Forage proportion and particle length affects the supply of amino acids in lactating dairy cows1

This study was conducted to evaluate the effects of dietary factors that alter ruminal fermentability on intake, duodenal flows and intestinal digestibility of individual amino acids (AA) in lactating dairy cows. The experiment was designed as a 4 × 4 Latin square using 4 ruminally and duodenally cannulated lactating dairy cows. Treatments were arranged in a 2 × 2 factorial design; 2 forage particle lengths (FPL) of alfalfa silage (short and long) were combined with low (35:65) and high (60:40) forage-to-concentrate ratio (F:C; dry matter basis). Four diets were formulated using 2 cuts of alfalfa silage [short (7.9 mm) and long (19.1 mm)], combined with 2 ratios of forage to barley grain concentrate (35:65 and 60:40). Overall, the interactions between dietary F:C and FPL on intake, duodenal flows, and intestinal digestibility of AA were marginal. Intakes of total AA and nonessential AA were not different between low- and high-F:C diets, whereas that of essential AA tended to be less with high-F:C diet as a result of lower intakes of Met, Phe, Arg, and His. The flows of total AA and microbial AA were reduced by 22 and 19%, respectively, with increasing F:C ratio in the diets due to consistently reduced flows of individual AA, whereas AA profiles (% of AA-N) of the duodenal protein were not different. Altering F:C from 35:65 to 60:40 decreased the intestinal digestibility of Ile, Leu, Thr, Val, Ala, Cys, and Ser, and consequently, tended to decrease the digestibility of total AA, essential AA, and nonessential AA. Intakes of total AA, essential AA, and nonessential AA were overall not affected by dietary FPL so FPL did not affect the flows or intestinal digestibility of AA. These results indicate that increasing dietary F:C ratio decreased overall AA supply because flow to the duodenum and intestinal digestibility of AA were decreased. However, increasing FPL had no effect on AA supply. The measured duodenal flows of AA were consistent with the predictions of the Cornell Net Carbohydrate and Protein System model for the low-forage diet, and were consistent with the National Research Council model for the high-forage diet. Furthermore, the digestibility of individual AA in the intestine varied considerably, regardless of dietary treatment. The results revealed the necessity to consider the both flows and digestibility of individual AA when optimizing ration formulation to meet AA requirements of dairy cows.