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

Authors: Freetly, Harvey; Nienaber, John; Brown-Brandl, Tami

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2007
Publication Date: 2/1/2008
Citation: Freetly, H.C., Nienaber, J.A., Brown Brandl, T.M. 2008. Partitioning of energy in pregnant beef cows during nutritionally induced weight fluctuation. Journal of Animal Science. 86(2):370-377.

Interpretive Summary: The cow's need for dietary energy changes throughout the year depending on her stage of pregnancy and the level of milk production. Typically, there are times in the year that nutrient availability from forages does not meet the nutrient requirements of the cow. When this discrepancy between need and availability occurs, cows enter into a negative energy balance and lose weight to support pregnancy and milk production. To prevent weight loss, mechanically harvested feeds such as hay or grain are fed. A potential way to reduce the cost of mechanically-harvested feed and increase the use of grazed feed is to allow cows to lose weight while forage availability is low and allow weight gain when forage availability is high. This study determined efficiency of feed use did not differ between cows that were fed to maintain maternal body weight and those allowed moderate loss of maternal body weight in the second trimester of pregnancy and fed to gain weight in the third trimester. These findings support that feeding systems can be developed that increase the use of grazed forage and reduce the amount of mechanically harvested feed offered.

Technical Abstract: 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 one calf before the study were fed a diet that consisted of chopped corn silage (67.3%), alfalfa hay (27.5%), corn (5.5%), and sodium chloride (0.2%; as dry matter). When cows were 87 +/- 0.6 d pregnant, the first nutrient balance measurements 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. 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 weight gain during the entire pregnancy, and 10 cows were feed restricted in the second trimester and realimented during the third trimester (L-H). Cows’ body weights 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 realimented cows than moderate cows (P < 0.001). The cow’s ability to adapt her energy metabolism during periods of moderate feed restriction and realimentation allows development of management strategies, which 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 what feed resources are being used.