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 4.0g 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−1 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.