New Mexico State University researchers sought to determine if providing a post-ruminal supply of a specific limited amino acid, methionine (Met), would help improve the efficiency of nitrogen usage for fetal development and maternal metabolism in late-gestating beef cows.

Five ruminally cannulated, multiparous late-gestation beef cows in their third trimester of pregnancy were used to study the effects of dl-Met supplementation. Treatments consisted of no urea (NU); urea (U; fed 0.053± 0.002 g/kg of body weight daily); U + 5 g of Met/day (5MU); U + 10 g of Met/day (10MU); and U + 15 g of Met/day (15MU).

Cows were adapted to the experimental diet 30 days before the beginning of the study, with periods lasting for 14 days; four days to allow for clearance of the previous treatment effects, four days for the adaptation to the treatments and six days for total fecal and urine collection.

Blood samples were collected every four hours on day 13 of each period for analysis of serum metabolites and plasma amino acid. Inclusion of urea increased dry matter and organic matter intakes, but no further improvement in intake was observed with the inclusion of Met. Serum urea concentrations increased with inclusion of urea and responded quadratically when Met was added; the lowest concentration was observed in the 5MU treatment. These data indicate that a bolus dose of urea alone did not influence N excretion, but inclusion of Met can influence the amount excreted.

More nitrogen was retained with the inclusion of urea, and nitrogen increased linearly with inclusion of Met. These results demonstrate that the requirement for Met in late-gestating beef cows was not satisfied with urea supplementation alone, and suggest that Met was a limiting amino acid and that supplementation of a combination of urea and 5g/day of rumen-protected Met in low-quality forage diets will improve nitrogen retention and promote protein accretion during late pregnancy.
— Waterman, et al. 2007.
Journal of Animal Science, 85:731.