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Effect of Stage of Lactation and Varying Available Energy Intake on Milk Production, Milk Composition, and Subsequent Tissue Enzymic Activity

D. Zaartu¹, C. E. Polan, L. E. Ferreri² and M. L. McGilliard

Department of Dairy Science, College of Agriculture and Life Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061

ABSTRACT

Twenty-four cows were allotted at parturition to receive one of four rations for 180 days. Rations were regular ad libitum (about 20% crude fiber), concentrate ad libitum (about 10% crude fiber), and restricted to recommended energy intake, and intermediate ration (14% crude fiber) ad libitum. Cows on concentrate consumed less dry matter, fiber, and protein than intermediate and regular cows. Cows fed more forage gained more weight, had milk with higher protein and fat content, but were similar in milk production to those fed high concentrate. Fat test decreased to 4 wk in all treatments, and by 8 or 9 wk cows fed concentrate decreased to their lowest fat test. Concentrate restriction enhanced fat test beyond 8 wk. Ruminal propionate was higher and similar for both concentrate groups. Ruminal volatile fatty acids were higher, glucose slightly higher, and blood acetate lower for cows on ad libitum concentrate compared to intermediate.

Mammary and adipose tissue from cows in midlactation were assayed for key enzymes of fatty acid synthesis. Activity of 6-phosphogluconate dehydrogenase tended higher for concentrate rations in adipose tissue, but mammary tissue was 21 to 28 times as active. Mammary tissue was 7 to 10 times as active as adipose tissue in fatty acid synthetase with no difference in rations.

¹ Federico Froebel 1645, Santiago 9, Chile.

² Department of Laboratory Medicine, SB10 School of Medicine, University of Washington, Seattle, WA 98195.