Phosphorus feeding and manure nutrient recycling on Wisconsin dairy farms

Recently approved nutrient management regulations for livestock operations focus on a farm's ability to recycle the phosphorus ( P) contained in manure. Most efforts to improve dairy manure management emphasize manure handling, storage, and land application techniques. Little is known about relationships between dairy feeding practices and manure P levels under farm conditions, or between herd size, cropland area and a farm's ability to recycle manure P through crops. A survey of 98 representative dairy farms in Wisconsin showed that most farms were self-sufficient in forage (alfalfa, corn silage) and grain production. Lactating dairy cows derived 90% of their feed dry matter (DM) and 78% of their P intake from these homegrown feeds. The P content (DM basis) of the dairy diet ranged from 2.3 to 8.5 with an average of 4.0 g P kg(-1). Approximately 85% of the surveyed dairy farms fed P in excess of the recently updated National Research Council (NRC) requirements. On these farms, amounts of P in manure were related to dietary P. Of the annual manure P excreted by cows fed a diet supplement, approximately two-thirds is derived from homegrown feeds and one-third from imported mineral and protein supplements. Stocking rates ranged from 0.19 to 1.68 AU ha(-1). Farms having stocking rates of less than 0.70 AU ha are self-sufficient in feed production. Approximately half of the farms are self-sufficient in feed production, 68% produce 90%, and 80% produce 80% of their annual feed requirement. Approximately 40% of the farms have a positive P balance ( manure P exceeds harvested crop P). On these farms, lowering dietary P to the levels recommended by NRC would reduce the number of farms having a positive P balance by 67%, and the land area in positive P balance by 60%. For farms having a high animal stocking rate, manure export, the addition of cropland for manure spreading, and/or reductions in livestock (cow and/or heifer) numbers may be the only feasible strategies for achieving P balance on a farm.