Efficiency of energy and nitrogen loss and gain in mature cows.

Our objective was to quantify the energy and nitrogen balances of mature cows fed a fixed amount of forage. Six cows were assigned to each of two treatments. At time 0, control cows received 83.55+/-.52 g of chopped brome hay x (BWkg)(-.75) x d(-1). Feed intake remained fixed (9,103+/-277 g/d) over the entire 224 d of the study. At time 0 treated cows received 82.10+/-1.26 g of chopped brome hay x (BWkg)(-.75) x d(-1) (9,083+/-113 g/d). After time 0, treated cows were offered 65% of the time-0 feed intake for the first 112 d (Phase 1) and 135% of the time-0 feed intake for the last 112 d (Phase 2). Treatments were designed so that the total amount of feed received during the 224 d was the same for each treatment. Additional balance measurements were made on d 28, 56, 84, 112, 140, 168, 189, and 224. Although treatment groups differed within phases for cumulative heat production, control cows did not differ from treated cows in total heat produced during the 224-d study (P = .60). Net retained energy over the entire 224-d period did not differ between treatments (P = .43). Treated animals retained more nitrogen than did control animals (P = .008). The increased efficiency of nutrient utilization during refeeding in cows allowed to fluctuate in weight offers the potential to develop feeding strategies that improve grazed forage utilization and reduce supplemental feed.