Effect of feed intake on metabolizable protein supply in Dorper × thin-tailed Han crossbred lambs

Accepted Manuscript Title: Effect of feed intake on metabolizable protein supply in Dorper × thin-tailed Han crossbred lambs Author: T. Ma K.-D. Deng Y. -Tu N.-F. Zhang C.-G. Jiang J. -Liu Y.-G. Zhao Q.-Y. Diao PII: DOI: Reference:

S0921-4488(15)30088-2 http://dx.doi.org/doi:10.1016/j.smallrumres.2015.10.016 RUMIN 5056

To appear in:

Small Ruminant Research

Received date: Revised date: Accepted date:

20-7-2015 11-10-2015 13-10-2015

Please cite this article as: Ma, T., Deng, K.-D., -Tu, Y., Zhang, N.-F., Jiang, C.-G., -Liu, J., Zhao, Y.-G., Diao, Q.-Y., Effect of feed intake on metabolizable protein supply in Dorper times thin-tailed Han crossbred lambs.Small Ruminant Research http://dx.doi.org/10.1016/j.smallrumres.2015.10.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of feed intake on metabolizable protein supply in Dorper  thin-tailed Han crossbred lambs T. Maa, K.-D. Dengb, Y.-Tua, N.-F. Zhanga, C.-G. Jianga, J. -Liua, Y.-G. Zhaoa, Q.-Y. Diaoa* a

Feed Research Institute, Chinese Academy of Agricultural Sciences, Beijing 100081,

China b

College of Animal Science, Jinling Institute of Technology, Nanjing, Jiangsu 210038,

China

Running title: Metabolizable protein supply in lambs

*Correspondence: Professor Q.-Y. Diao, Feed Research Institute, Chinese Academy of Agricultural Sciences, Beijing, China, tel +86(10) 8210 6055, fax +86(10) 6216 9105, email [email protected]

1

Highlights 

Metabolizable protein (MP) supply was measured in vivo in Dorper crossbred lambs at three levels of feed intake.



MP supplies for lambs with ADGs of 300, 150, and 0 g/day were 102, 91, and 75 g/day, respectively.



MP supply was predicted from intake of organic matter (R2 = 0.89) or crude protein (R2 = 0.89).



The MP requirement for Dorper crossbred lambs with an ADG of 300 g was lower than that for British or American evaluation systems.

2

Abstract: We investigated the metabolizable protein (MP) supply in lambs at different levels of feed intake. Twelve Dorper  thin-tailed Han crossbred ram lambs (41.3 ± 2.8 kg body weight) fitted with ruminal and duodenal cannulae were randomly assigned to one of three levels (n = 4 lambs each) of dry matter intake: ad libitum (AL) intake and 70% or 50% of AL intake. Digesta flow was measured using a dual-marker system (Yb and Co). A lower duodenal flow of rumen undegraded nitrogen (RUN) was measured for the 50% AL group (P < 0.05) compared with the other two groups. For lambs of the AL group, the ratio of microbial N/duodenal non-ammonia nitrogen (NAN) was lower (P < 0.05), and the ratio of RUN/duodenal NAN was higher (P < 0.05) compared with the other two groups. The ratio of RUN/N intake was higher for the 70% AL and 50% AL groups compared with that for the AL group (P < 0.05). Apparent post-ruminal N digestibility increased with decreasing feed intake (P < 0.05). A linear correlation was established to predict MP supply (g/day) from the intake of organic matter (kg/day) or crude protein (g/day): MP = 0.036 (±0.004) × organic matter intake + 50.47 (±4.43), R2 = 0.89; MP = 0.27 (±0.033) × crude protein intake + 49.88 (±4.93), R2 = 0.87. The current results provide preliminary data of MP requirements for growth of Dorper crossbred lambs. Key words: duodenal non-ammonia nitrogen; lamb; metabolizable protein; rumen undegraded nitrogen

