Efficient use of grassland nitrogen
By Jørgen Eriksen, Danish Institute of Agricultural Sciences, Department of Agroecology
In nitrate sensitive areas with agricultural production the recommended upper limit of nitrate in percolation water is 50 mg per litre. Organic farming has a lower nitrate leaching potential compared to conventional farming, but may occasionally exceed this limit, especially in the case of dairy crop rotations. In such rotations a considerable build-up of N takes place in grazed grassland. Therefore, organic dairy farming may only present an opportunity to reduce nitrate leaching if this grassland N is efficiently utilised. Losses may occur during grazing, and ploughing of grassland is followed by a large increase in the mineralization of N that often exceeds the need of the subsequent arable crop.
An experimental site at Research Centre Foulum has since 1997 been dedicated to studies on the utilization of grassland nitrogen in organic dairy systems. Different aspects have been studied and new investigations are in progress. Some of the main themes are described here in relation mainly to the effects observed on residual fertilizer effects and leaching losses.
Grassland composition and use
From the work it was found that grassland history (grass-clover or ryegrass, low or high N-level in feed) only moderately affected the precrop effect and the leaching losses. In contrast management greatly influenced both aspects. When using good management practices (spring ploughing, catch crops and reduced fertiliser application) the release of N from 3-year-old grasslands gave a considerable residual effect for 2 years after ploughing (Figure 1) without nitrate concentrations in leachates exceeding the upper limit of 50 mg per litre (Eriksen 2001).
Pasture age and composition
This study showed that nitrate leaching from grazed unfertilised grass-clover was always considerably lower than from grazed fertilised ryegrass. The effect of grassland age on nitrate leaching was insignificant in grass-clover, but clear in grazed ryegrass, where levels increased dramatically with sward age (Figure 2). The higher leaching losses from fertilised grass than from unfertilised grass-clover systems were caused by 1) a reduction in N2-fixation in grass-clover over time, and 2) a reduction in dry matter production in grass-clover over time lowering the grazing intensity and the recycling of grassland N via animal excreta (Eriksen et al. 2004). Following ploughing of the different grasslands we investigated the effect of these differences on the yield and quality of spring wheat. Without N-application, and with grass-clover residual effect as the sole source of N, the grain had 12.0% protein and 23.5% gluten in the flour, which are suitable values for good bread-making properties (Pedersen et al. 2003).
Frequency of grass in the crop rotation
If soil organic matter (including organic N) is build up during the pasture phase and mineralised following ploughing, what is the importance of, e.g., 25 or 75% grass-clover in the rotation for soil organic matter mineralization and leaching losses? One hypothesis is that the most grass-intensive systems may release unmanageable large amounts of N following ploughing leading to increased nitrate leaching compared to less grass-intensive systems. In this recently started project we will investigate the precrop effect and nitrate leaching in crop rotations similar during the last three years, but with widely different grassland frequency in the rotation (25, 38 and 75%). We expect that this provide information on the long-term effect of grass frequency in the crop rotation. These studies at the experimental site in Foulum are currently being supplemented by investigations on a private farm.
Eriksen J. (2001) Nitrate leaching and growth of cereal crops following cultivation of contrasting temporary grasslands. Journal of Agricultural Science 136: 271-281.
Eriksen J., Vinther F.P. & Søegaard K. (2004) Nitrate leaching and N2-fixation in grasslands of different composition, age and management. Journal of Agricultural Science. In press.
Pedersen L., Eriksen J. & Thomsen I.K. (2003) Baking quality of wheat from different cropping systems. Newsletter from Danish Research Centre for Organic Farming. Dec. 2003 no. 4. (http://www.darcof.dk/enews/dec03/bake.html)