Requirements of lactating dairy cows for leucine and methionine at the duodenum

Infusion experiments were conducted with nine lactating dairy cows of the German Black and White breed in mid-stages of lactation, with a daily milk production averaging 20–25 kg fat-corrected milk (FCM). The cows were fitted with rumen and duodenal cannulae. The objective of the study was to investigate the cows’ requirements for leucine and methionine at zero N-balance as supply to the duodenum. Over a period of 13 weeks, the animals received rations that were sufficient for maintenance plus 17 or 15 kg milk /day, regardless of the actual, higher yields of the animals. Differences between estimated requirements (according to recent knowledge) and energyand amino-acid supply by the ration were met by sucrose (hydrolysed)-, caseine (hydrolysed)and amino-acid infusion into the proximal duodenum. Alternatively, leucine and methionine were withheld from the infusion. Each experimental variation went on for five days. During this time, N-balances were established. Following the 11 infusion periods, the amounts of amino acids at the duodenum resulting from the rations were measured. The daily amounts of leucine sufficient to meet the requirements of dairy cows [600 kg body weight (BW)] for maintenance alone and for maintenance plus 15, 20, 25 or 30 kg milk /day were 38.1, 101.1, 124.7, 148.3 and 171.9 g, respectively. These values corresponded with those calculated from dose–effect correlations (55.1, 111.5, 130.6, 150.0 and 169.4 g) at zero N-balance. Daily methionine requirements, calculated at zero N-balances, were 6.3, 22.6, 28.4, 34.1 and 40.0 g. Again, these results corresponded well with estimations derived from dose–effect correlations (5.6, 22.7, 28.4, 33.1 and 39.8 g).  2000 Elsevier Science B.V. All rights reserved.