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1. Introduction Protein is the most expensive feed ingredient for livestock, and a clear understanding of the protein requirements of animals can be valuable for reducing costs and ensuring farm profitability. The crude protein (CP) system does not differentiate the requirements of ruminal microbes and the host animal. Furthermore, the CP system is based on an invalid assumption that the proteins in all feedstuffs are equally degraded in the rumen, with CP being converted to metabolizable protein (MP) with equal efficiency in all diets (NRC, 2000). Therefore, the requirements based on the CP system do not necessarily reflect the requirements of ruminants and may lead to protein deficiency. The current nutritional systems for sheep specify protein requirements as MP, including ruminally synthesized microbial CP, rumen undegraded protein, and a smaller proportion of endogenous CP, which contributes to the protein that is digestible in the small intestine (CSIRO, 2007; AFRC, 1993; NRC, 2007). Although MP may accurately reflect the utilization of protein by ruminants, evaluating MP in vivo is not easy because cannulated animals are required and surgical operations are laborious and expensive. Therefore, few in vivo studies have been conducted, and in vitro or in situ protein evaluation systems have been used in vivo measurements. However, in vitro results do not always accurately reflect the situation in vivo. Thus, we evaluated the MP supply in vivo using different levels of feed intake. Previously, our research team systematically investigated the requirements of energy (Deng et al., 2012, 2014; Xu et al., 2015) and minerals (Ji et al., 2013, 2014) in 4

Dorper × thin-tailed Han crossbred sheep, a dominant breed for lamb meat production in China. The present study provides preliminary data for evaluation of MP requirements of these crossbred lambs. 2. Materials and Methods 2.1. Animals and diets This study was conducted at the Experimental Station of the Chinese Academy of Agricultural Sciences (CAAS), Beijing, China from August 2010 to October 2010. The experimental procedures were approved by the Animal Ethics Committee of CAAS, and humane animal care and handling procedures were followed throughout the study. The trial was conducted for 25 days (Ma et al., 2013). Briefly, twelve 6-month-old Dorper  thin-tailed Han crossbred ram lambs (41.3 ± 2.8 kg body weight) were fitted with ruminal and duodenal cannulae and were randomly assigned to one of three levels of dry matter (DM) intake (4 lambs/group) according to a completely randomized design: ad libitum (AL), or restricted to 70% or 50% of the AL intake. The diet (Table 1) was offered as a single-pellet mixture (6.0 mm diameter) and fed once daily at 08:00 hours. The lambs had free access to clean water at all times. After adaption to dietary treatments for 7 days, lambs were then moved into individual metabolism crates for 12 days including 7 additional days of adaption and 5 days of a digestibility trial. Then, duodenal and ruminal digesta were sampled daily at time hours on 6 consecutive days (Ma et al., 2013). All lambs were weighed at the beginning and end of the digestibility trial. 5

The elements Yb and Co were used as markers to determine the duodenal digesta flow, and 15N was used as an external microbial marker (Ma et al., 2013). From days 8 to 25, the lambs were fed the experimental diet. Prior to offering the labeled feed, a priming dose that contained half the daily marker intake was administered through the rumen cannula of each animal. 2.2. Measurements and sample collection 2.2.1. Feed, orts, and feces In the digestibility trial, feces were collected daily at time hours from days 15 to 20. Feces were weighed daily, and then a sample of 10% was collected and pooled across days for each animal, dried at 65°C, and ground through a 1-mm sieve for analysis. Samples of feed were also collected daily, combined, dried at 65°C for 72 h, and ground through a 1-mm sieve. Feed refusals were weighed, sampled, dried, ground, and combined for each lamb before analysis. 2.2.2. Duodenal and ruminal digesta collection From days 21 to 23, a 100-ml sample of duodenal digesta was collected every 6 h, moving the collection time forward 2 h each day to obtain the samples at 2-h intervals. The samples were combined for each lamb and separated into particulate and liquid fractions for analysis of nutrient concentrations and markers (Ma et al., 2013). Samples of ruminal digesta were collected at 6-h intervals on days 24 and 25 for determination of microbial N yield (08:00, 14:00, and 20:00 hours on day 24; and 02:00, 05:00, 11:00, 17:00, and 23:00 hours on day 25; Ma et al., 2013). 2.3. Chemical analysis 6

