Estimation of energy requirements for maintenance and growth of male Afshari lambs from 40 to 50-kilogram using Relative Growth Index method

Document Type : Research Paper

Authors

• Mohsen Bayat ¹
• Ali Kiani ¹
• Ayoub Azizi ²

¹ Animal Science Group, Faculty of Agriculture, Lorestan University, Khorramabad, Iran

² Animal Science Group, Faculty of Agriculture, Lorestan University, Khorramabad, Iran.

Abstract

Keywords

• fat-tailed sheep
• growth energy requirement
• maintenance energy requirement
• Relative Growth Index

Main Subjects

• Animal Nutrition and poultry

Extended Abstract

Introduction

    Fat-tailed sheep are the dominant sheep breeds in many part of the word. Feeding systems designed for tailed breeds may not be suitable for fat-tailed breeds. Two commonly used approaches to estimate the energy requirements of livestock are comparative slaughter technique (CST) and indirect calorimetry. The CST method is practical for determining retained energy (RE), but it is cumbersome, expensive, and inconsistent with animal welfare standards in many countries. On the other hand, indirect calorimetry is a non-invasive method for measuring whole-body heat production from respiratory gas exchange. While it is considered a standard method, it is not available in many countries and requires relatively sophisticated research facilities and financial resources to perform the experiments. These two methods have limitations, especially in areas that are poor in terms of financial resources. An alternative method for assessing energy requirements of growing animals is to use gain as an indirect measure of energy retention. This approach involves regression equations for plotting average daily gain (ADG) against metabolizable energy intake (MEI). Alternatively, relative growth index (RGI; BW^0.75/g) of an animal can be calculated as the metabolic body weight (BW^0.75) divided by ADG (g). The RGI represents an index for the efficiency of gain in animals. The RGI is regressed against MEI per unit of gain to establish a linear equation (Y=a + bx) where intercept (a) and slope (b) are the requirement of metabolizable energy for maintenance (MEm; kJ/BW^0.75) and growth (MEg; kJ/g), respectively. Afshari sheep breed is a fat-tailed breed known for its meat production and high productive characteristics. This study hypothesis that energy requirement of Afshari growing lambs can be estimated using the RGI method. Thus, the aims of the study were to (1) estimate energy requirements for maintenance (MEm)and growth (MEg) of Afshari male lambs from 40 to 50 kg body weight, (2) to compare the observed ADG of Afshari growing lambs with predicted ADG values from CNCPS-S system providing insights into the accuracy of the CNCPS system for estimating nutrient requirements of fat-tailed sheep breeds.

Materials and Methods

     Thirty healthy Afshari male lambs (mean body weight: 40±1.3 kg, age: 170± 10 days) were used. The lambs were randomly assigned to three dietary intakes (10 lambs per treatment) in a completely randomized design. Thus, the lambs were either fed ad libitum (ADL, n=10) or restricted to 80% (R80, n=10) and 60% (R60, n=10) intake of ADL.The CNCPS-S model was used for predicting ADG of lambs. The predicted daily gain as output of the CNCPS-S model were 278, 181, and 91 g for ADL, 80R, and 60R respectively. The digestibility trials were conducted using eighteen lambs (six lambs of each group) close to the mean body weights (45 kg). Each trial lasted for seven days, with a two- days adaptation period and five-days of feces and urine collection. Relative growth index (RGI) was calculated as gram metabolic body weight (BW^0.75) divided by average daily gain (ADG) and total ME requirement = (MEm ×BW^0.75) + (MEg × gain).  The slope and intercept of this Equation were assumed as metabolizable energy requirement for maintenance (MEm; kJ per BW^0.75) and ME requirement for growth (MEg; kJ per g ADG). For comparison with observed data, the CNCPS-S was used to estimate ADG for each lamb in the present study.

