|Williams, Mary - Mimi|
|Chase, Chadwick - Chad|
Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2001
Publication Date: 9/1/2001
Interpretive Summary: Rhizoma peanut is a tropical forage legume that has high nutritional value for grazing beef cattle. Weed encroachment is a problem in rhizoma peanut pastures and is thought to be due to declines in the grass component of the pasture. Fertilizer, particularly nitrogen, and changes in stocking rate can be used to alter the relative proportion of a legumes and grasses in a pasture. Strategies that increase grass production can be detrimental to animal gains though, due to the generally lower feed value of grasses compared to legumes. In a study at the Subtropical Agricultural Research Station in Brooksville, FL, the effect of low levels of nitrogen fertilization and increased stocking rate on the forage production, diet selection, and performance of bulls grazing mixed grass and rhizoma peanut pastures were evaluated. Neither treatment affected the forage availability. As a consequence, although the bulls were found to preferentially graze the rhizoma peanut component of the sward, consume the grasses equivalent to their proportion in the pasture, and avoid the broad leaf weeds, neither nitrogen fertilization or increased stocking rate affected the performance of the bulls. This study suggests the long term nitrogen fertilization may be a means of maintaining or increasing the grass part of the mixed sward without compromising the legume component and subsequent animal performance.
Technical Abstract: Interaction of spring N (0 vs. 35 kg/ha), summer stocking rate (1.5 vs. 2.5 bulls/ha) and breed (tropically adapted [Senepol, Bos taurus, and Brahman, B. indicus] vs. temperate [Angus, B. taurus] or Senepol vs. Angus) was determined for mixed rhizoma peanut (RP; Arachis glabrata Benth.)- grass swards in 1995 and 1996. Pasture parameters measured were herbage mass (HM, kg/ha), nutritive value (CP and IVOMD), herbage allowance (HA, kg/kg BW), and diet composition. Animal parameters measured included total and seasonal (spring vs. summer) average daily gain (ADG), hip height (cm), scrotal circumference (SC, cm), and plasma urea nitrogen (PUN, mg/dl). Herbage mass (3.2 Mg/ha 2 yr avg.) was not affected by treatment. Stocking rate did affect HA, but it never fell below 3.0 kg/kg. Crude protein (200-140 g kg/kg) and IVOMD (650-540 g/kg) declined (P<.001) from spring until fall and were not affected by treatment. Treatment also had no effect on diet botanical composition. Season affected ADG with summer ADG about .2 kg/d lower than spring ADG. Because HA was never limiting, full season ADG was not affected by either treatment and averaged .60 kg/d for the two years. There were season by breed interactions (P<.05) for ADG due to greater declines during the summer for Angus than Senepol or Brahman. There were no differences in final body weight, SC, BCS, hip height or PUN due to treatments, but breed did affect (P<.05) all parameters accept BCS. Breed differences were consistent with reproductive development and/or frame size of the breeds studied.