Subterranean clover growth responded to grass suppression, phosphorus and molybdenum fertilisers in a summer dry hill environment

Pastures on the north-facing slopes of the Port Hills, Canterbury, are summer-dry, grass dominant, and have low soil phosphorous and sulphur fertility. The aim of this experiment was to improve the establishment of oversown subterranean (sub) clover by the addition of fertilizer and grass-supressing herbicide. Treatments were a factorial combination of phosphorous (0 or 60 kg P/ha, -P or +P), sulphur (0 or 50 kg S/ha, -S or +S), and molybdenum (0 or 0.06 kg Mo/ha, -Mo or +Mo), with or without autumn herbicide (plus or minus, +H or -H). Fertilisers were applied in 2021, with P and S reapplied in 2023, and herbicide applied in 2022 and 2023. Pasture yields were measured annually from 2021 to 2023. Measurements in 2021 found no fertiliser effect on total pasture yield of 3650±83 kg DM/ha that included 770±43 kg of sub clover. In 2022, P addition increased total pasture yield from 2900 to 3270 kg DM/ha. Herbicide application doubled the sub clover yield from 310 to 645 kg DM/ha and reduced grass from 1650 to 1410 kg DM/ha. In 2023, the herbicide application did not affect the total accumulated pasture yield (10180±160 kg DM/ha) but increased sub clover content from 10 to 25% and reduced the grasses from 63 to 47%, with about 4% white clover. Where herbicide was applied without fertiliser (+H), total clover yield increased from 1460 to 2930 kg DM/ha. The application of P and Mo, (i.e., +H+P+Mo treatment) increased it further to 4010 kg DM/ha. There was no yield response to +S or +Mo unless added with P. Applying herbicide or phosphorous increased total metabolisable energy from 38.5 to 42.6 GJ ME/ha and crude protein from 500 to 595 kg/ha in pastures sampled in November 2023. The treatment combination providing the highest feed quality values was +H+P+Mo. These results suggest that to optimise sub clover content of pastures on the Port Hills competing grasses must be controlled initially, followed by P fertiliser if soil tests indicate it is required along with molybdenum.