Effects of diet quality on energy expenditure by 20-month-old Alpine, Angora, Boer, and Spanish wethers

Six Alpine (47.7 ± 1.36 kg initial BW), Angora (28.8 ± 1.11 kg), Boer (87.5% Boer and 12.5% Spanish; 49.3 ± 2.17 kg), and Spanish (38.7 ± 0.51 kg) wethers (initial age of 19 months) were used to determine effects of genotype and diet quality on energy expenditure (EE) when fed near maintenance and fasted. The experiment consisted of four simultaneous crossovers, with 21 days for adaptation before measures. Diets were 65% concentrate (CON) or coarsely ground alfalfa hay (FOR). EE was determined from O2 consumption and production of CO2 and CH4 with a head-box respiration calorimetry system, along with urinary N excretion, over 2-day periods in fed and fasted states (4 days fast). EE was expressed on the basis of average BW during the fasted measurement period. There were only significant interactions between genotype and diet in DM and gross energy intakes, which were due to differences in magnitude. Intake of ME was similar among genotypes and slightly greater (P < 0.05) for CON than for FOR (450 kJ/kg versus 424 kJ/kg BW0.75). Neither diet (373 and 371 kJ/kg BW0.75 for CON and FOR, respectively; S.E. = 5.9) nor genotype (377, 377, 361, and 373 kJ/kg BW0.75 by Alpine, Angora, Boer, and Spanish, respectively; S.E. = 9.3) influenced fed EE (P > 0.10). Fasted EE was similar between diets but was greatest among genotypes (P < 0.05) for Alpine (251, 224, 217, and 225 kJ/kg BW0.75 by Alpine, Angora, Boer, and Spanish, respectively; S.E. = 7.2). In summary, based on fasting measures, it would not appear that dairy goat breeds, such as Alpine can minimize EE to the extent of other genotypes in response to very severe nutrient restriction, such as fasting. With mature, nonlactating goats and a level of feed intake near the MEm assumed for a constant and nonlimiting plane of nutrition, MEm may not differ markedly among genotypes. Although, higher levels of intake to support growth, milk production, or fiber growth could conceivably elicit differences among goat genotypes in MEm.