Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 24, 1997
Publication Date: N/A
Interpretive Summary: Optimization of a cow-calf production system requires synchronization of the cow's nutrient requirements with feed availability. Typically, ranges have a period during which grazed forage is of poor nutritional quality or low availability. During these periods of reduced forage quality, cows typically lose weight unless supplemental feed is provided. In order to devise appropriate realimentation programs for cows that have lost weight during the periods of nutrient shortage, it is imperative to understand the efficiency of nitrogen and energy loss and gain. It was determined that efficiency of energy and nitrogen retention increases during refeeding when cows are allowed to fluctuate in weight. This increase in nutrient retention appears to result from an increase in the efficiency with which nutrients are used rather than from differences in available nutrients. Weight cycling does not have a biological advantage in retained energy; however, the increased efficiency of nutrient utilization during the weigh gain period offers the potential to develop feeding strategies that improve grazed forage utilization and reduce supplemental feed. Matching the weight gain period with availability of low cost forage may result in a management strategy that is more economically efficient than management strategies that maintain weight.
Technical Abstract: It was the objective of this study to quantify the energy and nitrogen balance of mature cows fed a mixed amount of forage either in a manner to maintain weight or in a manner that allowed for weight loss and gain. Twelve mature nonpregnant, nonlactating cows were fed 82.57 g of chopped brome hay/(BW kg)**.75/day and energy and nitrogen balances were determined. Six cows were then assigned to each of two treatments. Control cows continued to receive 82.57 g of chopped brome hay/(BW kg)**.75 /day over the 224 d of the study. Treated cows were switched to 53.67 g of chopped brome hay/(BW kg)**.75/day after the first collection period and remained at this feed intake for 112 d. At the end of 112 d, treated cows received 111.47 g of chopped brome hay/(BW kg)**.75/d for an additional 112 d. All feed intakes were based on the initial weight of the cows, and 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 the 28, 56, 84, 112, 140, 168, 189, and 224 d. While 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. Net retained energy over the entire 224-d period did not differ between treatments. Treated cows retained more nitrogen than did control cows. 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.