Comparison of different models to estimate purine bases absorbed in goats

The purine bases absorbed (PBa) and microbial nitrogen flow (MNF) in the duodenum were estimated with five models proposed for sheep (1, 4), goats (2), cattle (5) and a goat-sheep mixed model (3). Data of purine derivatives excreted (PDe) in urine of eight digestibility trials were used. In these trials, eight adult male castrated Murciano-granadino goats (30.5 ± 4.95 kg of live weight) were used. Feed was offered as a total mixed ration and was based on alfalfa hay or barley straw and concentrates (50:50). The range in variation for daily DM intake, OM digestibility and nitrogen balance of the different rations were: 38 to 42 g/kg LW, 68 to 71% and 0.35 to -0.11 g/d respectively. The estimation to PBa, MNF and microbial yield, were different between equations. The estimation of PBa in equation 4 was underestimated by too high recovery of endogenous fraction of PDe in urine considered in this equation. The ratio between N content in the microbial population of the rumen (MNR) and PBa was lower in the equation 2 (0.55 mmol/day) than equations 1, 3, 4 and 5 (0.727 mmol/day). The variability of the factors used in the equations to estimate PDa and MNF within and among species limits its use. _______________________________________________________________________________________