A High Roughage System for Raising Calves Based on the Early Development of Rumen Function. V. Some Effects of Feeding Aureomycin with Different Ratios of Hay to Grain

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Abstract
Twenty-four Jersey calves were used in a study of the effects of feeding Aureomycin to calves raised on the high roughage system using 3 ratios of hay to grain (4:1, 3:2, and 2:3). Aureomycin in the form of Aurofac 2A was fed at the rate of 15 mg./day for the 1st 7 wks. during milk feeding and at the rate of 20 mg./lb. of dry feed ingested from the end of the 7th wk. to 12 wks. of age. Aureomycin increased growth both in body wt. and withers height and resulted increased feed intake and efficiency of feed utilization at all 3 hay to grain ratios. After the termination of Aureomycin feeding, both growth and efficiency of feed utilization declined below the level of the controls so that no prolonged growth benefit resulted at either hay to grain ratio fed, 2:1 or 4:1. The percentage of dry matter, cellulose, and protein digested was not influenced by Aureomycin feeding. When the grain was increased from the 4:1 to the 2:3 hay to grain ratio, growth increased in accordance with the increased total digestable nutrients (TDN) intake, cellulose digestion decreased, and protein digestion and nitrogen retention increased. Piboflavin appeared to be higher in both rumen juice and urine when Aureomycin was fed. Thiamine was not affected by Aureomycin in either rumen juice or urine. Although the total volatile fatty acid and acetic acid content of rumen juice was not changed by Aureomycin feeding, a shift occurred in the relative proportion of propionic and butyric acids. In the Aureomycin-fed group, the avg. butyric acid content of the rumen juice was higher and propionic acid was lower than in that of the control group. The stepwise increase in the proportion of hay to grain fed resulted in lower percent acetic and propionic acids and increased percent butyric acid. During the period 8-12 wks., blood sugar was maintained at a higher level in the Aureomycin-fed group than in the control group. Ratings of indicator microorganisms, made from wet preparations and Gram stains, showed that Aureomycin feeding practically eliminated the establishment of the Hay II group of rumen bacteria, which were well established in the control group. Increasing the amt. of grain fed in relation to hay resulted in a progressive decrease in rumen bacteria of the Hay II group, which is in agreement with previously published results. The evidence obtained suggests that the growth stimulation due to Aureomycin feeding resulted from an alteration in energy metabolism, possibly involving the microflora of the rumen.