Fallowing and wheat production in southern NSW

Abstract

The response of wheat to the length of fallow, to the number of cultivations, and to pre-planting chemical control of weeds on high fertility soils was determined under winter rainfall conditions at Wagga Wagga, New South Wales, over four years commencing 1960-61. Length of fallow had little influence on the conservation of rainfall except in 1961-62, when approximately twice the average summer rainfall added 1.8 inches of soil moisture per acre 48 inches. Long fallow increased available nitrogen accumulation, but this did not increase yield over either the mechanically prepared shorter fallows, or pre-planting chemical weed control. This was due to losses during the autumn-winter period of some of the excess mineral nitrogen that accrued during the long fallow. High mineral nitrogen concentrations also occurred during the summer on weed-free, uncultivated soils. Grain yield after a single autumn cultivation was as high as after a long fallow except in 1961-62. In this year the long fallow (September to May) significantly increased yields over all other treatments. The absence of any positive yield response to the application of 60 to 80 lb nitrogen an acre to short fallows suggests that mineral nitrogen concentrations were generally adequate for grain production. The addition of nitrogen to long fallows often depressed yields. The dependence of grain yield on adequate weed control is illustrated by a highly significant negative correlation (r = -0.849 ; P<0.001) of grain yield with weed growth. It is concluded that in the Wagga Wagga environment weed control is more important than moisture conservation and mineral nitrogen accumulation through fallowing.