The Effects of Density and the Timing of Removal on Interference Between Barley, white Mustard and Wild Oats

Abstract

An experiment was designed to determine the effects of various densities of barley, white mustard and wild oat on the growth of each other in mixed cultures. The time for which a species was allowed to grow in the presence of another was varied in order to assess the period in growth at which interference occurred. The experiment was designed in such a way that it might have some relevance to the problem of crop growth in the presence of a mixture of weeds. Most experiments on interference between a weed and a crop have concerned the effects of removing a single weed species from a crop-weed mixture. A common situation in agricultural practice is that a crop grows in a background of a mixed weed population and the question arises whether removal of one of these weeds will result in improved crop growth, or whether the second weed then increases its activity and maintains the depression in crop yield. There is some evidence that the density of a weed population is, within limits, relatively unimportant in its effects on a crop. Shadbolt and Holm (1956), for example, found that the variations in the density of a weed population had relatively little effect on the yields of carrots and onions. Naylor (1972), found that the depression of yield in winter wheat was linearly related to the log of the density of Alopercurus myosuroides Huds.-each ten-fold increase in weed density reduced crop yield by c. 25 0. It is certainly possible to envisage situations in which intraspecific interference between plants in a mixed population is much stronger than interspecific effects-under such circumstances variation in the density of one species (e.g. the weed) might be expected to be reflected in the growth of that species rather than in its associates (e.g. the crop). It is of considerable importance in weed-control practice to know the time at which a weed population begins to affect the growth of a crop. How late in the growth of a weed-crop mixture can weed control be delayed before the weed has produced an irreversible depression of yield? What degree of crop recovery is possible if a weed population is removed after it has started to reduce the growth of the crop? Hammerton (1967) showed that a weed infestation may depress the yield of kale by up to 89 % if the weeds remained with the crop to maturity but only by about 12 % if the weeds were removed when the crop was at the four-leaf stage. Two recent reviews (Nieto, Brondo & Gonzalez (1968) and Kasasian & Seeyave (1969)) show that the yield of many crops is only slightly affected by weed populations that establish more than 30 days after the crop. Provided that the crop is weed-free during the period 10-30 days after sowing, subsequent weed development is quite unimportant in maize, beans, tomatoes, sweet potatoes and pigeon peas. It has been argued from these