Crude protein and rumen undegradable protein effects on reproduction and lactation performance of Holstein cows

We conducted a study to determine the effects of excess dietary crude prote in (CP) and rumen undegradable protein (RUP) on reproduction and lactation performance of Holstein cows. During each of three yearly replicates, cows were blocked by previous mature equivalent milk production and randomly ass igned at calving (n = 47; partum group) or at 42 +/- 21 d postpartum (n = 1 34; postpartum group) to the following dietary treatments: 1) ryegrass past ure supplemented with a corn and soybean meal grain mix (high CP, moderate RUP); 2) ryegrass pasture mornings and corn silage evenings, supplemented w ith grain as in diet 1 (moderate CP, moderate RUP control diet), and 3) rye grass pasture mornings and corn silage evenings, supplemented with a grain mix containing corn, soybean meal, corn gluten meal, and blood meal (modera te CP, high RUP). Dietary CP and RUP concentrations were approximately 23.1 , 5.8; 17.7, 5.0; and 17.2, 6.8% of dry matter for diets 1 to 3, respective ly. Plasma urea N concentrations were highest in cows fed diet 1 (25.0 mg/d l), intermediate in cows on diet 2 (20.1 mg/dl), and lowest in cows on diet 3 (18.5 mg/dl). Cows fed excess dietary protein (diet 1) exhibited lower f irst breeding pregnancy rates (24.1 vs. 41.0%) and lower overall pregnancy rates (53.4 vs. 75.4%) than did cows fed diet 2, increasing time nonpregnan t by an average of 15.1 d per cow. Reproductive performance was similar bet ween cows fed diets 2 and 3. Mean fat-corrected milk (FCM) yield was not af fected by protein concentration (diet 1 vs. 2); however, partum group cows that received supplemental RUP (diet 3) produced more 3.5% FCM than control s in early lactation. Feeding grain diets that contained excess dietary pro tein impaired the reproductive performance of dairy cows grazing ryegrass.