title: Stocking rate and monensin supplemental level effects on growth performance of beef cattle consuming warm-season grasses.
abstract: The objective of this study was to evaluate the effects of monensin supplementation on animals receiving warm-season grass with limited supplementation. In Exp. 1, treatments were a factorial combination of 2 stocking rates (1.2 and 1.7 animal unit [AU] [500 kg BW]/ha) and supplementation with monensin (200 mg/d) or control (no monensin) distributed in a complete randomized design with 3 replicates. Thirty Angus × Brahman crossbred heifers (Bos taurus × Bos indicus) with initial BW of 343 ± 8 kg were randomly allocated into 12 bahiagrass (Paspalum notatum) pastures and supplemented with 0.4 kg DM of concentrate (14% CP and 78% TDN) daily for 86 d. Herbage mass (HM) and nutritive value evaluations were conducted every 14 d, and heifers were weighed every 28 d. There was no effect (P ≥ 0.97) of monensin on HM, herbage allowance (HA), and ADG; however, animals receiving monensin had greater (P = 0.03) plasma urea nitrogen (PUN) concentrations. The stocking rate treatments had similar HM in June (P = 0.20) and July (P = 0.18), but the higher stocking rate decreased (P < 0.01) HM and HA during August and September. Average daily gain was greater (P < 0.01) for the pastures with the lower stocking rate in August but not different in July and September (P ≥ 0.15). Gain per hectare tended to be greater on pastures with the higher stocking rate (P ≤ 0.06). In Exp. 2, treatments were 3 levels of monensin (125, 250, and 375 mg/animal per day) and control (no monensin) tested in a 4 × 4 Latin square with a 10-d adaptation period followed by 5 d of rumen fluid collection and total DMI evaluation. Blood samples were collected on d 4 and 5 of the collection period. Ground stargrass (Cynodon nlemfuensis) hay (11.0% CP and 52% in vitro digestible organic matter) was offered daily. The steers received the same supplementation regimen as in Exp. 1. Total DMI was not different among treatments (P = 0.64). There was a linear increase (P ≤ 0.01) in propionate and a tendency for decreased acetate (P ≤ 0.09) concentrations in the rumen with increasing levels of monensin; however, there was no effect (P ≥ 0.19) of monensin levels on ruminal pH and ruminal concentrations of butyrate and ammonia. In addition, there was no effect (P ≥ 0.73) of monensin levels on plasma concentrations of glucose, insulin, IGF-1, and PUN. In summary, monensin supplementation effects were not detected at either stocking rate and may not be effective in increasing performance of beef cattle grazing low-quality warm-season grasses with limited supplementation.
authors.forename: J M B
authors.initials: JM
authors.lastname: Vendramini
authors.forename: J M D
authors.initials: JM
authors.lastname: Sanchez
authors.forename: R F
authors.initials: RF
authors.lastname: Cooke
authors.forename: A D
authors.initials: AD
authors.lastname: Aguiar
authors.forename: P
authors.initials: P
authors.lastname: Moriel
authors.forename: W L
authors.initials: WL
authors.lastname: da Silva
authors.forename: O F R
authors.initials: OF
authors.lastname: Cunha
authors.forename: P D S
authors.initials: PD
authors.lastname: Ferreira
authors.forename: A C
authors.initials: AC
authors.lastname: Pereira
mesh_terms: D000821:Animal Feed / Q000032:analysis*; D000818:Animals; D001806:Blood Urea Nitrogen; D002417:Cattle / Q000502:physiology*; D031744:Cynodon / Q000737:chemistry*; D005260:Female; D006799:Housing, Animal; D007328:Insulin / Q000097:blood; D007334:Insulin-Like Growth Factor I; D008985:Monensin / Q000494:pharmacology*; D009753:Nutritive Value; D031781:Paspalum / Q000737:chemistry*; D061209:Proton Ionophores / Q000494:pharmacology; D012621:Seasons
chemical_list: D007328:Insulin; D061209:Proton Ionophores; D007334:Insulin-Like Growth Factor I; D008985:Monensin