The effect of lactation and diet on milk lipoprotein lipase activity (LPL), cell count, serum albumin concentration and free fatty acid levels, expressed in terms of acid degree value (ADV), was examined in 2 experiments. In addition, the ability of blood plasma from cows on different diets to activate LPL was also investigated.
Changes in ADV were not consistently associated with changes in LPL, cell count or serum albumin throughout lactation, although results indicated that ADV appeared to be influenced by cell count in early lactation, and by the amount of blood plasma proteins reaching the milk during the remainder of lactation. LPL activity was low in colostrum, but increased rapidly between d 2 and 12 post-partum, reaching a peak at 5 months. Activity was unaffected by diet.
There was no effect on ADV of feeding concentrate: hay ratios of 75:25 and 60:40, but ADV was higher in cows fed a low protein diet (crude protein 9%) between 10 and 60 d post-partum, than in cows fed a high protein diet (crude protein 18 %) during the same period. In addition, the blood plasma for cows on the low protein diet had a greater ability to activate LPL although, on the basis of the millk: blood serum albumin ratio, a smaller proportion of blood constituents had reached the milk. The ability of blood plasma to activate LPL was also found to increase with increasing levels of lipoprotein lipid in the blood.