Partitioning of energy in pregnant beef cows during nutritionally induced body weight fluctuation.

The purpose of this study was to determine if the efficiency of energy retention in pregnant cows was dependent on the time during the pregnancy that feed was offered. Our hypothesis was that restricting feed intake during the second trimester of gestation and providing the saved feed during the third trimester was less energetically efficient than providing the feed during the second trimester. Twenty cows (4 breed composite: 1/4 Hereford, 1/4 Angus, 1/4 Red Poll, and 1/4 Pinzgauer) that had produced 1 calf before the study were fed a diet that consisted of (DM basis) 67.3% chopped corn silage, 27.0% alfalfa hay, 5.5% corn, and 0.2% NaCl. When the cows were 87 +/- 0.6 d pregnant, the first nutrient balance measurement was conducted. Six subsequent nutrient balance measurements were taken on d 122 +/- 0.6, 143 +/- 0.6, 171 +/- 0.6, 206 +/- 0.6, 241 +/- 0.6, and 262 +/- 0.6 of gestation. Each nutrient balance measurement consisted of a 96-h total collection of feces and urine and a 24-h indirect calorimetry measurement. Ten cows were fed for moderate BW gain during the entire pregnancy, and 10 cows were feed-restricted in the second trimester and realimented during the third trimester (low-high, L-H). The BW of the cows at parturition (559 +/- 14 kg) did not differ between treatments (P = 0.20). There was a general trend for the proportion of ME intake retained to decrease in moderate cows as pregnancy progressed. The proportion of ME intake retained in L-H cows decreased during the first 49 d of feed restriction, but the proportion of ME retained after 77 d of restriction was greater than that retained at 49 d of restriction. During realimentation, there were no time effects for efficiency of ME conversion to retained energy, but efficiency was greater for L-H cows than moderate cows (P < 0.001). The ability of the cow to adapt its energy metabolism during periods of moderate feed restriction and realimentation allows development of management strategies that alter the time interval of the production cycle during which supplemental feed is offered. Total savings in feed offered during the production year are minimal, but management strategies can be developed that shift which feed resources are being used.