Effects of duration of grazed lucerne on long-term yields and nitrogen uptake of subsequent wheat

Abstract

The long-term effects of varying durations of lucerne Icy, extended fallowing, and continuous wheat growing on the growth, yield, and nitrogen uptake of subsequent wheat crops were determined on two contrasting soils in northern New South Wales. Durations of lucerne ley were 3+, 2+ and 1+ years on a black earth and 5+, 3+ and 1+ years on a red-brown earth. With the exception of the first wheat crop, wheat production for several years following lucerne exceeded that following extended fallow or continuous wheat growing, whether measured as vegetative yield at anthesis, grain yield, nitrogen uptake, or grain protein. The beneficial effects of lucerne on vegetative yield, nitrogen uptake, and grain protein reached a maximum in the second crop after lucerne, and the effects of 2+ or more years of lucerne remained significant for the next five crops on the black earth and the next two crops on the red-brown earth. Grain yields fluctuated widely with season, the magnitude of the lucerne effect being much more dependent on rainfall, but the duration of the effect was similar for grain and vegetative parameters. The shorter duration of the lucerne effect on the red-brown earth appeared to be associated with its more freely draining nature and consequent loss of accumulated nitrogen. The optimum duration of lucerne for maintaining nitrogen-dependent wheat yields was 3+ years on both soil types. It eliminated the need for nitrogen fertilizer for the following five wheat crops on the black earth and three wheat crops on the red-brown earth. Extended fallowing also had a beneficial effect on all parameters, particularly in the first and second crops after the fallow ended. Its effect was generally significantly smaller than the lucerne effect except in the first crop after fallow.