Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2002
Publication Date: 6/1/2002
Citation: Williams, M.J., Chase, C.C., Jr., Hammond, A.C. 2002. Diet quality and performance of heifers in the subtropics. Agronomy Journal. 94:88-95. Interpretive Summary: Dependence on tropical forages and lack of significant grain production in the Gulf Coast region of the U.S.A. limits beef cattle producers in this region largely to the cow-calf segment of the cattle industry. Tropical grasses generally have lower nutritive value than temperate grasses. For replacement heifers this often means low post-weaning growth rates and delayed onset of puberty with lower lifetime productivity because the majority of heifers do not calve until they are 3 yr old. Different systems of grazing management such as rotational or continuous stocking can affect forage quality and alter (improve or reduce) animal gains on the same forage. Little is known about the advantages or disadvantages of rotational or continuous stocking in the Gulf Coast region where both cool temperatures and seasonal rainfall both can limit plant growth. In a three year study, heifer gains were equal to or better with continuous stocking than with rotational stocking on bahiagrass in Florida. This was due to the heifers' ability to select a diet that had higher protein levels when they were allowed to continuously graze compared to rotational grazing. This indicates that some type of continuous stocking should be used to maximize individual animal gains on tropical grasses.
Technical Abstract: The effect of continuous (CON) vs. rotational (ROT) grazing management of bahiagrass (Paspalum notatum Flugge), previous winter nutritional regimen (corn + soybean meal [CS] vs. soybean meal [SBM] alone), and breed of heifer (Angus, Hereford, Brahman, and Senepol) was determined for three grazing seasons. Pasture dry matter production (DM), crude protein (CP), and in vitro organic matter digestibility (IVOMD) were determined. Average daily gain (ADG), body weight (BW), body condition score (BCS), and plasma urea nitrogen (PUN) and glucose (PGLU) were also determined. Winter supplementation affected (P<0.05) BW, BCS, and PGLU with CS>SMB. In contrast PUN levels for SBM>CS at the start of the summer grazing season, but this difference did not persist. Breed also affected (P<0.01) most of the animal variables studied with the breed responses generally segregating based on temperate (Angus and Hereford) vs. tropical (Brahman and Senepol) adaptation. Forage DM varied from a seasonal average of 1000 kg/ha in 1990 to 2000 kg/ha in 1992 due to rainfall distribution during the grazing season. Neither DM, CP, or IVOMD differed due to grazing management, but CON heifers had higher (P<0.001) PUN levels than ROT heifers. Only during 1990, when forage growth was restricted by dry weather, did this nutritional advantage in diet selection translate into improved ADG (0.30 +/- 0.02 vs. 0.25 +/- 0.02 kg/d for CON vs. ROT, respectively) and BW (314 +/- 4 vs. 301 +/- 4 kg for CON vs. ROT, respectively).