Lactational response of dairy cows to oral administration of L-tyrosine

Tyrosine was administered to lactating cows to determine if the lack of response to canola meal treated with formaldehyde could be due to failure to increase uptake of tyrosine. Disappearance of free tyrosine from nylon bags suspended in the rumen of a fistulated steer was 27.4% after 12 h. Oral administration of tyrosine to sheep increased the free tyrosine concentration of duodenal fluid but did not affect tyrosin concentration in blood plasma. Tyrosine (50 g/cow per day) was fed to six lactating Holsteins in a switchback design. Milk yield was increased by 1.0 kg/day. In a second trial with a switchback design, tyrosine was fed to 12 lactating cows that were consuming diets containing formaldehyde-treated canola meal. Supplementing tyrosine to the formaldehyde-treated diet increased milk production by 1.6 kg/day, but adding tyrosine to the untreated diet did not affect milk yield. Results suggest that availability of tyrosine can limit milk production in some circumstances and that the lack of response to canola meal treated with formaldehyde may be partly due to failure to increase the uptake of tyrosine.