female lambs at ad libitum (AL) or restricted to 70 or 40% of ad libitum (AL) intake Items1) Initial SBW (kg) Final SBW (kg) OM intake (g kg–1 SBW0.75) N intake (g kg–1 SBW0.75) N retention (g kg–1 SBW0.75) MP intake (g kg–1 SBW0.75)2)

AL 20.3 30.4 a 102.3 a 1.94 a 0.30 a 7.11 a

Level of feed intake 70% 20.3 29.6 a 91.0 b 1.72 b 0.27 a 6.30 b

40% 20.3 21.6 b 51.4 c 0.97 c 0.01 b 3.56 c

SEM

P-value

0.20 0.89 4.39 0.08 0.03 0.31

0.995 <0.001 <0.001 <0.001 <0.001 <0.001

1)

SBW, shrunk body weight; OM, organic matter; N, nitrogen; MP, metabolizable protein. Calculated from the ratio of MP supply to OM intake reported by Ma et al. (2015). Means within a row with different letters differ (P<0.05).

3.2. Protein requirements for maintenance Nitrogen retention (NR) was linearly correlated with N intake (NI; P<0.001, Fig. 1). The endogenous and metabolic loss of N, estimated as the intercept of the linear regression, was (280±47) mg kg–1 SBW0.75, which corresponds to an NPm of (1.75±0.29) g kg–1 SBW0.75. When N retention was regressed with MP intake, a linear equation was obtained (P<0.001, Fig. 2), and the MP required for maintenance by extrapolating the linear regression to zero N retention was 3.37 g kg–1 SBW0.75. Consequently, the kpm was 0.52 for Dorper×thin-tailed Han crossbred female lambs from 20 to 35 kg of BW.

3.3. Protein requirements for growth The partial efficiencies of MP use for body weight gain were estimated using a multiple regression model and assuming that the metabolizable N intake above maintenance (MNIg) is partially recovered as body N for growth (RNg, g kg–1 SBW0.75). The multiple regression equation obtained to explain this relationship was: RNg (g kg–1 SBW0.75)=0.001(±0.023)+[0.521(±0.047)× MNI g (g kg –1 SBW 0.75)] (r 2=0.82, n=28, RMSE=0.063, P<0.001) The intercept of this last equation did not differ from 0 (P=0.97), indicating a non-existent N retention when MNIg was 0. The slope of the regression equation (0.52) represents kpg and thus, NP and MP requirements for body weight gain (MPg) are presented in Tables 3 and 4.

0.60 0.50 0.40 0.30 0.20 0.10 0.00 –0.10

0.50

1.00

1.50

2.00

2.50

N intake (g kg–1 SBW0.75 d–1)

Fig. 1 Relationship between nitrogen (N) retention and N intake of Dorper× thin-tailed Han crossbred female lambs from 20 to 35 kg of body weight (BW). N retention (g kg–1 shrunk body weight, SBW0.75)=−0.280(±0.047)+[0.304(±0.028)×N intake (g kg–1 SBW0.75)], r2=0.82, root mean square error (RMSE)=0.064, n=28, P<0.001.

N retention (g kg–1 SBW0.75 d–1)

other groups (P<0.001). The N retention did not differ between lambs fed with AL and 70% of the AL (P=0.209).

N retention (g kg–1 SBW0.75 d–1)

2)

0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 –0.10 3 –0.20

4

5

6

7

8

9

Metabolizable protein intake (g kg–1 SBW0.75 d–1)

Fig. 2 Relationship between nitrogen retention (NR) and metabolizable protein intake (MPI) of Dorper×thin-tailed Han crossbred female lambs from 20 to 35 kg of BW. NR (g kg–1 SBW0.75)=−0.282(±0.047)+[0.083(±0.007)×MPI (g kg–1 SBW0.75)], r2=0.83, RMSE=0.063, n=28, P<0.001.

by counterbalancing the inevitable losses of urinary, fecal,

4. Discussion

and dermal N (CSIRO 2007). AFRC (1992) suggested daily

4.1. Protein requirements for maintenance

for lambs nourished by intra-gastric infusions. However,

The NPm is the amount of protein to sustain tissue proteins

quirement due to the lack of conservation of protein by the

endogenous and metabolic N losses of 350 mg kg–1 BW0.75 this method might overestimate the endogenous N re-

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Table 3 Net protein (NP) required for growth (g d –1) of Dorper×thin-tailed Han crossbred female lambs from 20 to 35 kg of body weight (BW) ADG (g)1) 100 200 300 350 1)

20 12.1 24.2 36.3 42.4

BW (kg) 25 11.6 23.2 34.8 40.6

30 11.2 22.4 33.6 39.2

35 10.9 21.8 32.7 38.1

ADG, average daily gain.

