Nitrogen and phosphorus utilization and excretion by beef cows fed a low quality forage diet supplemented with dried distillers grains with solubles under thermal neutral and prolonged cold conditions

A digestibility and balance study was conducted to evaluate N and P utilization by cows kept under thermal neutral and prolonged cold conditions and fed poor quality forage with and without protein delivered as dried distillers grains with solubles (DDGS). The study also examined forms of N and fractions of P excreted by cows when fed these diets. Twenty-four mature, non-pregnant and non-lactating beef cows (680 ± 57 kg BW) were fed a low-quality forage diet (deficient CP, 60 g CP/kg; 0 g/kg DDGS) and a low-quality forage diet supplemented with 100 g/kg DDGS (sufficient CP, 87 g CP/kg) or 200 g/kg DDGS (excess CP, 116 g CP/kg). The study was conducted from October through February, thus allowing assessment under thermal neutral and prolonged cold conditions. Fecal N excretion (g/d), fecal ammonium N, and urine ammonium N increased linearly (P < 0.001) with DDGS supplementation. Fecal ammonium N concentration was 25% greater (P = 0.002) in cows exposed to prolonged cold compared to thermal neutral conditions. The proportion of urine urea N (% of total N) increased from 29% to 54% and urine organic N decreased from 66% to 39% with DDGS supplementation. Nitrogen retention was influenced by diet and season (P < 0.001) increasing with increasing DDGS supplementation under thermal neutral conditions with minimum change under prolonged cold exposure conditions. Fecal P excretion (g/d), urine P concentration, and urine P excretion increased linearly (P < 0.001) with DDGS supplementation. Phosphorus retention decreased with increasing DDGS supplementation in both seasons but the decline was more drastic in cold-exposed cows. Feeding DDGS increases total N and P content of the resulting manure, as well as the forms of N and fractions of P that have the potential to increase field runoff when manure is field-applied. Cows exposed to prolonged cold utilize feed N and P differently from cows in thermal neutral conditions. The N and P requirements of cows exposed to prolonged cold in the northern agricultural regions of North America require further examination to meet animal metabolic requirements without increasing environmental risk.