Effects of excess degradable protein on postpartum reproduction and energy balance in dairy cattle.

The objectives of this study were to observe endocrine and reproductive responses of cows and heifers fed two diets (16 and 19% CP), which met undegradable protein requirements but differed in rumen degradable protein. Cows (n = 33) and heifers (n = 32) were randomly assigned within parity to diets at calving and remained on diets for 20 d after first breeding. Energy balance was determined twice weekly through the first luteal phase. Blood and milk samples were taken three times per week. Diet did not affect average daily energy balance, days to negative energy balance nadir, days to first ovulation, days to first service, or plasma glucose concentrations. First service conception rate was lower (31% vs. 48%) and plasma urea higher in animals fed the high protein diet. Days to energy balance nadir was correlated with days (r = .75) to first ovulation. Luteinizing hormone pulse frequency increased and pulse amplitude decreased in frequent samples (12-min intervals for 8 h) collected at 14 d postpartum versus sampling after the energy balance nadir. These data suggest that energy balance status plays an important role in determining the postpartum return of cyclic ovarian activity. Feeding excess CP as rumen degradable protein elevated plasma urea concentrations and decreased first service conception rate.