Effects of cattle slurry on the growth potential and clover proportion of organically managed grass-clover leys

The growth potential of grass-clover leys as influenced by cattle slurry application and clover content was studied in field experiments during three growing seasons at four locations. A grassland growth model was used to quantify growth potential and to separate the influence of weather variables from factors related to the plant stand. Application of cattle slurry increased the initial relative growth rate (R-s), on average from 0.21 to 0.23 g g(-1) day(-1) of the spring growth. In regrowth the effect of cattle slurry was less pronounced. The effect of cattle slurry declined with increasing R-s of the unfertilized treatment and rose with increasing rate of ammonium N applied by the slurry. The positive response to cattle slurry application on R-s was unaffected by the clover proportion in the spring growth, while in regrowth it tended to diminish with increasing clover proportion. A rising clover proportion enhanced the growth potential of the ley in both growth periods. In spring growth, highest R-s was found at an average clover content of approximately 55%, while in regrowth R-s increased linearly with the clover content. On average, slurry application reduced the clover proportion of dry matter yield by 7%-units in spring growth and 13%-units in regrowth.