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

Abstract

Pastures on the north-facing slopes of the Port Hills, Canterbury, are summer-dry, grass dominant, with soils low in phosphorus and sulphur. The aim of this experiment was to assess the effect of P, S and Mo fertiliser and grass-supressing herbicide on the establishment and growth of oversown subterranean (sub) clover. Fertiliser treatments were first applied in 2021, with P and S reapplied in 2023, and herbicide applied in 2022 and 2023. Pasture yields were measured from 2021 to 2023. Total pasture yield in 2021 was not affected by fertiliser treatment and averaged 3650±83 kg DM/ha with 770±43 kg of sub clover. In 2022, pasture yield increased from 2900 to 3270 kg DM/ ha with P. The sub clover yield doubled from 310 to 645 kg DM/ha when herbicide reduced the grass from 1650 to 1410 kg DM/ha. However, in 2023, the total accumulated pasture yield (10180±160 kg DM/ha) was not affected by herbicide but sub clover content increased from 10 to 25% as grasses were reduced from 63 to 47%, with ∼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 increased it further to 4010 kg DM/ha. There was no yield response to +S or +Mo unless added with P. Applying herbicide or phosphorus 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 highest feed quality values were from treatments that had herbicide suppression of the grass and additional P and Mo fertilisers. These results suggest a pasture sub clover content of 25% on the Port Hills can be achieved using grass suppression followed by P fertiliser, if indicated by soil tests, along with molybdenum.