Feed and orts were analyzed for DM, organic matter (OM), and N. DM was determined by drying samples in an oven at 135°C for 2 h (method 930.15; AOAC, 1995). OM was measured as the difference between DM and ash content (g/kg DM) and the ash content was measured by placing the samples into a muffle furnace at 600 ºC for 6 h (method 924.05; AOAC, 1990). Nitrogen was determined by the Kjeldahl method using Se as a catalyst (Marshall and Walker, 1978), and CP was calculated as 6.25 × N. The isotopic abundance of

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N in bacterial N and duodenal fractions was

determined by isotope ratio mass spectrometry (Finnigan Mat 251, Thermo Fisher Scientific Inc., USA). The concentrations of Yb and Co in both feed and digesta were separately determined by inductively coupled plasma emission spectrometry (X series 2 ICP-MS, Thermo Fisher Scientific Inc., USA). 2.4. Calculations and statistical analysis The duodenal flows of nutrients were determined by reconstitution of the duodenal digesta based on Yb and Co concentrations and the content of the nutrients in the particulate and whole fractions (Faichney, 1975). Endogenous N was calculated as 0.10 of duodenal N flow according to the NRC (1985), and rumen undegraded nitrogen (RUN) was calculated by subtracting microbial N and endogenous N from duodenal non-ammonia nitrogen (NAN). Apparent post-ruminal N digestibility was calculated as the disappearance of N (%) as the digesta N flowed from the duodenum to the anus. The MP was calculated as duodenal NAN × apparent post-ruminal N digestibility. The data were analyzed as a completely randomized design using SAS version 7

9.1 (SAS Institute, Inc., Cary, NC, USA). All results were analyzed using PROC GLM, and the comparison of the means was performed using the least squares means option of SAS. Besides, all data were analyzed by a regression including the linear and quadratic effects of level of intake. Besides, the linear regressions analyses were conducted with PROC REG Statistical significance was accepted if P < 0.05. 3. Results Lesser duodenal flow of RUN was observed for the 50% AL group (P < 0.05; Table 2). The ratio of microbial N/duodenal NAN was lower (P < 0.05), whereas the ratio of RUN/duodenal NAN was higher (P < 0.05) in lambs in the AL group compared with the other two groups. The ratio of RUN/N intake was higher for the 70% and 50% AL groups compared with that of the AL group (P < 0.05). Apparent post-ruminal N digestibility increased with decreasing feed intake (P < 0.05; Table 3). Both MP/OM intake (P < 0.05) and MP/CP intake increased (P < 0.05) with increasing feed intake. A linear or quadric correlation was observed between MP supply (g/day) and the intake of OM (kg/day) or CP (g/day) (Table 4). 4. Discussion We observed a lower ratio of duodenal NAN/N intake in the AL group (0.87) compared with the 70% (1.04) and 50% (1.05) of AL groups. The lambs in the AL group may have had less recycled N in the form of urea than the lambs in the feed-restriction groups because the capacity for ammonia absorption was limited and excess ammonia could not be efficiently absorbed by the rumen. This may further 8

