Grazing reduces biomass and N2 fixation in grass-clover
By Finn P. Vinther, Danish Institute of Agricultural Sciences, Department of Agroecology
Significant amounts of plant biomass and fixed N are incorporated into soil as roots, nodules and root exudates during the growth of grass-clover, which is exposed to soil microbial degradation when field is ploughed. This has major implications for our understanding of the potential contributions of legumes in the N economics and losses in grass-clover cropping systems.
In this study, which was a part of the DARCOF-project DINOG, we have studied the effects of grass-clover pasture age and cutting frequency (simulated grazing) on the development of the above ground as well as the below ground biomass, with emphasis on below-harvest contribution to the total N2 fixation.
Methodology
N2 fixation in grass-clover mixtures was determined using the 15N dilution method with accompanying ryegrass as reference plant. Aboveground biomass (AGB) under mowing conditions was determined from cuttings at 3-4 weeks intervals, whereas the AGB under simulated grazing conditions ('grazing') was determined from cuttings at 1-2 weeks intervals. Additionally, the effect of cutting frequency on clover growth was studied in a greenhouse experiment with red and white clover. Belowground biomass (BGB) was determined by inserting a steel cylinder (diameter 20 cm and height 20 cm) into the soil. Afterwards, the soil was carefully washed off and the plant material separated into grass stubble, clover stubble including stolons, grass roots and clover roots.
Effects of age on biomass production and N2 fixation
During the growing season of 2001 the total dry matter production was 5.9, 7.1 and 5.6 t/ha in the 1st, 2nd and 8th year grass-clover, respectively, with a significantly (P<0.05) higher production in the 2nd than in the 1st and 8th year swards. The proportion of clover showed a typical temperature dependent seasonal variation with values between 5 and 15% in the spring, increasing to 40 to 60% in mid-summer followed by a decrease to about 10% in the late autumn. As average over the entire period, the proportion of clover was significantly (P<0.05) lower in the 8th year grass-clover than in the 1st and 2nd year.
The proportion of N derived from the atmosphere (pNdfa) also showed a significant seasonal variation, with values ranging from 0.79 to 0.86 in the spring, from 0.93 to 0.96 in mid-summer and 0.74 to 0.77 in late autumn. This resulted in a N2 fixation in the harvested clover biomass of 59, 79 and 49 kg N/ha in the 1st, 2nd and 8th year grass-clover, respectively.
In total, the amounts of nitrogen harvested during the growing season of 2001 were 183, 231 and 181 kg N per ha in the 1st, 2nd and 8th year grass-clover pastures, respectively. This nitrogen was removed from the field in the experimental plots, but would under grazing conditions - to a great extend - be returned to the pastures via urine and excreta. In addition to the biomass and nitrogen removed from the plots during the growing season, a considerable amount was present in roots and stubble at the end of the growing season, with a major part present in the grass roots. The amount of N in clover stubble and roots and in grass stubble did not seem to be affected by pasture age, whereas the content in grass roots increased considerably with age.
Cutting reduces biomass production and N2 fixation
Effects of cutting frequency on biomass production was studied in a greenhouse experiment with red and white clover plants grown in pots filled with sand and supplied with nutrient solution. In the 'uncut' treatment plants were grown for 2 months before harvested, whereas plants in the 'cut' treatment were cut to about 2 cm five times during the 2-month growing period. Cutting significantly reduced the AGB as well as the BGB in both red and white clover.
Effects of cutting frequency on biomass production and N2 fixation were also studied in a field experiment with a 2nd year grass-white clover crop. A normal mowing strategy (mowing) with 3 cuts was compared with a simulated grazing strategy ('grazing'), where cuts were made with 1-2 weeks intervals. The dry matter production was in 2003 significantly lower than what is generally obtained in a 2nd year grass-clover, which was the case for both grass and clover dry matter. As for the greenhouse experiment, the total production tended to be lower after frequent cuttings (grazing) than after only 3 cuts (mowing). However, the proportion of clover was significantly higher in the ‘grazing’ treatment than under mowing conditions.
As for the harvested biomass, the below harvest biomass, left in the field at the end of season, was affected by cutting frequency. Both the amount of stubble/stolons and roots was significantly higher after mowing than after 'grazing', resulting in a below-harvest biomass being 4 and 2.5 times higher than the harvested biomass, respectively. This means that the total amount of nitrogen left in the field at the end of the growing season was about 150 N per ha in the mowed and 100 kg N per ha in the 'grazed' treatment.
Conclusions
A summary of results is presented, which relates to the below-harvest plant biomass, i.e., stubble/stolons and roots, which is left in the field at the end of the growing season and eventually will be incorporated into the soil after ploughing. Two major conclusions were that:
- the total below-harvest plant biomass and amount of nitrogen increases with increasing pasture age. The grass components contribute more to the increase than the clover
- frequent cutting (grazing) reduces biomass production in the harvested as well as the below-harvested components.
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