PROTEIN-CONTENT IN DAIRY-CATTLE DIETS AFFECTS AMMONIA LOSSES AND FERTILIZER NITROGEN VALUE

Altering feeding strategies for dairy cattle (Bos taurus) may reduce m anure N excretion and NH3 emission from the manure. We determined the effect of dietary formulation on NH3 emission immediately following ma nure excretion and the availability of manure N for plant growth. Manu re (urine and feces) was collected from dairy cattle fed diets contain ing crude protein levels of 16.4% (T1High), 15.3% (T1Med), and 12.3% ( T1Low) in trial 1, and 15.3% (T2High), 16.7% (T2Med), and 15.3% (T2Low ) in trial 2. Ammonia emission was measured in the laboratory for up t o 48 h. Emissions during the first 24 h following manure excretion wer e 38 and 23% of the total manure N from Diets T1High and T1Low, and 22 , and 15% of the total manure N from Diets T2High and T2Low, Manure NH 4+ concentration and pH mere positively related to the dietary crude p rotein level. Manure from cattle-fed diets T1Med and T1Low were applie d to soil at two rates to determine short-term N availability for thre e plantings of corn (Zea mays L.) grown for 30 d each in the greenhous e. The recovery of manure and fertilizer N in the plants and the soil for the first two plantings was 48, 31, and 103% for the T1Med, T1Low, and inorganic N treatments, respectively. Whole farm N utilization fo r diets T2High and T2Low were estimated at 18 and 23%, respectively. F eeding diets lower in crude protein increases efficiency of N on the d airy farm, decreases NH3 emissions following excretion but also decrea ses the short-term N availability of the manure.