Influence of Fertilizer Treatment on the Intake, Digestibility and Palatability of Tall Fescue Hay

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Abstract
Feeding trials were carried out to determine the influence of N. P and K fertilization on the intake, digestibility and palatability of Kentucky 31 tall fescue hay. The fertilized hays were fed to sheep in either a conventional trial, with digestibility determined at the ad libitum level, or under a cafeteria system, where the animals had a free choice of the different hays. In the conventional trials level of N fertilizer had a significant effect on protein digestibility in both the first and aftermath cuttings. There were no differences in dry matter or cellulose digestibility between fertilized and control treatments in the first cutting, but a significant depression in digestibility in the non-nitrogen fertilized aftermath hays was noted. Ad libitum intake levels for the fescue hays were not significantly different in either cutting, and intake and NVI values were generally lower than for a standard alfalfa. When the first-cutting hays were offered to sheep on a cafeteria basis, the animals showed a decided, and individual ability to discriminate among treatments. The sheep tended to select either one or two major choices and to maintain this selection pattern fairly consistently over a 40-day period. Individuals differed in their feeding behavior, but as a group the primary selection was for fescue treated with phosphate fertilizer, and the second selection was for fescue fertilized at a low (50 lb.) level of nitrogen. The sheep tended to reject the fescue hays fertilized with medium and high levels of nitrogen. While significant differences in preference patterns or palatability due to fertilizer treatment were obtained, these differences appeared to bear no consistent relationship to intake as determined in the conventional system. An attempt was made to relate intake and palatability values to chemical composition, rates of in vitro fermentation and production of VFA from the forages. Differences in ad libitum intake between the fescues and alfalfa were associated with differences in structural components, more particularly with the cell-wall and lignin fractions. These differences were also reflected in the rates of breakdown of cellulose under in virto conditions. The phosphate-fertilized fescue hay was degraded at a significantly faster rate during the early stages of fermentation than the other fescue hays. Molar ratios of VFA determined in rumen fluid from sheep in the conventional trials showed no significant changes due to fertilizer treatment, but the acetic:propionic acid ratio in rumen fluid from sheep on a standard alfalfa was markedly lower. Intake and NVI values showed a significant negative correlation with the acetic:propionic acid ratio in rumen fluid. Little explanation can be offered for the observed differences in palatability. Variations in the level of carbohydrate, nitrogen and mineral components of the experimental hays were not related consistently to selection behavior. There was some indication of a sub-optimum level of phosphorus nutrition in the cafeteria animals, but the theory of “nutritional compensation” which is suggested requires further investigation.