Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/19/2006
Publication Date: 2/1/2007
Citation: Waterman, R.C., Loest, C.A., Bryant, W.D., Petersen, M.K. 2007. Supplemental methionine and urea for gestating beef cows consuming low quality forage diets. Journal of Animal Science online doi:10.2527/jas.2006-425. J. Anim Sci. 85:731-736. Interpretive Summary: Amino acids are the building blocks of protein and methionine is often an amino acid considered deficient in grazing livestock diets. Forages consumed by grazing livestock are altered in the rumen (largest compartment of a ruminant stomach) once microorganisms living there consume them. Generally, the profile of amino acids presented to the small intestine remains consistent, and is comprised of microbial protein and forage protein that escapes rumen degradation. However, if this profile of amino acids being presented to the small intestine is deficient of one or more amino acids livestock performance may be compromised. Therefore, the present study evaluated the response of providing an essential amino acid (methionine) to the gastric stomach of ruminants (abomasum) thereby circumventing the microorganisms. Late gestating cows were utilized in the present study and were on a wheat straw and alfalfa diet. Urea was added to the diet to insure that the microorganisms in the rumen were adequate in nitrogen for microbial synthesis (microbial growth). As a result, of increasing methionine supply to the small intestine pregnant cows retained more nitrogen (excreted less nitrogen in feces and urine) or in other words utilized amino acids more efficiently for needs of maternal tissues and that of the developing fetus. The importance of this finding is the recognition that grazing livestock production can be improved by meeting not only a protein requirement but maybe more importantly an amino acid requirement.
Technical Abstract: Studies conducted to evaluate Met requirements for late gestating beef cows consuming low quality forages. Inadequate supply of metabolizable AA may limit protein accretion during pregnancy in beef cows. In Exp. 1, two ruminally cannulated non-gestating non- lactating cows were utilized in a flow study to evaluate the supply and AA composition reaching the duodenum. Cows were allowed ad libitum access to water, mineralized salt, and basal diet comprised of 67% wheat straw (1.9% CP and 78.7% NDF, OM basis) and 33% alfalfa (17.0% CP and 43.2% NDF, OM basis). In Exp. 2, Five ruminally cannulated gestating beef cows (490 ± 27 kg) were used in a 5 ' 5 Latin square to evaluate the effects of post ruminal DL-Met supplementation on nitrogen (N) retention, serum metabolites, and plasma AA concentrations during the third trimester of pregnancy. Basal diet was individually fed and refusal weights recorded for N intake determination. Treatments consisted of no urea (NU), urea (U; 0.053 ± 0.002 g/kg BW/d), U + 5 g/d Met (5MU), U + 10 g/d Met (10MU), and U + 15 g/d Met (15MU). Cows were adapted to the diet 30 d before to the start of the experiment with periods lasting 14 d: 4 d to allow clearance of previous treatment effects, 4 d for adaptation to treatments, and 6 d for total fecal and urine collection. Serum and plasma samples were collected every 4 h for 24 h on d 13 of each period for analysis of serum metabolites and plasma AA. When DM and OM intake was compared between the NU vs. U the inclusion of urea improved intake (P = 0.05) however, no further improvement in intake were observed with the inclusion of Met. Serum urea N concentrations increased with inclusion of urea (P = 0.04) and were lowered with the inclusion of Met (P = 0.09). Nitrogen retention was improved with the inclusion of urea (P = 0.06) and 5 g/d Met (P = 0.04). Plasma Met concentration increased (P < 0.01) for U vs. 5MU + 10MU + 15 MU by 29.9%, 51.9% and 67.5% respectively when compared to U fed cows. Methionine flow to the duodenum was 1.97 g/kg OM intake and OM intake for U treated cows was 6.61 kg/d providing 11.05 g/d of Met to the duodenum. Thus, we conclude that the requirement for beef cows grazing low-quality forages in our study would require between 11.05 and 16.05 g/d Met (11.05 g/d from the basal diet + 5 g/d of supplemental Met). These observations support our hypothesis that Met is a limiting AA for gestating beef cows consuming low quality forages. Key words: Beef Cows, Gestation, Methionine, Nitrogen Retention, Plasma Amino Acids