Amino Acid Supply and Metabolism in Relation to Lactational Performance of Dairy Cows Fed Grass Silage Based Diets

The main objective of the thesis was to study AA supply of dairy cows fed grass silage based diets with specific emphasis on the impact of changing individual and total AA supplies arising from individual AA supplementations or changes in the diet on milk production and AA utilisation. Mammary AA metabolism was also studied to broaden the understanding of the mechanisms underlying altered lactational performance. Experiments documented in publications I and II were AA infusion studies in which the utilisation of infused His (I) and the role of BCAA as the second limiting AA on grass silage-cereal based diets (II) were investigated. Graded infusion of His (0, 2, 4 and 6 g/d) linearly increased milk and milk protein yields and the utilisation of infused His remained constant across infusion levels. This indicates that mammary gland is able to regulate nutrient uptake and that arterial supply is not the sole factor affecting mammary metabolism. Infusions of BCAA did not affect lactational performance suggesting that they are not second limiting AA on grass silage-cereal based diets. The effects of barley and rapeseed meal supplementation of grass-red clover silage on omasal canal AA flow and the AA profile of liquid and particle associated bacteria, protozoa and the entire microbial protein were studied in publication III. Barley increased microbial AA flow and rapeseed meal increased the flow of dietary AA entering the omasal canal. The AA profiles of individual microbial fractions were different but the effect of diets on AA profiles was negligible. Diet had no effect on the AA profile of the entire microbial protein. Under the dietary conditions used, the assesment of microbial AA flow was more dependent on the accuracy of microbial protein flow measurements than the AA profile of individual AA fractions. The effects of AA profile and rumen undegradable protein content of protein supplements on postruminal AA supply and lactational performance were investigated in publication IV. All protein supplements (fish meal, soybean meal and maize gluten meal) increased omasal canal AA flow and milk production. Higher dietary protein flow was also reflected in the AA profile of omasal digesta. Lactational responses and increases in AA flow were lower for soybean meal compared with fish meal and maize gluten meal owing to higher N losses in the rumen. The mammary gland appeared to be capable of regulating AA utilisation by changing the rate of extraction of individual AA. The purpose of publication V was to assess the effect of silage harvest date (primary and secondary cuts) and the level of concentrate on postruminal AA supply and lactational performance. Lactational performance was higher for diets based on the secondary cut silage owing to greater nutrient supply. Increasing the concentrate level increased microbial protein supply but did not increase N capture in the rumen. Differences in AA flows between the concentrate levels were lower than predicted by the AAT/PBV system while ruminal degradation of barley was higher than current predictions. Amino acid profiles of omasal digesta and microbial protein were similar between experiments. Some variation in total AA supply exists in spite of all basal diets were based on grass silage and cereal. This appears to be associated with differences in silage quality. Results demonstrated the importance of AA profile of absorbed protein in utilisation of dietary AA and also that AA supply can be altered by changing the AA profile and rumen undegradable protein content of protein supplements. Total AA supply appears in part, to compensate for the incomplete AA profile of digested protein because mammary gland is cabable to regulate AA uptake.