Effects of breed and diet on growth and body composition of crossbred Boer and Spanish wether goats

Sixty growing 3/4 Boer x 1/4 Spanish (BS) and Spanish (SP) wethers were used to determine influences of diet and breed on growth and body composition. A 50% concentrate pelleted diet (CON) and one based on grass hay (HAY) were fed free-choice. Six wethers of each breed were harvested at 0 wk (total of 12) and 6 of each diet-breed combination were harvested at 14 and 28 wk (24 per time). Initial BW of fed wethers was 21.6 and 18.8 kg for BS and SP, respectively (SEM = 0.67). Average daily gain during the entire experiment was influenced by an interaction (P < 0.05) between breed and diet (199, 142, 44, and 50 g for BS:CON, SP:CON, BS:HAY, and SP:HAY, respectively). Carcass mass was greater (P < 0.05) for CON vs. HAY (56.2, 56.2, 53.2, and 54.0% empty BW for BS:CON, SP:CON, BS:HAY, and SP:HAY, respectively). Mass of the liver (2.11, 1.92, 2.00, and 1.98% empty BW; SEM = 0.048) and gastrointestinal tract (5.50, 4.83, 8.43, and 8.36% empty BW for BS:CON, SP:CON, BS:HAY, and SP:HAY, respectively; SEM = 0.158) tended (P < 0.07) to be influenced by an interaction between breed and diet. Mass of internal fat (12.2, 12.1, 3.4, and 3.4% empty BW for BS:CON, SP:CON, BS:HAY, and SP:HAY, respectively; SEM = 0.28) differed (P < 0.05) between diets. Energy in the carcass (320, 236, 87, and 79 MJ), noncarcass tissues (318, 237, 77, and 72 MJ), and empty body (638, 472, 164, and 150 MJ) ranked (P < 0.05) BS:CON > SP:CON > BS:HAY and SP:HAY. Empty body concentration of protein was 18.3, 17.5, 18.3, and 19.7% (SEM = 0.29) and of fat was 24.0, 23.4, 10.8, and 10.3% for BS:CON, SP:CON, BS:HAY, and SP:HAY, respectively (SEM = 0.59). Energy concentration in accreted tissue was 17.0, 18.7, 16.3, and 6.4 MJ/kg for CON:wk 1-14, CON:wk 15-28, HAY:wk 1-14, and HAY:wk 15-28, respectively (SEM = 1.39). In conclusion, relatively high growth potential of growing Boer goats with a moderate to high nutritional plane does not entail a penalty in realized growth when the nutritional plane is low. Body composition of growing Boer and Spanish goats is fairly similar regardless of growth rate. For growing meat goats other than with a prolonged limited nutritional plane, an average energy concentration in accreted tissue is 17.3 MJ/kg.