The effects of management for aphid control on lucerne production and persistence : a dissertation submitted in partial fulfilment of Bachelor of Agricultural Science Honours in the University of Canterbury [Lincoln College]

Abstract

In the second year of a two year grazing management trial on Wairau Lucerne, plots were grazed under a 3 or 6 weekly rotation. Half of each plot was nard grazed in June to control over wintering aphids. In the spring/autumn an aphicide treatment was applied. Plant roots excavated in the autumn, were used for determination of plant populations, root dry weights and analysis for non-structural carbohydrate root reserves.

Cool season grazing management reduced spring aphid populations from 37 to 3/stem and showed no significant difference in dry matter yield. Three weekly spelling, although reducing aphid numbers, halved plant populations, root weights, and markedly reduced dry matter yield. Associated with this decline, was the dramatic increase in weed populations, notably Scotch thistle. Weeds contributed 60% to the total yield of 10,400 kg/ha under 3 weekly spelling. However, the shading effect of the lucerne canopy and harsh conditions following grazing, reduced weed populations and prevented thistle establishment throughout the growing season, under 6 weekly spelling, resulting in only 23% of 18,800 kg/ha total yield being weeds.

Aphid control by spraying throughout the growing season increased lucerne yield, but not significantly. Spraying of lucerne in the autumn reduced aphid populations from 21/stem to 1/stem under 6 weekly spelling and resulted in high plant root reserves, with no significant effect on top yield. Only under spraying, was there a significant difference in root reserves between 3 and 6 weekly spelling.

The results show that cheap methods of controlling aphids in lucerne can be developed using grazing management.