Comparison of biomass production and biological nitrogen fixation of multi-species pastures (mixed herb leys) with perennial ryegrass-white clover pasture with and without irrigation in Canterbury, New Zealand

Legume-based pastures in Australasia are predominantly perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.). These pastures have disadvantages such as limitations in the production and persistence of white clover and ryegrass in drought prone regions. With the increasing interest in organic agriculture, more complex pasture mixtures are seen as an alternative to the standard ryegrass-white clover (RWC) pasture. Although anecdotal evidence exists to support the benefits of alternative pastures, there is insufficient scientific evidence to support or refute these claims. The present study was conducted to compare three different alternative multi-species (mixed herb leys) (MSP) pastures with the standard RWC pasture with and without irrigation, in terms of biomass and biological nitrogen fixation (BNF) over a period of 1 year under field conditions in Canterbury, New Zealand. Isotopic dilution technique involving field 15N-microplots were used to measure BNF. Irrigation doubled dry matter yields (DMY) of all pastures compared with those under dryland. There was no significant difference between DMY of all pastures under dryland. However, under irrigation, MSP pastures out-yield RWC pasture in total and legume DMY. Red clover MSP produced significantly lower DMY than the comprehensive or lucerne MSP treatment, probably due to the ability of lucerne in extracting water from deeper soil depth. Seasonal changes in plant composition varied according to the species sown, soil moisture status and temperature. Total and seasonal amounts of N2 fixed followed the same trend as the DMY and were related to the DMY of legumes. On average, about 26-34 kg N was fixed per tonne of legume DMY, showing a uniformity across all pastures. Overall, based on the present results of 1-year study, the MSP pastures tend to result in higher DMY, BNF and legume growth compared with that of the RWC pasture, provided irrigation is available. With increased BNF, these irrigated MSP pastures may provide high quality feed to grazing animals.