Table 4 Metabolizable protein (MP) required for growth (g d–1) of Dorper×thin-tailed Han crossbred female lambs from 20 to 35 kg of body weight (BW) ADG (g)1) 100 200 300 350 1)

20 23.3 46.6 69.8 81.5

BW (kg) 25 22.3 44.7 67.0 78.1

30 21.6 43.1 64.7 75.5

35 21.0 41.9 62.8 73.3

ADG, average daily gain.

microbial capture of N. CSIRO (2007) suggested the NPm of the sheep was the sum of the endogenous urinary loss (0.147×BW+3.375) and fecal loss (15.2 g kg–1 DM intake). Thus, the NPm for a lamb of 28 kg consuming 1.02 kg of DM daily (this is the average DM intake of the 28 lambs used in the current study) is about 23.0 g, which is 10% greater than the current value (1.75 g kg–1 SBW0.75). In the current study, the NPm calculated by partial regression N retained on N intake (1.75 g kg–1 SBW0.75 or 1.68 g kg–1 BW0.75) was slightly greater than that of the Ile de France (1.56 g kg–1 SBW0.75; Silva et al. 2003) and Texel crossbreeds (1.52 g kg–1 SBW0.75; Galvani et al. 2009), but lower than that of Morada Nova (1.83 g kg–1 SBW0.75; Costa et al. 2013) lambs measured using the same methods. The variations in NPm could be explained by the differential utilization efficiency of protein or amino acids by the tissues during the growth of lambs. On the other hand, the above studies were all conducted in a tropical or sub-tropical zone with high humidity, while the current study was conducted in a warm temperate zone where the weather is always dry during summer. As reviewed by Marai et al. (2007), both temperature and humidity could have influence on nutrient digestibility and degradation in the rumen; thus, the experimental condition could be another important factor that contributed to the differences in NPm apart from methods or animal breeds. The MPm obtained in the current study was 3.37 g kg–1 SBW0.75 or 3.24 g kg–1 BW0.75. Our result is greater than that suggested by AFRC (1993; 2.1875 g kg–1 BW0.75), INRA (1989; 2.50 g kg–1 BW0.75), but lower than that reported by NRC (1987; 3.72 g kg–1 BW0.75) and Liu et al. (2005) who found the MPm of 4.41 g kg–1 BW0.75 from a linear multiple

regression of MP requirements against the body weight, live weight gain, and wool growth of sheep (n=213) in a feeding study. In a more recent study, where a comparative slaughter trial was also used, an MPm of 2.31 g kg–1 SBW0.75 was observed in Texel crossbred lambs (Galvani et al. 2009). The methods adopted by the USA (NRC 2007), UK (AFRC 1993), France (INRA 1989), and Australia (CSIRO 2007) are all based on a common overall model, although requirements are expressed in different terms. In the current study, MP was calculated based on the method reported by Ma et al. (2015), who conducted an in vivo study and measured MP using sheep with ruminal and duodenal cannula. Therefore, the discrepancy in the calculation of MP is inevitably associated with the methodologies adopted. As there is still a lack of simple and robust methods for calculating MP, this area requires further investigation and examination.

4.2. Protein requirements for growth The body protein contents of Dorper crossbred female lambs (range from 187.1 to 195.2 g kg–1 EBW; Table 5) was comparable to those of Texel crossbred (range from 175.2 to 176.2 g kg–1 EBW; Galvani et al. 2009) and Morada Nova lambs (range from 167.3 to 175.3 g kg–1 EBW; Costa et al. 2013), but greater than those of hair and wool lambs (range from 112.6 to 122.9 g kg–1 EBW; Silva et al. 2003). Body protein composition decreased as lambs grew from 20 to 35 kg of BW in the current study. The relative loss in body protein could be explained by the increasing proportion of fat during the fattening period, as proved by Xu et al. (2015) that fat deposition was the dominant factor increasing body energy composition. Our findings were in accordance with studies on Ideal×Ile de France wool, Santa Inês hair lambs (Silva et al. 2007), and Morada Nova lambs (Costa et al. 2013). Galvani et al. (2009) also observed a decreasing proportion of protein in fleece-free bodies. However, in their study, the whole body protein content of the lamb was nearly constant, as the increasing protein content of the wool compensated for the loss in the body. The Dorper is regarded as a nonwool breed and it was reported that Dorper ewes could only produce 0.66 kg of greasy wool over an 8-month period (Basson et al. 1969), which was lower than that of the Texel crossbreed (around 2.40 kg; Wuliji et al. 1990). Therefore, it is unexpected that a compensation for body protein from wool growth in Dorper crossbred lambs will be observed. The NPg values (12.1, 24.2, and 36.3 g d–1) determined in the current study were extensively lower than those of early maturing growing lambs (23.5, 30.5, and 50.0 g d–1) of 20 kg BW gaining 100, 200, and 300 g d–1 recommended by NRC (2007), assuming a kpg of 0.50. AFRC (1993) used two equations proposed by ARC (1980), NPf (g d–1)=ADG×(160.4–1.22×BW+0.0105×BW2) and NPw (g

