Effect of herbage on N intake and N excretion of suckler cows

Abstract

The release of nitrogen on pastures by the grazing animal is a significant source of N load to soil and water and hence a possible source of nitrate pollution. Here we studied the effect of forage quantity and quality related to N intake to examine excretion and loss. A two-year grazing experiment on an intensively managed Lolio-Cynosuretum was established in Rengen Research Station, Germany. Two groups of 4 suckler cows each were grazed along a gradient of forage mass and quality consisting of 16 plots each in 4 replicates per year. Our results show that in the final plots of the gradient, forage mass exceeded by far the recommended optimum of herbage allowance. At the same time, herbage quality decreased along that gradient from about 11 to 9.5 MJ metabolisable energy per kg dry matter, but daily N intake increased from 140 to 600 g N per animal. The related N excretion per animal was constant, with faeces at about 60 to 75 g N. In contrast, daily N excretion with urine increased linearly depending on N intake from less than 100 g to more than 500 g per animal. Thus, N loss was estimated to increase in the same way and was determined as about 22% of total N intake. We conclude that low quality in tall swards, including partly senesced plant material, does not imply low herbage and N intake and N excretion. Our findings indicate that in grassland extensification schemes, where late turning out to pasture is common, N intake and hence excretion might disagree with the intention of limiting forage N conversion and subsequent release to the environment. We also provide a simple spreadsheet with which farmers can quantify the N excretion as a factor of herbage allowance, faeces N content, daily N retention and stocking density. This appears useful not only under standard conditions on improved grassland, but also in environmentally sensitive areas where N release is required to be kept under strict control.