Effects of stage of lactation and level of feed intake on energy utilization by Alpine dairy goats.

Thirty-six lactating Alpine does were used to determine effects of stage of lactation and level of feed intake on energy utilization. Twelve does were assigned to measurement periods in early, mid, and late lactation (wk 5, 13, and 27, respectively). For 6 does of each group, after ad libitum consumption of a 60% concentrate diet, feed intake was restricted to near the metabolizable energy (ME) requirement for maintenance (ME(m)) for 8 d followed by fasting for 4 d. For other does, fasting immediately followed ad libitum consumption. Intake of ME was similar among stages of lactation with ad libitum intake (22.1, 22.1, and 19.8 kJ/d in early, mid, and late lactation, respectively). The efficiency of ME use for maintenance determined with does fed near ME(m) averaged 81%. Fasting heat energy was greater for ad libitum consumption than for near ME(m) consumption [368 vs. 326 kJ/kg of body weight (BW)(0.75)] and was numerically lowest among stages in late lactation with near ME(m) intake (334, 350, and 295 kJ/kg of BW(0.75) in early, mid, and late lactation, respectively) and ad libitum consumption (386, 384, and 333 kJ/kg of BW(0.75) in early, mid, and late lactation, respectively). The efficiency of use of dietary ME for lactation was greater for consumption near ME(m) than for consumption ad libitum (67.9 vs. 58.6%) and with ad libitum consumption tended to decrease with advancing stage of lactation (63.9, 57.3, and 54.5% for early, mid, and late lactation, respectively). Estimated ME(m) was greater for ad libitum intake than for near ME(m) intake and was lowest during late lactation (429, 432, and 358 kJ/kg of BW(0.75) for near ME(m) intake and 494, 471, and 399 kJ/kg of BW(0.75) for ad libitum intake in early, mid, and late lactation, respectively). However, because of increasing BW as the experiment progressed, ME(m) (MJ/d) was similar among stages of lactation with both levels of intake. The efficiency of ME use for maintenance and lactation was similar among stages of lactation and greater with near ME(m) intake than ad libitum intake (77.1 vs. 67.7%). In conclusion, the ME(m) requirement (kJ/kg of BW(0.75)) of does in late lactation was less than in early and mid lactation. A marked effect of restricted feed intake subsequent to ad libitum consumption on estimates of efficiency of energy use for maintenance and lactation was observed compared with use of nonlactating animals. Level of feed intake can have substantial effect on estimates of energy utilization by lactating dairy goats.