No abstract

Chemical composition of the empty body was determined in 159 animals slaughtered at weights ranging from 121 to 706 kilograms. Holstein and Angus bulls, steers and heifers were fed at two energy levels: ad libitum and 65 to 70% ad libitum. The allometric equation, Y = aXb, was used to determine the effect of energy intake and the influence of breed and sex on the accretion rates of the chemical components relative to the growth of the empty body or fat-free empty body. Group comparisons for chemical composition were made after adjustment by regression to a common empty body weight. The expression of the sex influence on the accretion rates of water, protein and ash relative to the empty body depended on the breed and the energy intake level considered. The accretion rate of chemical fat was not influenced by sex. Genetic differences in the accretion rate relative to the empty body were detected only among animals in the high energy intake group. Regardless of sex, the accretion rates of protein and ash were more rapid (P < .05) in Holsteins than in Angus. However, a breed influence on the accretion rate of chemical fat was detected only among bulls, where Angus had a more rapid accretion rate. Feeding animals at two energy levels resulted in different accretion rates relative to the empty body. In the Angus breed, regardless of sex, the accretion rates of water, protein and ash were more rapid (P < .05) in the low intake group, whereas the accretion rate of chemical fat was slower (P < .05). Among Holsteins, the low energy intake level had a less definite effect; for bulls, the accretion rates of water and chemical fat were more rapid (P < .05) and slower (P < .05), respectively; for steers, and accretion rate of protein was more rapid (P < .05), and for heifers, none of the accretion rates was altered.

In a series of experiments we investigated effects of several factors on intake of urea by lactating dairy cows. Cows given an unfamiliar ingredient or altered moisture in the ration reduced their intake, and this effect was attributed to a newness factor. Addition of urea to a ration may have a similar effect. An experimental design providing both no-choice and two-choice data was devised to minimize the effect of newness. When urea was isolated physically by pellets coated with ground corn, cows selected against urea-containing pellets on a two-choice basis and reduced intake on a no-choice basis. When the choice was between two urea-containing rations, cows preferred the pellets in which urea flavor and odor should have been most evident. Urea odor in the feedbox or urea in the drinking water did not reduce feed intake. Eating rate of a 2.5% urea-containing ration over two daily 30-min feeding periods was about one-half that of a non-urea ration. This effect was evident during the first 5-min eating interval. Administration of urea solution into the rumen prior to feeding a nonurea ration markedly reduced subsequent intake. Reticulum pH and ammonia indicated possible sublethal ammonia toxicity. Intake and eating rate were depressed and reticulum pH was elevated when cows were fed a ration with 2.5% urea versus 1% urea or nonurea rations. Elevated reticulum ammonia occurred on both 1 and 2.5% urea-containing rations. Cows not previously exposed to urea-containing rations reduced intake and eating rate when fed rations with 2.0 and 2.5% urea. Intake reduction was not comparable to that by cows preconditioned to urea rations. Upon third exposure to 2.5% urea in the ration, cows reduced and ceased intake but readily consumed a nonurea ration. Cows require preconditioning to develop a negative aversion to rations containing high urea, perhaps through a mechanism of sublethal ammonia toxicity.

No abstract

Milking rations containing 16 (control), 13.2, and 10.4% protein were fed to four midlactation, rumen-fistulated Holstein cows. Ammonium chloride was infused ruminally for 5 consecutive days after morning feeding when cows were fed milking rations containing 13.2 and 10.4% protein. Amount infused was equivalent to the ammonia in 1 or 2% dietary urea. Rumen and blood samples were taken prior to and following morning feedings. Intake of milking ration was the same across treatments. Initial rumen pH was higher for ammonium chloride treatments. It then declined, as did the control, to the same, lowest pH at 1.5 to 3 h postfeeding. Rumen ammonia increased rapidly for cows receiving both ammonium chloride treatments to .5 h postfeeding and then declined rapidly. Blood urea nitrogen was highest for cows fed the control ration, peaked .5 h postfeeding for cows infused with the low ammonium chloride, then dropped and peaked again 6 h postfeeding. Blood ammonia was highest among treatments for control cows and differed by sampling time only for control cows with a peak .5 h postfeeding. Because lower rumen pH traps ammonia preventing rapid absorption into blood, interpretation of high rumen ammonia must consider effect of rumen pH.