Nutrient balances on organic pig farms

Summary
To develop outdoor pig production systems, elucidation of the environmental impact is required. Moreover, it is of importance how the nutrients are controlled and managed in the outdoor systems so a high level of utilisation is obtained. Results from private farms describe the level of nutrients, whereas model calculations identify management initiatives, which decrease the potential nutrient load. On the private farms, the level of nutrients were higher than assumed in the Danish legislation. The high level was caused by a high level of feed supplied to the sow herds. In addition, the crop rotations were not adapted at the same paste as increases in herd sizes. The model calculations indicated that changes in feed level, protein content of feed and/or stocking density reduced the nutrient load significantly. Whereas changes in number of piglets produced per sow only had limited impact. The area with pasture constituted only a minor part (10-30%) of the crop rotation, and the nutrient surplus for the entire crop rotation was comparable to other farming systems.

Maintenance of grasscovered soil was another area of focus. The level of grass cover depends on eg. estab-lishment of pasture, first use in spring, reproduction stage of animals, stocking density, supplementary feed, rooting, and season. In addition, choice of grass specie appears to be important. On-going research focuses on comparison of the suitability of different grass varieties.

We quantified spatial variation and temporal dynamics of mineral N, ammonia volatilization, denitrification and nitrate leaching from outdoor pig production at three commercial farms in Denmark. From paddocks containing farrowing sows substantial nitrogen losses were observed, particularly around the feeding areas. Gross ammonia emission rates varied between 0.08 and 1.26 kg N ha–1 per day and increased with animal density and N intake. Denitrification rates averaged 0.52 kg N ha–1 in autumn following summer grazing with lactating sows, and 0.10 kg N ha–1 per day in the following spring. Cumulated nitrate leaching during sow grazing and in the following year was 126–276 kg N ha–1. A total of 81 to 100% of the N input in feed to the paddock could be accounted for in piglets (44%), as ammonia volatilization (13%), as denitrification (8%), or as nitrate leaching (16 to 35%). The necessary development towards increasing N-use efficiency in outdoor pig production may include changes in management, such as lowering the stock density and reducing the level of dietary N. Also, a more uniform distribution of nutrients in the field may be achieved by manipulating the excretory behavior of the animals.