explain the similarity in duodenal RUN flow between lambs in both the AL (10.2 g/day) and 70% (10.9 g/day) of AL groups, as RUN was calculated by subtracting microbial N and endogenous N from the duodenal NAN flow. Microbial protein, ruminal undegraded protein, and endogenous protein entering the small intestine are efficiently digested and absorbed (Annison et al., 2002). The large intestine also supports microbial populations that ferment digesta, similar to the rumen. A net NH3-N absorption equivalent to 0.5 or 5.3 g N/day was measured for sheep fed a low- or high-N diet, respectively (Dixon and Nolan, 1982), but this contribution could be relatively low in our study. The apparent post-ruminal N digestibility averaged 0.63 ± 0.02 (standard error) in the current study, which is close to the combined results from five experiments on sheep (0.64 ± 0.04; NRC, 1985). Assuming that the intestinal digestibilities of microbial N and endogenous N are 0.64 (assuming that 80% of the microbial protein was true protein and 80% of microbial true protein was digested; NRC, 2000) and 0.67 (Varga, 2007), respectively, the intestinal digestibilities of dietary RUN could range from 41% to 55%, which is lower than those reported by Van Straalen and Tamminga (1990). This group used the nylon bag technique and reported that the intestinal digestibility of protein in different dietary materials (concentrate and roughage) varied from approximately 60% to 90%. Those results also suggested that intrinsic degradation characteristics obtained from in situ or in vitro conditions are inadequate to assess the real protein value, as factors including the type of N, energy source, and flow rate could influence the energy cost of microbial protein synthesis. 9

The NRC (2000) indicated that the dietary MP concentration or intake ranges from 0.6 to 0.8 of CP, depending on the concentration of undegraded dietary protein. An equation, CP = MP/(64 + (0.16 × %undegraded dietary protein)/100), was established to convert MP to CP. Using that equation, the calculated MP supply should be 135, 98, and 72 g/day for lambs fed AL, 70% AL, and 50% AL, respectively. The calculated MP supply for lambs with an average daily gain (ADG) of 300 g was higher than the measured value (102 g/day), suggesting a possible overestimate of MP supply in Dorper crossbred lambs from the NRC (2000) equation. In the current study, the MP supply for lambs with ADGs of 300, 150, and 0 g/day was 102, 91, and 75 g/day, respectively. The AFRC (1993) recommends that 91 or 133 g MP is required for housed intact male lambs (40 kg body weight) with an ADG of 150 or 300 g. The NRC (2007) recommends 114 g MP for growing lambs (age = 8 months, early maturing; 40 kg body weight) with an ADG of 300 g. The INRA (1989) recommends 115 g protéines digestibles dans l'intestin (PDI) for male growing/fattening lambs. Our current results suggest that lambs with an ADG of 300 g need less MP compared with other protein evaluation systems. This discrepancy could be mainly attributed to the animal species. Dorper × thin-tailed Han crossbred sheep are a dominant breed for meat production in China, and previous studies showed that the energy requirements of these sheep are lower than those recommended by the British and American nutritional systems (Deng et al., 2012, 2014). Other factors such as methodology and feeding conditions may have also contributed to the difference. Our current results provide preliminary data for MP requirements for growth of Dorper crossbred lambs. 10

Further studies are required to verify our reported values. Daily CP requirements can be obtained by dividing MP amounts by a value between 0.6 and 0.8, depending on the digestibility of the protein in the feed (NRC, 2007). A constant of 0.7 was assumed by the NRC (2007) to calculate MP from dietary CP. In our current study, the ratio of MP/CP intake decreased from 0.7 to 0.5 with increasing feed intake. Although the same diet was fed to all groups, the lowest ratio was measured for the AL group. This was not unexpected because a greater feed intake is associated with lesser nutrient digestibility owing to a high passage rate (Seo et al., 2006), which could be reflected by the decreased post-ruminal N digestibility (56–70%). Other factors, such as ruminal pH and microbial proteolytic activity, may also affect ruminal digestibility of feed protein and, therefore, microbial CP and MP. Our results show that microbial CP contributed between 0.58 and 0.94 of MP, which is consistent with previous studies reporting that bacterial CP can supply 50–100% of the MP required by ruminants (NRC, 1985; Spicer et al., 1986). The ratio of microbial CP to MP decreased with an increasing level of intake of theses protein typs. Santos et al. (1998) showed that in most of the metabolism studies, a greater proportion of rumen undegraded protein in feed decreased the delivery of microbial CP to the small intestine by negatively affecting microbial production. Fermentable organic matter (FOM) was calculated as the sum of OM apparently digested in the rumen and bacterial OM synthesized in the rumen. The ratio of microbial CP/FOM in the current study averaged 107 g/kg and was not affected by feed intake. This value was ~35% lower than the values of feedstuffs reported by the INRA (1989; 145 g/kg). 11

