Effects of prepubertal growth rate and diet on lipid metabolism in lactating Holstein cows.

The objectives were to determine the effects of rate of BW gain and type of silage fed before puberty on the partitioning of excess dietary energy between synthesis of milk and BW gain in second or third lactation. Accordingly, 41 Holstein heifers weighing 175 kg were fed diets containing either alfalfa silage or corn silage to gain either 725 or 950 g/d until BW was 325 kg and two estrous cycles were observed. Puberty occurred near 281 kg of BW. During second (n = 36) or third (n = 5) lactation, the cows were fed a control diet (60% forage and 40% concentrate) and a high energy diet (20% forage and 80% concentrate) in a double-reversal experimental design with three 6-wk periods. The rate of BW gain before puberty did not affect the magnitude of changes in DMI, milk yield, milk composition, or concentrations of thyroid hormones, insulin, bST, glucose, or lipids in serum when cows were switched from a control to a high energy diet during second or third lactation. However, compared with cows fed a corn silage diet, cows fed alfalfa silage between 175 and 325 kg of BW had more depressed yields of fat, total solids, and FCM when fed the high energy diet than when fed the control diet during second or third lactation. Increased deposition of fat in adipose and mammary tissues of cows with mean BW gain in excess of 950 g/d or fed a corn silage diet between 175 and 325 kg of BW did not result in more pronounced depression of milk fat percentage when cows were switched from a control to a high energy diet during second or third lactation. Overall, neither rate of BW gain nor type of silage fed between 175 and 325 kg of BW had a major influence on partitioning of excess dietary energy between synthesis of milk and BW gain during second or third lactation.