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The objective of these studies was to elucidate the causes underlying the reduction of fertility associated with feeding of excess ruminally degradable protein to cattle. Holstein heifers were fed total mixed rations that either met (Normal) or exceeded (High) ruminally degradable protein requirements. In Exp. 1 heifers (n = 80) were inseminated at estrus after being fed the experimental diets for 4 wk. First-service conception rates were 82 and 61% for the Normal and High groups, respectively (P < .05). Plasma urea nitrogen (PUN) levels were elevated (10.2 +/- .17 vs 14.8 +/- .19 mg/dL) in the High group throughout the experiment (P < .05). In Exp. 2, heifers (n = 32) receiving the same diets as in Exp. 1 were fitted with catheters in a jugular vein, in the bladder, and in the uterus on either the day before expected estrus or d 6 after estrus. On estrus or d 7, plasma and urinary NH4 and urea nitrogen and uterine pH were determined at 4-h intervals for 24 h beginning just before feeding. Plasma NH4 was variable and did not differ between treatments or days of the estrous cycle. During the 24-h period, urinary urea nitrogen and PUN were elevated (P < .05) in the High groups and did not differ between days of the cycle. Urinary NH4 excretion was elevated (P < .05) during the 24-h period only in the High group on d 7. Uterine pH did not change over time after feeding.(ABSTRACT TRUNCATED AT 250 WORDS)
Elrod et al. (Mon,) studied this question.