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 ty pe of silage fed before puberty on the partitioning of excess dietary energy between synthesis of milk and BW gain in second or third lactat ion. Accordingly, 41 Holstein heifers weighing 175 kg were fed diets c ontaining either alfalfa silage or corn silage to gain either 725 or 9 50 g/d until BW was 325 kg and two estrous cycles were observed. Puber ty 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% conce ntrate) and a high energy diet (20% forage and 80% concentrate) in a d ouble-reversal experimental design with three 6-wk periods. The rate o f BW gain before puberty did not affect the magnitude of changes in DM I, milk yield, milk composition, or concentrations of thyroid hormones , insulin, bST, glucose, or lipids in serum when cows were switched fr om a control to a high energy diet during second or third lactation. H owever, compared with cows fed a corn silage diet, cows fed alfalfa si lage between 175 and 325 kg of BW had more depressed yields of fat, to tal solids, and FCM when fed the high energy diet than when fed the co ntrol diet during second or third lactation. Increased deposition of f at 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 c ows were switched from a control diet to a high energy diet during sec ond or third lactation. Overall, neither rate of BW gain nor type of s ilage fed between 175 and 325 kg of BW had a major influence on partit ioning of excess dietary energy between synthesis of milk and BW gain during second or third lactation.