Doctoral Dissertations

Date of Award

6-1968

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Animal Science

Major Professor

Charles S. Hobbs

Committee Members

K. M. Barth, R. R. Shrode, J. K. Bletner, R. H. Feinberg

Abstract

The objectives of this study were: (a) to evaluate and compare several methods of estimating average daily gain (ADG) of steers grazing on two types of pasture, (b) to investigate factors that affect ADG of these steers during the spring-summer grazing season, (c) to study the changes of volatile fatty acid (VFA) concentration in the rumen of these steers and their relation to animal performance and (d) to study selective grazing and its relation to ADG of steers. Two types of pasture, fescue-lespedeza (F-L) and orchardgrass-ladino clover (O-C) were used in this study. A grazing trial with steers was conducted to determine the body weight changes on these two types of pasture during various periods of the grazing season. Esophageal-fistulated steers were used to sample the diet of grazing animals (grazed sample) while hand-clipped samples were used to represent the forage available to the animals. Rumen-fistulated steers were used for in vivo VFA production studies. Chemical composition of grazed and clipped forage samples and in vivo and in vitro VFA production were determined at 28-day intervals during the spring-summer grazing season. These measures which affect quality of pastures were correlated with average daily gain of steers grazing the two types of pasture. Measures which were highly correlated with ADG were then used to develop equations for predicting ADG by multiple regression analysis. The results of this study were as follows: 1. ADG of steers grazing O-C pastures was higher than that of steers grazing F-L pastures. This was especially pronounced in the early part of the grazing season. 2. Grazed samples were significantly higher (P<.05) with respect to crude protein content in both types of pasture. 3. A consistently lower protein content, higher acid detergent fiber (ADF) content and acid insoluble lignin (AIL) content in the diet of the steers grazing F-L pastures may have accounted for the lower ADG of these steers. 4. There was no significant correlation between ADG and percent of protein in the grazed or the clipped samples from either type of pasture. The percent of AIL in the clipped samples was negatively correlated with ADG. 5. Total VFA production reached a peak about one hour after the morning grazing and tended to decline thereafter. However, the molar percent of individual VFA in rumen liquor varied little in both types of pasture during the hours after the morning grazing. The pH values and total VFA concentration were negatively correlated. Total VFA concentration was higher in the summer than in the fall. 6. Total and individual VFA concentration in the dorsal area of the rumen was significantly higher than in the ventral area. However, there were no significant differences in VFA ratio between these two locations. In both types of pasture, ADG was more highly correlated with total VFA concentration in samples from the dorsal area than with the same variable in samples from the ventral area of the rumen. 7. For the prediction of ADG of steers grazing F-L pastures, the equation including the variables in vivo total VFA concentration, AIL and crude protein percentage was the most useful of several similar equations. This multiple regression equation explained 98 percent of the variability in ADG of steers. 8. In O-C pastures, the equation containing the variables in vivo total VFA concentration, percent of AIL and in vitro DDM was most valuable in predicting ADG of steers. This equation accounted for 99 percent of the variation in ADG of steers. 9. Results of this investigation indicate that total or indivi-dual VFA production may be an important factor in the prediction of ADG of steers grazing F-L or O-C pastures. Together with other variables considered in this study, VFA concentration accounted for most of the variation in ADG of the steers.

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