Effects of small ruminant type and restricted protein intake on metabolism

Boer goat (BG), Spanish goat (SG), and Rambouillet sheep (RS) wethers, ≥2 yr of age, were used in a crossover experiment with 28-day periods. Diets were ad libitum consumption of wheat straw alone (CON) or with a 90% soybean meal, 10% molasses supplement given at 0.22% BW (SBM). Initial BW was 35, 55, and 32 kg for BG, RS, and SG, respectively. NDF digestibility was similar among animal types and between diets. BW change tended to be lowest for RS (−92, −158, and −107 g/day for BG, RS, and SG, respectively; SE = 22.6). ME intake was similar among animal types (244, 230, and 259 kJ/kg BW0.75 for BG, RS, and SG, respectively; SE = 16.6) and greater (P < 0.05) for SBM vs. CON (320 vs. 168 kJ/kg BW0.75). Total energy expenditure (EE) was greater (P < 0.05) for RS than for BG (362, 415, and 402 kJ/kg BW0.75 for BG, RS, and SG, respectively) and for SBM vs. CON (413 vs. 374 kJ/kg BW0.75). EE by the portal-drained viscera (PDV) (1.34, 1.33, and 1.17 MJ/day; SE = 0.122) and liver (1.48, 1.44, and 1.32 MJ/day; SE = 0.133) was similar among animal types. Liver EE was greater (P < 0.05) for SBM vs. CON (1.60 vs. 1.22 MJ/day), but PDV EE was similar between diets. Net fluxes of ammonia N (AMN) and urea N (UN) across the PDV (AMN: 3.4, 2.4, and 3.2 g/day (SE = 0.69); UN: −5.2, −3.3, and −4.6 g/day (SE = 1.19)) and liver (AMN: −3.6, −3.2, and −4.3 g/day (SE = 0.78); UN: 7.6, 4.8, and 4.2 g/day for BG, RS, and SG, respectively (SE = 1.17)) were similar among animal types. In conclusion, the magnitude of any difference in N recycling among animal types was less than necessary to affect fiber digestibility. Nonetheless, some findings suggest a lesser ability of sheep to modify metabolic functions to cope with limited nutritional planes elicited by feeding crop residue-based diets, perhaps relating to metabolism by extra-splanchnic tissues.