Of note, FOM is calculated from total tract OM digestibility in a protein evaluation system using in vitro methods. Although Gosselink et al. (2004) reported that the accuracies of the direct and indirect estimates of FOM of forages are similar, the digestibility property of combined feedstuffs may differ from a single feedstuff. 5. Conclusions Our current study evaluated the MP supply to lambs fed three levels of feed intake and provide preliminary data for MP requirements for growth of Dorper crossbred lambs. The MP requirement for these lambs with an ADG of 300 g was lower than that specified by the British or American evaluation system, and further study is required to verify our reported values. Acknowledgments This study was supported by the earmarked fund for China Agriculture Research System

(CARS-39)

and

the

National

Key

Technology

R&D

Program

(2012BAD39B05-3). We thank Prof. Z. C. Feng for his assistance with surgical operations on lambs. We also thank Y. C. Wang and Y. F. Zhang for their technical assistance. All authors participated in the writing of the final draft of the manuscript and agreed with the final format. The authors state that there are no conflicts of interest.

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References AFRC, 1993. Technical Committee on Responses to Nutrients. Nutritive requirements of ruminant animal: protein. Nutr. Abstr. and Reviews. Series B. 62(12), 787–835. Annison, E.F., Lindsay, D.B., Nolan, J.V., 2002. Digestion and metabolism of protein, in: Freer, M. and Dove, H. (Eds.), Sheep Nutrition. CAB International, Wallingford, UD, pp. 95–118. AOAC, 1995. Official Methods of Analysis. 15th ed. Washington, DC: Association of Official Agricultural Chemists. CSIRO, 2007. Nutrient Requirements of Domesticated Ruminants. CSIRO Publishing, Collingwood, Australia. p. 296. Deng, K.-D., Diao, Q.-Y., Jiang, C.-G., Tu, Y., Zhang, N.-F., Liu, J., Ma, T., Zhao, Y.-G., Xu, G.-S., 2012. Energy requirements for maintenance and growth of Dorper crossbred ram lambs. Livest. Sci. 150, 102–110. Deng, K.-D., Jiang, C.-G., Tu, Y., Zhang, N.-F., Liu, J., Ma, T., Zhao, Y.-G., Xu, G.-S., Diao, Q.-Y., 2014. Energy requirements of Dorper crossbred ewe lambs. J. Anim. Sci. 92, 2161–2169. Dixon, R.M., Nolan, J.V., 1982. Studies of the large intestine of sheep. 1. Fermentation and absorption in sections of the large intestine. Brit. J. Nutr. 47, 289–300. Faichney, G.J., 1975. The use of markers to partition digestion within the gastro-intestinal tract of ruminants. In Digestion and Metabolism in Ruminant. 13

University of New Publishing Unit, Armidale. p. 277–291. Gosselink, J.M.J., Dulphy, J.P., Poncet, C., Tamminga, S., Cone, J.W., 2004. A comparison of in situ and in vitro methods to estimate in vivo fermentable organic matter of forages in ruminants. Njas-Wagen. J. Life SC. 52, 29–45. INRA, 1989. Ruminant Nutrition: Recommended Allowances and Feed Tables. R. Jarrige, ed. John Libbey Eurotext, Paris. Ji, S.-K., Xu, G.-S., Jiang, C.-G., Deng, K.-D., Tu, Y., Zhang, N.-F., Ma, T., Lou, C., Diao, Q.-Y., 2013. Net phosphorus requirements of Dorper × thin-tailed Han crossbred ram lambs. Asian Australas. J. Anim. Sci. 26(9), 1282–1288. Ji, S.-K., Xu, G.-S., Jiang, Diao, Q.-Y., Jiang, C.-G., Deng, K.-D., Tu, Y., Zhang, N.-F., 2014. Net zinc requirements of Dorper × thin-tailed Han crossbred lambs. Livest. Sci. 167, 178–185. Ma, T., Deng, K.-D., Jiang, C.-G., Tu, Y., Zhang, N.-F., Liu, J., Zhao, Y.-G., Diao, Q.-Y., 2013. The relationship between microbial N synthesis and urinary excretion of purine derivatives in Dorper × thin-tailed Han crossbred sheep. Small Ruminant Res. 112, 49–55. Marshall, C.M., Walker, A.F., 1978. Comparison of a short method for Kjeldahl digestion using a trace of selenium as catalyst, with other methods. Journal of the Science of Food and Agriculture. 29, 940–942. NRC, 1985. Ruminant Nitrogen Usage. Washington, D.C.: National Academy Press. NRC, 2000. Nutrient Requirements of Beef Cattle, 7th rev. ed. National Academy of Sciences, Washington, DC. 14

