Sex effects on net protein and energy requirements for growth of Saanen goats

Requirements for growth in the different sexes remain poorly quantified in goats. The objective of this study was to develop equations for estimating net protein (NP_G) and net energy (NE_G) for growth in Saanen goats of different sexes from 5 to 45 kg of body weight (BW). A data set from 7 comparative slaughter studies (238 individual records) of Saanen goats was used. Allometric equations were developed to determine body protein and energy contents in the empty BW (EBW) as dependent variables and EBW as the allometric predictor. Parameter estimates were obtained using a linearized (log-transformation) expression of the allometric equations using the MIXED procedure in SAS software (SAS Institute Inc., Cary, NC). The model included the random effect of the study and the fixed effects of sex (intact male, castrated male, and female; n = 94, 73, and 71, respectively), EBW, and their interactions. Net requirements for growth were estimated as the first partial derivative of the allometric equations with respect to EBW. Additionally, net requirements for growth were evaluated based on the degree of maturity. Monte Carlo techniques were used to estimate the uncertainty of the calculated net requirement values. Sex affected allometric relationships for protein and energy in Saanen goats. The allometric equation for protein content in the EBW of intact and castrated males was log₁₀ protein (g) = 2.221 (±0.0224) + 1.015 (±0.0165) × log₁₀ EBW (kg). For females, the relationship was log₁₀ protein (g) = 2.277 (±0.0288) + 0.958 (±0.0218) × log₁₀ EBW (kg). Therefore, NP_G for males was greater than for females. The allometric equation for the energy content in the EBW of intact males was log₁₀ energy (kcal) = 2.988 (±0.0323) + 1.240 (±0.0238) × log₁₀ EBW (kg); of castrated males, log₁₀ energy (kcal) = 2.873 (±0.0377) + 1.359 (±0.0283) × log₁₀ EBW (kg); and of females, log₁₀ energy (kcal) = 2.820 (±0.0377) + 1.442 (±0.0281) × log₁₀ EBW (kg). The NE_G of castrated males was greater than that of intact males and lower than that of females. Using degree of maturity for estimating NP_G and NE_G, we could remove the differences between sexes. These results indicate that NP_G and NE_G differ among sexes in growing Saanen goats, and this difference should be accounted for by feeding systems. Including the degree of maturity as predictor cancels out those differences across sexes in protein and energy requirements.