The effect of varying the energy content of the ration on the voluntary feed intake of ruminants with different energy requirements

Four experiments were conducted to study the proposed theory that ruminants are capable of adjusting their voluntary feed intake in order to meet physiological energy needs if fill is not a limiting factor. Three experiments were conducted using 84 ruminant animals. These animals were used to study the effect of varying the energy content of the ration on voluntary feed intake and the digestibility of various ration components. The digestibility values were determined by the chromic oxide and acid detergent lignin techniques. The energy content of the rations were varied by feeding rations composed of various forage-to-concentrate ratios. The following forage-to-concentrate ratios were fed: (1) 100:0, (2) 82.5:17.5, (3) 65:35, (4) 47.5:52.5, and (5) 30:70. Thirty Holstein calves were fed rations 1, 2, and 3 in Experiment I. There were no significant differences in dry matter intakes between the three rations. Significant differences between rations did exist in the digestibility of dry matter and gross energy when determined by the chromic oxide indicator method. These differences were not noted when the digestibility values were determined by the acid detergent lignin indicator method. Significant differences were found in digestible energy intakes when determined by the acid detergent lignin method but not when determined by the chromic oxide method. No significant differences were observed between the average daily gains of the calves on the three rations. Thirty Holstein heifers were fed rations 2, 3, and 4 in Experiment II. Decreases in dry matter intakes were noted as the amount of concentrate increased in the ration. Significant differences were found in the digestibility of dry matter as determined by both indicator methods. No significant differences were found however in the digestibility of gross energy as determined by both methods. Significant differences did exist in the digestible energy intake of the animals on the three rations. No significant ration differences were noted in average daily gains or blood glucose values, whereas significant differences did exist in rumen pH values. Fifteen Holstein cows and nine Jersey cows were fed rations 3, 4, and 5 in Experiment III. There were no significant ration differences in dry matter intakes or the digestibility of dry matter and gross energy when determined by the chromic oxide method. Significant differences between rations were observed in these digestibility values when determined by the acid detergent lignin method. There were no significant differences in digestible energy intakes when determined by both methods. Significant ration differences were also not observed in average daily gains, rumen pH, actual milk production, fat test, and fat-corrected milk production between the three rations. Three fistulated cows were used in Experiment IV to study the effect of increasing the energy content of the ration on voluntary feed intake. The energy content of the ration was increased by the intraruminal infusions of volatile fatty acid mixtures. The two volatile fatty acid mixtures used contained 57 percent acetic, 22 per cent propionic, and 21 percent butyric acids and were neutralized to pH 6.15 with sodium hydroxide. One of the mixtures allowed 500 milliliters of the acids to be infused per day, whereas the other mixture allowed 1000 milliliters to be infused per day. When the two acid mixtures were infused, they reduced dry matter intakes but also caused unphysiological conditions. Acid infusion appeared to have no effect on rumen pH and rumen volatile fatty acid concentrations.