NRC, 2007. Nutrient Requirements of Small Ruminants. Sheep, Goats, Cervids and New World Camelids. National Academy Press, Washington, DC. p. 384. Santos, F.A.P., Santos, J.E.P., Theurer, C.B., Huber, J.T., 1998. Effects of rumen-degradable protein on dairy cow performance: A 12 year literature review. J. Dairy Sci. 81, 3182–3213. SAS, 2005. Institute Inc. SAS OnlineDoc ® 9.1.3. SAS Institute, Cary, NC. Seo, S., Tedeschi, L.O., Schwab, C.G., Garthwaite, B.D., Fox, D.G., 2006. Evaluation of the Passage Rate Equations in the 2001 Dairy NRC Model. J. Dairy Sci. 89(6), 2327–2342. Spicer, L.A., Theurer, C.B., Sowe, J., Noon, T.H., 1986. Ruminal and postruminal utilization of nitrogen and starch from sorghum grain, corn, and barley based diets by beef steers. J. Anim. Sci. 65, 521–529. Van Straalen, W., Tamminga, S., 1990. Protein degradation of ruminant diets. in Feedstuff Evaluation. J. Wiseman and D. J. A. Cole, ed. Butterworths, London, UK. p. 55. Varga, G.A., 2007. Why use metabolizable protein for ration balancing. Proceedings of Pennsylvania State Dairy Cattle Nutrition Workshop, Grantville, PA. P. 51–57. Xu, G.-S., Ma, T., Ji, S.-K., Deng, K.-D., Tu, Y., Jiang, C.-G., Diao, Q.-Y., 2015. Energy requirements for maintenance and growth of early-weaned Dorper crossbred male lambs. Livest. Sci. 177, 71–78.

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Table 1. Ingredients and chemical composition of the experimental diet Item

Composition (g/kg DM)

Ingredients† (g/kg DM) Chinese wild rye hay

553

Corn

292

Soybean meal

138

Calcium carbonate

9.5

Salt

5.6

Mineral/vitamin premix‡

1.8

Chemical composition (g/kg DM, determined) DM (g/kg, as fed)

906

OM

916

CP

112

NDF

380

ADF

241

DM, dry matter; OM, organic matter; CP, crude protein; NDF, neutral detergent fiber; ADF, acid detergent fiber. †

All ingredients were pelleted (6.0 mm in diameter).

‡ The premix contained (per kg): 22.1 g Fe, 13.0 g Cu, 30.2 g Mn, 77.2 g Zn, 19.2 g

Se, 53.5 g I, 9.10 g Co, 56.0 g vitamin A, 18.0 g vitamin D3, and 170 g vitamin E.

