Cool season responses of Caucasian and white clover pastures to nitrogen fertiliser and defoliation frequency

The response of Caucasian and white clover / ryegrass pastures, maintained at two fertility levels, to autumn defoliation frequency and the application of 100kg N ha⁻¹ in autumn and early winter was examined in an experiment conducted at Lincoln University. The objective of the experiment was to establish the suitability of Caucasian clover as a means to increase the clover / content of dairy pastures were frequent applications of nitrogen fertiliser are commonly applied. Although the mean autumn clover content of Caucasian clover pastures was greater than white clover pastures (13.5 vs 9.1%) neither the clover species nor soil fertility level significantly affected the response to nitrogen or defoliation frequency. The application of nitrogen fertiliser significantly (p< 0.01) increased pasture productivity in both autumn and spring, however, the nitrogen response of 7.9 kg DM / kg N in autumn and 4.7 kg DM / kg N in spring was less than the commonly expected response of 10 kg DM / kg N. The lack of response to the applied nitrogen was attributed primarily to the availability of mineralised nitrogen from clover nitrogen fixation. Chemical analysis of ryegrass foliage from the zero nitrogen plots indicated that the mean nitrogen content in the herbage (3.94%) was only marginally deficient. Winter response may have also been affected by environmental conditions. Heavy rain following nitrogen fertiliser application may have resulted in leaching losses while cool temperatures may have reduced plant uptake of applied nitrogen. Frequent defoliation (14 days) significantly (p< 0.01) reduced autumn dry matter yield compared with infrequent (28 day) defoliation. This was attributed to the reduced leaf area index of the frequently defoliated plots limiting light interception. Also measured was pasture clover content and the autumn liveweight gain of lambs grazing Caucasian or white clover pastures. Caucasian clover pastures had significantly (p< 0.001) more clover than white clover pastures at the commencement of both rotation 1 (29.1 vs 15.4%) and rotation 2 (14.1 vs 9.8%). The increased clover content resulted in the autumn liveweight gain of lambs grazing Caucasian clover (124 g hd⁻¹ d⁻¹) being significantly (p< 0.009) greater than the liveweight gain of lambs grazing white clover (111 g hd⁻¹ d⁻¹). Although this short term experiment gave inconclusive results a long term study to determine the suitability of Caucasian clover for dairy pastures is warranted. The greater legume content of Caucasian clover pastures has the potential to improve milk solids yield if it can persist in nitrogen fertilised dairy pastures.