Dry matter production and botanical composition of multi-species pasture mixtures during the second year after establishment

Abstract

A pasture mixture experiment was conducted, growing four species; perennial ryegrass, plantain, white clover and red clover under irrigation in Canterbury, New Zealand. The experiment set out to identifty an optimal pasture seed mixture that maximised annual dry matter (DM) yield by quantifying species identity and diversity effects from pasture mixtures. The plots were sown containing one- to four-species, based on a simplex mixture design containing all possible combinations. The annual and seasonal DM production were statistically analysed with a special cubic model created from the simplex mixture design, enabling predictions to be made on the mixture that would produce maximum yield for the second year of growth (May 2016 to May 2017). The DM production and botanical composition of each monoculture and mixture were measured, compared and analysed. To explain differences in yield between mixtures, N content, leaf area index and light interception were also measured. The modelled analysis showed that five two-species mixtures and one three-species mixture showed significant species interactions and subsequent yield increases over the mean of their component monocultures. The seed mix that was predicted to maximise yield consisted of 0.29 ryegrass, 0.20 plantain and 0.51 red clover in terms of seeds/unit area, predicted to yield 16.44 t DM/ha for the second year of growth. This mix produced 5.19 t DM/ha more than the rygrass and white clover (RG*WC) mixture, which yielded 11.25 t DM/ha. The greatest diversity effects were shown by the two species mixtures that included red clover, and produced 4.93 t DM/ha (RG*RC) and 5.31 t DM/ha (P*RC) above the mean yield of the monocultures. Species interactions and botanical compositions within the mixtures were not consisitent over time; the optimal mixture (ryegrass, plantain and red clover) displayed transgressive overyielding over six of the eight sampling dates. There were no significant species interactions across any of the mixtures at the early spring harvest (August 2016) and the late autumn harvest (May 2017). The botanical composition of the highest yielding mixture RG*P*RC vaired in botanical composition over the season. It deviated from its sown proportions of 0.33 RG -0.33 P - 0.33 RC to 0.24-0.54-0.22 of RG, P, RC respectively at the time of first harvest (2/08/2016). By the second harvest (22/09/2016) the botanical composition of the mixture had changed to 0.10-0.26-0.64; halving the proportion of ryegrass and plantain and tripling the proportion of red clover. There were significant differences in N content between the four monocultures, ranging from 2.30% (ryegrass) to 3.75% (red clover). The mixture N% was lower when white clover was included in the mixture (2.71% for RG*WC, 2.82% for P*WC) than when red clover was included (3.22% for RG*RC, 3.49% for P*RC). The N content of the mixtures can be used to explain yield as the average N concentration of the mixtures increased by 1%, the annual yield increased by 6.60 t DM/ha. Leaf area index and light interception analysis within the seventh and eighth harvests showed that significantly higher yielding mixtures such as RG*RC, P*RC and RG*P*RC reached critical light interceptance and had a significantly higher leaf area index than their component monocultures and mixtures that produced lower yields, such as RG*P, P*WC and WC*RC