Effects of cutting frequency on plant production, N-uptake and N2 fixation in harvested and below-harvest plant biomass of grass clover

Summary

Nitrogen (N) accumulating in stubble, stolons and roots is an important component in N balances in perennial ryegrass-white clover swards, and the effects of cutting frequency on the biomass of above- and below-harvest height were studied during two consecutive years. Total dry matter (DM) and total N production, and N2 fixation, were measured at two cutting frequencies imposed in the summers of two years either by cutting infrequently at monthly intervals to simulate mowing or by frequent cutting at weekly intervals to simulate grazing. Total DM production harvested was in the range 3000 - 7000 kg DM ha-1 with lower DM production associated with the frequent cutting treatment, and it was significantly affected by the different weather conditions in the two years. The higher cutting frequency also reduced the biomass below harvest height, but the different weather conditions between years had less effect on stubble and, in particular, root biomass. The biomass of white clover roots was significantly lower than that of perennial ryegrass roots, and remained at a relatively constant level (200-500 kg DM ha-1) throughout the experiment, whereas the biomass of perennial ryegrass roots increased from 2 400 kg DM ha-1 in the year of establishment to 10 200 kg DM ha-1 in the infrequent cutting treatment and 6 650 kg DM ha-1 in the frequent cutting treatment by the end of the experiment, giving shoot: root ratios of 4.7 – 16.6 and 0.5 – 1.6 for white clover and perennial ryegrass, respectively. Annual N2 fixation was in the range 28 – 214 kg N ha-1, and the proportion of N fixed in stolons and roots was on average 0.28. However, since the weather conditions affect the harvested DM production and the shoot: root ratio, care must be taken when estimating total N2 fixation based on an assumed or fixed shoot: root ratio