Combined free-range piglets and energy crop production: Impact on nitrate leaching

Summary

Keeping sows in the outdoor pasture conveys high risk on environmental pollution through nitrate leaching which might be tremendously high in autumn and winter. Integrating grassland based free-range pigs with selected energy crops has been proposed as one possible approach to reduce nutrient leaching; however the effect has never been quantified. The ecoefficient pig farming that takes care of pig’s animal welfare and possible reduction of nitrate loss into environment may be achieved which will contribute to the Danish Green Growth Agreement of reducing up to 19,000 tons of NO3-N between 2010 and 2015.
Apart from the main focus on NO3-N leaching potential, this experiment also investigated Nmin and phosphorus distributions at various soil depths and distances from willow plus estimating farm N balance from a commercial organic pig farm. In the experimental paddocks, two willow rows each from one side of the paddock are separated by 18.5 m distance covered by grass. In each of the four measurement rows established, the ceramic suction cups were installed at 1.45 m depth at 0.5, 2.5, 4.5, 6.5 and 9.5 m from willow while the two soil samples were taken at each cup at three soil layers, 0-25 cm, 25-50 cm and 50-100 cm. Through a 1 m soil column, the factorial ANOVA found significant highest (p<0.001) Nmin close to sow’s huts with 149 Kg N/ha followed by 101 and 100 Kg N/ha adjacent to feeders. The Nmin content however differed with distance with NH4-N of up to 90% near feeders while up to 84% of Nmin was NO3-N close the huts. The phosphorus near feeders had the highest level with 53 Kg P/ha when considering 1 m soil depth. In both Nmin and P, the lowest levels were found closest to the willow (0.5 m) with 41 Kg N/ha and 39 Kg P/ha through 1 m soil depth. In addition, the N balance estimation of the farm where the total input was 404 Kg N/ha, about 104 Kg N/ha (26%) was converted into piglets while 39 Kg N/ha (10%) was estimated for willow N uptake.
Nitrate-N leaching as expected was the highest near the huts with an average of 37 mg NO3- N/litre followed by 28 Mg NO3-N/litre at 6.5 m. Of all the distance points, the leaching at only 4.5 and 6.5 were significantly to 0.5 m (p<0.01). Since excretory behaviour of pigs was not part of the experiment, the lower NO3-N leaching closest to willow could be due to both lower excretion and also high water and nutrients uptake by trees. However, even though the NO3-N from soil samples at 2.5 m was the second highest, the leaching wasn’t as high as expected, which was attributed to high uptake by the trees whose significant uptake by roots may extend as far as 3.5 m away from willow. The 9.5 m close to feeders had the low leaching which could be due to low NO3-N as NH4-N dominated with about 90 % of the total Nmin with 79% of this being in top soil. With their long growing season and deep root system, willow could substantially reduce N loss through nitrate leaching.
The results from this study suggest that, paddocks should be designed so as to maximize the potential nutrients uptake by willow as pigs are known to have high excretion activities near shelter zones such as trees. In addition to that, analysis of pig excretory behaviour will enable the findings to establish a clear-cut relationship of whether lower nitrate leaching close to willow is due to low excretion or high nutrients demand by trees. The hotspot areas on the other hand as observed in our experiment could still be carrying high N loss potential and therefore in addition to perennial crops, frequent reallocation of feeders & huts, rotation of pigs into new paddocks and regulate stocking density could be done together. These measures could both improve grass cover and reduce nutrient losses into the environment.