Results and Discussion

DM intake of control, R80 and R60 group was 3.9, 3.2 and 2.6% of body weight respectively. The DM intake of control lambs was 31 g per kg body weight and 82.2 g/BW^0.75. These values were within the range of sheep DMI with similar BW, i.e. 2.86-3.91% BW as recommended by NRC (2007). The metabolizability values obtained in this experiment were between 61 and 63.7% which was in the range (40 to 64%) proposed in several reports (ARC. 1980).. Relationship between   MEI and ADG indicated that ADG increased linearly with increases in the metabolizable energy concentration in the diet: [ME (kJ/BW^0.75) = 1.13 ADG (g) + 500 (R² = 0.58)]. The estimated ME requirement for maintenance of Afshari growing lambs based on ADG was 500 kJ/BW^0.75. This value of MEm was in line with the value (560 kJ/BW^0.75 of Shrunk BW) recommended by NRC (2007) and close to the value of 542 kJ/BW^0.75 for tropical sheep breeds (Salah et al.,. 2014).. MEm in the present study was significantly differ from tailed sheep breeds. Around 31% higher than that obtained for Texel crossbred lambs (381 kJ/BW^0.75 )(Galvani et al.,. 2008)., and 9% higher than the value of 460 kJ/BW^0.75 for English sheep breeds (Dawson and  Steen. 1998). and 24% higher than the value of (403 kJ/BW^0.75) reported for Dorper crossbred ram lambs(Deng et al.,. 2012).. The inconsistency of MEm values found in the present study with those reported in the literature could be possibly explained by differences between breeds, age, bodyweight, body composition, feed ingredients, nutritional quality of diets and environmental effects, as well as methodological differences (Ben Ettoumia et al., 2022). The metabolisable energy requirement for gain (MEg) in finishing Afshari lamb based on ADG and RGI was estimated to be 20.9 kJ ME and 17.4 kJ ME per g of gain respectively. These estimates are inside the range of values published in the literature (13.7 to 27.9 kJ ME per g of gain These estimates are inside the range of values published in the literature (13.7 to 27.9 kJ ME per g of gain). The present values were slightly lower than 24.2 ME for gain reported for sheep in warm environment condition (Salah et al.,. 2014). Discrepancies might be explained by different used methods, livestock genotypes, animal age and body composition of animals(Salah et al.,. 2014).. Based on RGI method, MEm was 439 kJ/BW^0.75 [ME (kJ/g ADG) = 2.47 RGI (BWg^0.75) + 24.2 (kJ/g ADG), R²=0.85] when the RGI was as metabolic body weight divided by average daily gain (BW^0.75/g).  The predicted MEm for a growing male lamb with 40 kg body weight and q_m=0.64 and 250 g gain based on RGI equation would be 440×15.9 + 24.2×250 = 6.985 (MEm) + 6050(MEg) = 13035 (13.0 MJ/d) and based on ADG equation is [ME (kJ/BW^0.75) = 493 (kJ/BW^0.75) + 20.0 ADG (g/BW^0.75), R²=0.64] is 493×15.9 + 20.0×250 = 7839 (MEm) + 5000 (MEg) =12839 (12.8 MJ/d). According to the NRC (2007) and the AFRC system (AFRC. 1993). tables, the energy requirements  for the same lamb is 15.1 and 18.6 MJ/d respectively. It means that the energy required to grow a 40 kg  male Afshari lamb with a daily growth rate of 250 g in the NRC (2007)  and the English system (AFRC. 1993) is about 20% and 30% higher than the present estimated value respectively. As comparison, the ADG of the present lambs were predicted by the CNCPS-S. Observed values were regressed on CNCPS-S model-predicted values (observed ADG, g/d = 1.04 (±0.085) - [36.9 (±0.079) × predicted ADG, g/d]; R² = 0.88). The ADG was markedly underestimated by the CNCPS-S for Afshari lambs both in control and restricted feed level. Differences between model predicted and observed ADG was 37 g. This difference might be attributed to the lower energy requirements for gain in Afashri lambs.

Conclusion

In this study, the energy requirement for maintenance and growth of a fat-tailed sheep was calculate based on ADG and relative growth intake. The Estimated MEm (440 kJ/BW^0.75) and MEg (24.2 kJ/g ADG) were in consistent of those reported in other studies using comparative slaughter technique and indirect calorimetry. The ADG values predicted from CNCPS-S model were underestimated as compared to observed ADG in fat-tailed Afshari lambs which suggests that a single recommendation for all sheep breeds under diverse management conditions cannot be feasible.

Author Contributions

“Conceptualization, A.K. and M.B.; methodology, M.B.; software, A.K.; validation, A.K., A.A. and M.B.; formal analysis, M.B.; investigation, M.B.; resources, M.B.; data curation, M.B.; writing—original draft preparation, M.B.; writing—review and editing, A.K.; visualization, A.A.; supervision, A.K.; project administration, M.B.; funding acquisition, A.K. All authors have read and agreed to the published version of the manuscript.”

All authors contributed equally to the conceptualization of the article and writing of the original and subsequent drafts.

Data Availability Statement

Data available on request from the authors.

Acknowledgements

The authors would like to thank Deputy of Research of Lorestan University for its financial support, and Riyhan Farm for their technical support and material used for experiment.