MA Tao et al. Journal of Integrative Agriculture 2017, 16(5): 1138–1144

Table 5 Body composition of Dorper×thin-tailed Han crossbred female lambs from 20 to 35 kg of BW1) Items EBW (kg) Water (g kg–1 EBW) Protein (g kg–1 EBW) Fat (g kg–1 EBW) Ash (g kg–1 EBW) Energy (MJ kg–1 EBW) 1)

20 16.2 654.5 195.2 76.1 40.6 7.36

SBW (kg) 25 30 35 20.1 24.0 28.0 632.0 614.1 599.3 192.0 189.3 187.1 95.1 114.1 133.2 40.4 40.3 40.2 8.26 9.08 9.84

EBW, empty body weight; SBW, shrunk body weight.

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levels (AL, 70 and 40% AL) in which the ratio of MP supply to OM intake (g kg–1) were 69.4, 88.9, and 102.4, respectively. As there is still lack of a simple method for the calculation of MP, this could be a reasonable way to calculate MP in the current study. Nevertheless, considering the difference in animal physiology status (BW, age, and cannulation) between our previous and current studies, further study is still needed to examine the utilization efficiency of MP for both the maintenance and growth for early-weaning lambs.

5. Conclusion d )=3+0.1×NPf, to predict the protein requirement for the growth of body and fleece in lambs, respectively. According to those equations, the NPg (NPf+NPw) was approximately 26 to 49% greater (ranged from 17.4 to 57.0 g d–1) than values determined in the current study. Therefore, caution should be taken before applying certain evaluation systems to avoid the overestimation of NPg of Dorper crossbred lambs. By using the same method, our NPg values were 26% greater than those of the Texel crossbreed (Galvani et al. 2009) growing from 20 to 35 kg of SBW gaining 100 and 200 g d–1, but 20% lower than that of Morada Nova lambs (Costa et al. 2013) growing from 20 to 30 kg of BW gaining 100, 200, and 300 g d–1, respectively. Many factors could be associated with such discrepancies, including breed, physiological stage, and experimental conditions. –1

4.3. Efficiency of MP utilization The partial use efficiency of MPm to NPm (kpm) was calculated to be 0.52 in the present study. This value was lower than previously adopted 1.0 by AFRC (1992) or 0.67 by CSIRO (2007), as well as lower than that of Texel crossbred lambs (0.66, Galvani et al. 2009). The partial efficiency of use of MPg to NPg (kpg) obtained in the current study (0.86) was greater than that adopted by AFRC (1993; 0.59), CSIRO (2007; 0.70), and Galvani et al. (2009) in Texel crossbred lambs (0.71). Those discrepancies could be attributed to animal factors, including breed, maturity, and physiological status. The method for calculating or determining MP may be another factor contributing to the variability of the efficiency of MP use. Our previous study showed that a decreased feed intake could increase total-tract N digestibility without affecting ruminal N digestibility (Ma et al. 2013), and the increased duodenal N digestibility could be due to the prolonged gastric empty time. Thus, it could not be expected that the ratio of MP to CP were identical under a different feeding level. In the current study, MP was calculated from OM intake based on the results of our previous study using 6-month-old Dorper×thin-tailed Han lambs (41.3±2.8 kg BW) with both ruminal and duodenal cannula fed at three different

The current study suggested that the protein requirements for the maintenance and growth of Dorper×thin-tailed Han early-weaned crossbred female lambs were lower than the recommendations of AFRC (1993) and NRC (2007).

Acknowledgements This research was supported by the earmarked fund for the China Agriculture Research System (CARS-39). We thank the staff (Li Yanling, Zhang Yongfa, Liu Jie, Zhao Yiguang, Ji Shoukun, Zhang Litao, and Lou Can) of Feed Research Institute of Chinese Academy of Agricultural Sciences for their technical assistance.

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