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Effects of High Grain Feeding and Stage of Lactation on Serum Insulin, Glucose and Milk Fat Percentage in Lactating Cows^{1 ,2}

Departments of Dairy Science and of Human Nutrition and Foods, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061

Twelve lactating Holstein cows were used to study the interrelationships of serum insulin, milk fat percentage, serum glucose and ruminal volatile fatty acids during adaptation to a high grain ration. Six cows, one-half in early (39.3 kg milk/day) and one-half in midlactation (23.6 kg milk/day) were fed a control (C) ration ( 35% grain) while six comparable cows were adjusted over a 30-day period to a 85%, high grain ration (HG). Feed intake and milk production were recorded daily. Biweekly milk samples were analyzed for fat, rumen fluid for volatile fatty acids and serum for glucose and insulin. Based on regression analysis of each mentioned parameter (Y) regressed on days (X), there was no difference in response to rations during early lactation. However, in midlactation, all values, except ruminal butyrate, changed significantly with time. Likewise, correlations of insulin with fat, glucose and ruminal propionate were significant in midlactation but not early lactation. Although ruminal acetate and propionate were responsive to HG feeding, the correlations between fat, glucose and insulin were significant and changed responsively only during midlactation when HG was fed. At this stage of lactation, production is lower and energy demands are reduced allowing glucose to increase in the circulation.

KEY WORDS: • high grain ration • serum insulin • serum glucose • volatile fatty acids • milk fat

¹ Approved for publication by the Dean, College of Agriculture and Life Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Va. 24061.

² A preliminary report was presented at the 57th Annual Meeting of the Federation of American Societies for Experimental Biology, 15–20 April. 1971, Atlantic City, N. J., Federation Proc. 32: 3835 (abstr.). These data are from a dissertation submitted by the senior author in partial fulfillment of the requirements for the Ph.D. degree in 1973 at the Virginia Polytechnic Institute and State University, Blacksburg, Va. 24061.

Manuscript received 3 July 1973.