Effects of Fescue Type and Sampling Date on the N Disappearance Kinetics of Autumn-Stockpiled Tall Fescue

Authors: FLORES, RENE - UNIV. OF ARKANSAS; Coblentz, Wayne; OGDEN, ROBIN - UNIV. OF ARKANSAS; COFFEY, KENNETH - UNIV. OF ARKANSAS; Looper, Michael; WEST, CHARLES - UNIV. OF ARKANSAS; ROSENKRANS, CHARLES - UNIV. OF ARKANSAS

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/14/2007
Publication Date: 3/19/2008
Citation: Flores, R., Coblentz, W.K., Ogden, R.K., Coffey, K.P., Looper, M.L., West, C.P., Rosenkrans, C.F. 2008. Effects of fescue type and sampling date on the N disappearance kinetics of autumn-stockpiled all fescue. Journal of Dairy Science. 91:1597-1606.

Interpretive Summary: Recently, there has been increased interest in autumn-stockpiling tall fescue for grazing during winter months. Assessment of ruminal disappearance kinetics, which is essential for most newer nutritional models, have not been evaluated for these stockpiled forages. Most of the tall fescue throughout the southern Ozarks is infected with the wild type endophyte, but novel endophytes that produce minimal ergot alkaloids are becoming increasing popular with producers. Two tall fescue types were autumn-stockpiled in order to assess nitrogen partitioning within cell-soluble and cell-wall fractions, and ruminal disappearance kinetics of nitrogen for these forages sampled during winter. Generally, endophyte status (wild type or novel) had little practical effect on nutritive value or kinetics of ruminal disappearance. In addition, most N-related nutritional indices remained relatively stable over winter, were affected only minimally by grazing, and are likely to be superior to those from most tall fescue hays used commonly as supplemental winter forage. Overall, the nutritional characteristics exhibited by autumn-stockpiled tall fescue forage appear to be suitable for developing dairy heifers in the southern Ozark Highlands; however, lingering questions concerning suboptimal voluntary intakes by grazing cattle must be resolved before these findings can be used to develop management strategies.

Technical Abstract: Two tall fescue [Lolium arundinaceum (Schreb.) Darbysh] forages, one an experimental host plant/endophyte association containing a novel endophyte that produces low or nil concentrations of ergot alkaloids (HM4), and the other a typical association of Kentucky 31 tall fescue and the wild-type endophyte (Neotyphodium coenophialum; E+), were autumn-stockpiled following late-summer clipping and fertilization with 56 kg/ha of N in order to assess N partitioning and ruminal disappearance kinetics of N for these autumn-stockpiled tall fescue forages. Beginning on 4 December 2003, sixteen 361 ± 56.4-kg replacement dairy heifers were stratified by weight and breeding, and assigned to one of four 1.6-ha pastures (two each of E+ and HM4) that were strip-grazed throughout the winter. Pastures were sampled before grazing was initiated (4 December), each time heifers were allowed access to a fresh pasture strip (26 December, 15 January, and 4 February), and when the study was terminated (26 February). Generally, fescue type and the fescue type x sampling date interaction exhibited only minor effects on total forage N, or partitioning of N within either the cell solubles (NDSN) or the cell wall (NDIN). However, concentrations of N and NDSN within pregrazed forages declined in a strongly linear relationship with sampling dates. In addition, concentrations of NDIN changed in erratic and often higher-ordered relationships with time, but the magnitude of these responses generally was limited. Unlike the partitioning of N within cell-wall and cell-soluble fractions, kinetic characteristics of ruminal N disappearance frequently exhibited interactions of fescue type and sampling date. For pregrazed forages, these included interactions for all response variables, and for postgrazed forages, fractions B and C, as well as rumen degradable protein (RDP). Ruminal disappearance rate (Kd) for pregrazed E+ and HM4 exhibited quadratic (range = 0.057 to 0.082/h) and cubic (range = 0.057 to 0.075/h) relationships with time, respectively. For postgrazed E+ and HM4 forages, Kd was unaffected (mean = 0.066/h) or only tended to be affected by sampling date (mean = 0.065/h), respectively. Concentrations of RDP exhibited various curvilinear relationships with sampling dates, but disappearance was consistently extensive, and the overall range was relatively narrow (71.3 to 78.9 % of N). These findings suggest that ruminal disappearance of N for autumn-stockpiled tall fescue forages remains extensive throughout the winter months, and is only affected minimally by fescue type, sampling date, and grazing status.