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Table 2. Duodenal N flow in Dorper  thin-tailed Han crossbred lambs fed a total mixture diet at three levels of feed intake Level of feed intake1

Item

P-value SEM

AL

70%

50%

Treatment

Linear

Quadratic

ADG, g2

324a

148b

29c

10.5

<0.05

<0.05

<0.05

NI, g/day3

31.2a

22.0b

16.0c

3.42

<0.05

<0.05

<0.05

DM

757a

538b

381c

48.1

<0.05

<0.05

<0.05

OM

706a

481b

355c

45.4

<0.05

<0.05

<0.05

NAN4

27.2a

22.8b

16.8c

1.30

<0.05

<0.05

<0.05

Microbial N5

14.4a

9.60b

6.78c

0.97

<0.05

<0.05

<0.05

% of NAN

0.53a

0.42b

0.40b

0.01

<0.05

<0.05

<0.05

Endogenous N6

2.72a

2.28b

1.68c

0.13

<0.05

<0.05

<0.05

RUN7

10.2a

10.9a

8.46b

0.35

<0.05

<0.05

<0.05

0.37b

0.48a

0.50a

0.02

<0.05

<0.05

<0.05

Duodenal flow, g/day

% of NAN

17

MN/NI, %

0.46

0.44

0.42

0.01

0.163

0.052

0.130

RUN/NI, %

0.33b

0.49a

0.53a

0.03

<0.05

<0.05

<0.05

RUN (endogenous N excluded)/NI, %

0.41b

0.60a

0.63a

0.03

<0.05

<0.05

<0.05

SEM, standard error of the mean; NI, N intake; NAN, non-ammonia nitrogen; RUN, rumen undegraded nitrogen; MN, microbial nitrogen. 1

Ad libitum (AL) or restricted to 70% or 50% of the ad libitum intake.

2, 3, 4, 5

Ma et al., 2013.

6

Calculated as 0.10 of duodenal N flow, according to the NRC (1985).

7

Calculated as duodenal NAN − microbial N − endogenous N.

a,b,c

Mean values within a row with different superscript letters were significantly different (P < 0.05).

Table 3. Metabolizable protein supply in Dorper  thin-tailed Han crossbred lambs fed a total mixture diet at three levels of feed intake Item 2

DMI, kg/day OMI, kg/day CPI, g/day Duodenal N flow, g/day3 Fecal N, g/day4

Level of feed intake1 AL 70% 50% a b 1.62 1.16 0.81c a b 1.47 1.02 0.74c 195a 137b 100c a b 28.3 23.5 17.2c 11.9a 8.07b 5.16c 18

SEM 1.00 0.91 3.42 1.41 1.19

Treatment <0.05 <0.05 <0.05 <0.05 <0.05

P-value Linear <0.05 <0.05 <0.05 <0.05 <0.05

Quadratic <0.05 <0.05 <0.05 <0.05 <0.05

Apparent post-ruminal N digestibility, %5 60.0c 64.0b 71.4a 0.02 <0.05 <0.05 <0.05 6 a b MP supply, g/day 102 91 75c 3.24 <0.05 <0.05 <0.05 c b a MP/OMI, g/kg 69.4 88.9 102.4 4.42 <0.05 <0.05 <0.05 c b a MP/CPI, g/g 0.52 0.65 0.76 0.03 <0.05 <0.05 <0.05 SEM, standard error of the mean; DMI, dry matter intake; OMI, organic matter intake; CPI, crude protein intake; MP, metabolizable protein. 1

Ad libitum (AL) or restricted to 70% or 50% of the ad libitum intake.

2, 3, 4

Ma et al., 2013.

5

Calculated as (duodenal N flow − fecal N)/duodenal N flow × 100%.

6

Calculated as duodenal NAN flow × apparent post-ruminal N digestibility × 6.25.

a,b,c

Mean values within a row with different superscript letters were significantly different (P < 0.05).

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Table 4. Prediction of metabolizable protein supply (g/day) in Dorper  thin-tailed Han crossbred lambs fed a total mixture diet at three levels of feed intake Variables

Equation

R-square

RMSE

n

P-value

OMI (kg/day)

MP = 0.036 (±0.004) × OMI + 50.47 (±4.43)

0.89

4.15

12

<0.05

CPI (g/day)

MP = 0.27 (±0.033) × CPI + 49.88 (±4.93)

0.87

4.51

12

<0.05

OMI, organic matter intake; CPI, crude protein intake; MP, metabolizable protein; RMSE, root mean square error.

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