Supplemental methionine and urea for gestating beef cows consuming low quality forage diets

¹ Department of Animal and Range Sciences, New Mexico State University, Las Cruces, NM

^* To whom correspondence should be addressed. E-mail: richard{at}larrl.ars.usda.gov.

	Abstract

A study was conducted to evaluate Met requirements of late-gestation beef cows consuming low quality forages on the premise that inadequate supply of metabolizable AA may limit protein accretion during pregnancy. Five ruminally cannulated multiparous late-gestating predominantly Angus ( 75%) with Hereford and Simmental breeding beef cows (490 ± 27 kg) were used in a 5 x 5 Latin square designed experiment to evaluate the effects of post-ruminal DL-Met supplementation on 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 daily), U + 5 g/d Met (5MU), U + 10 g/d Met (10MU), and U + 15 g/d Met (15MU). Cows were adapted to experimental diet 30 d before the start of the study 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. Blood samples were collected every 4 h on d 13 of each period for analysis of serum metabolites and plasma AA. Inclusion of urea increased DM and OM intakes (U vs. NU; P = 0.05), but no further improvement in intake was observed with inclusion of Met. Serum urea concentrations increased with inclusion of urea (P = 0.03) and responded quadratically (P = 0.06) when Met was added with the lowest concentration observed with the 5MU treatment. More N was retained with the inclusion of urea (P = 0.04) and increased linearly with inclusion of Met (P = 0.07). Plasma Met concentration increased linearly (P < 0.01) with inclusion of Met. These data suggest that Met was a limiting AA and that supplementation of a combination of urea and 5 g/d of rumen-protected Met to low quality forage diets will improve N retention and promote protein accretion during late pregnancy.

Key Words: Beef Cows, Gestation, Methionine, Nitrogen Retention, Plasma Amino Acids

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