Four ruminally and duodenally cannulated multiparous Finnish Ayrshire cows were fed on diets consisting of grass silage (0·6 kg/kg DM) and one of four concentrates: barley, barley + urea, barley + rapeseed meal and barley + rapeseed cake. The objective of the present study was to compare omasal canal and duodenal digesta flows. Values for digesta flow into the omasal canal and duodenum were determined using a triple-marker method based on Co-EDTA, Yb-acetate and indigestible neutral-detergent fibre (NDF) markers. Microbial non-NH3 N (NAN) flow was assessed by purine flow. Microbial samples to determine the bacterial purine: N ratio were harvested from the rumen, omasum and duodenum. Organic matter flow was significantly lower into the omasum than the duodenum, indicating an endogenous organic matter secretion into the abomasum. In contrast, NDF and acid-detergent fibre flows were significantly higher into the omasum indicating digestion of fibre in the omasum. Microbial NAN flows were significantly different (P < 0·001) when estimates were based on bacterial samples harvested from different sites. Differences in total NAN, microbial NAN and dietary NAN flows entering the omasal canal and duodenum were non-significant. The results indicated that the omasal sampling technique provides a promising alternative to the duodenal sampling technique to investigate forestomach digestion in dairy cows and offers an alternative means to study rumen N metabolism.

The response of dairy cows fed grass silage-based diets to the abomasal infusion of water (control) or 6.5 g of His alone or in combination with either 6.0 g of Met or 19.0 g of Lys or both was studied in an incomplete 4 x 5 Latin square experiment with 14-d periods. Each cow received a basal diet of 8 kg/d of cereal concentrate [12.1% crude protein (CP)] and free access to grass silage (14.1% CP) ensiled with an acid-based additive. Postruminal infusions increased arterial plasma concentrations of the amino acids (AA) infused, but compared with control, only the infusion of His (18 vs. 57 mumol/L) was associated with significant increases in milk and milk protein yields. Infusions of His did not affect dry matter intake of grass silage, rumen fermentation, or diet digestibility. Milk protein content was unchanged by treatments, but His infusions decreased lactose and fat contents. The combinations of AA did not produce any further responses compared with His alone. However, milk protein percentage was slightly higher, and milk fat percentage tended to be higher when Met rather than Lys was infused with His. We concluded that His is the first-limiting AA when grass silage-based diets are supplemented with cereal concentrates, while neither Met nor Lys are the second-limiting AA with grass silage feeding.

Our previous study showed that His was the first-limiting amino acid (AA) for milk protein production in cows fed grass silage and cereal-based supplement. The aim of this study was to identify the second-limiting AA and determine whether glucose was limiting responses to His. Abomasal infusion of His (6.5 g/d), glucose (250 g/d), His (6.5 g/d) + glucose (250 g/d), His 6.5 g/d) + Leu (12 g/d) and His (6.5 g/d) + Leu (12 g/d) + glucose (250 g/d) on milk production and utilization of amino acids by mammary gland was in an incomplete 5 x 6 Latin square design with 14-d periods. The diet was based on restrictively fermented grass silage fed ad libitum and 8 kg/d of concentrate comprised of barley, oats, unmolassed sugar beet pulp, urea, and minerals. The infusions did not affect feed intake, diet digestibility, or rumen fermentation pattern. The molar proportion of propionate in rumen VFA was low (15.5%), suggesting that glucose supply from the basal diet could be limiting. Milk and milk protein yields were increased by His infusion. Infusion of His increased plasma His concentration from 19 to 52 microM but decreased extraction efficiency of His. Infusion of glucose increased plasma glucose concentration, milk lactose concentration, and yield and tended to increase milk protein yield. Responses in milk protein yield to combined infusions of His and glucose were additive, suggesting that the utilization of the first-limiting AA His was limited by glucose supply. Infusion of Leu increased plasma Leu concentration but did not produce any further milk protein yield response compared with the infusions without Leu. It was concluded that the efficiency of utilization of the first-limiting AA His could be improved by increasing the supply of glucose, when the basal diet produces a rumen fermentation pattern low in propionate. Leu was not the second-limiting AA in cows fed grass silage-based diets.

Five Finnish Ayrshire cows were used in a 4 x 4 Latin square experiment designed to study the effects of graded doses of postruminal His infusion on milk production, arterial concentrations, and mammary uptakes of plasma amino acids (AA) as well as utilization of added His. Grass silage (16.9% CP) was given ad libitum with 8 kg of cereal-based concentrate per day (11.3% CP). Treatments were abomasal or duodenal infusions of 250 g of glucose/d in combination with 0, 2, 4, or 6 g of His/d. Infusions did not affect dry matter intake (mean 18 kg/d). Infusion of His increased milk yield linearly from 27.0 to 28.8 kg/d, protein yield from 861 to 919 g/d and lactose yield from 1345 to 1457 g/d. Milk fat yield and content changed in a cubic manner (1240, 1167, 1296, and 1177 g/d and 4.60, 4.16, 4.60, and 4.09). Infusion of His had no influence on milk protein or lactose concentrations. Arterial Lys and His concentrations increased linearly, but other AA concentrations were unaffected as well as calculated arteriovenous differences and mammary AA uptakes. The extraction of His decreased linearly with an increasing amount of His. The utilization of added His (28%) was not affected by the level of infusion, and mammary AA uptake seemed to be regulated by an inverse relationship between arteriovenous difference of essential AA and calculated mammary plasma flow. This experiment confirmed that His is the first-limiting AA on grass silage-cereal based diets.

Five rumen-cannulated Finnish Ayrshire cows were used in two 5 x 5 Latin square experiments designed to study the lactation and metabolic responses to increasing doses of DL-Met or L-Lys infused into the abomasum. The cows were fed grass silage ensiled with a formic acid additive for ad libitum intake. A supplement with barley and oats was given at a rate of 9 kg/d (Experiment 1) or 7 kg/d (Experiment 2). The experimental treatments were 0, 10, 20, 30, or 40 g of Met/d (Experiment 1) and 0, 15, 30, 45, or 60 g of Lys/d (Experiment 2). The infusion of Met did not significantly affect feed intake or daily milk yield, but increased milk fat content, ECM yield, and C4 to C14 and C18 to C20 fatty acid production in milk. The infusion of Met caused an increase in arterial plasma Met concentration and a decline in branched-chain amino acids (AA). Mammary gland uptake of Met was not related to plasma AA concentration. The infusion of Lys did not affect feed intake, milk yield, or milk composition, except for increases in milk urea and NPN contents. The infusion of Lys increased plasma Lys, BCAA, EAA, and the EAA to TAA ratio. Uptake of plasma BCAA and NEAA by the mammary gland decreased, which suggests that Lys was used as a substrate for milk NEAA synthesis. These data demonstrate that Met is important in the milk fat synthesis, and Lys is important in mammary gland AA metabolism. However, neither Met nor Lys is the first-limiting AA in the milk protein yield of cows fed a grass silage and cereal diet.