Effect of varying forage to concentrate ratio of isonitrogenous rations on feed intake by ruminants

Numerous theories have been proposed concerning the control of voluntary feed intake. One of these theories is that ruminants tend to regulate feed intake to satisfy physiological energy needs when fill does not limit consumption of the diet. The purpose of this study was to determine if ruminants are able to regulate feed intake to meet physiological energy needs when fed isonitrogenous rations varying in forage:concentrate ratios.

A digestion trial using six non pregnant Holstein heifers in a 3 by 3 Latin square design was conducted. The effect of various alfalfa hay:concentrate ratios on energy intake, digestibility and volatile fatty acid concentration in the rumen was studied. The three rations used contained 100, 67 and 33 percent forage and were pelleted into three fourths by three fourths inch range cubes.

Voluntary feed intake decreased with added concentrate to the ration, whereas digestibility of gross energy increased. The net effect was to provide equivalent energy intakes (measured by digestible energy intake and nutritive value index) for the three alfalfa hay: concentrate ratios. Digestible dry matter intakes were also essentially the same for all rations. Significant increases in dry matter, gross energy and crude protein digestibility were observed, but no significant differences were observed for acid detergent fiber digestibility.

Highly significant correlations were found between digestible dry matter and dry matter intake, digestibility of gross energy and dry matter intake and digestible dry matter and gross energy digestibility. Non significant correlations were found between dry matter intake and pH and dry matter intake and acetate:propionate ratio. The percent of acetic acid decreased, and the percent of propionic and butyric acids increased in the rumen with the addition of concentrate to the ration. The acetate propionate ratio and rumen pH decreased as the amount of concentrate increased in the ration. Density measurements of the rations were determined and were found to increase as the percent concentrate increased in the ration.