Some Quantitative Aspects of Weed Competition in Vegetable Crops

rpHE tremendous losses brought about by the presence of weeds in T agricultural crops have been much underrated in the past. Recently it has been estimated, probably conservatively, that the total losses to agricultural producers in the United States approaches four billion dollars annually (18). Unless the basic causes of these losses and the places in which they occur can be determined, their significance cannot be fully evaluated. When plants are grown in competition with one another, several factors of the environment may be altered which will adversely affect the growth processes of the plants. The most important of these factors are: light intensity, soil moisture, and soil nutrient. It is extremely difficult under conditions of actual competition to determine the role which each factor plays in the injury to the crop. There is seldom only one factor involved, but more often there is a complex interaction between factors. Under normal conditions of plant growth, for instance, the light intensity may be severely reduced. It has been determined that these reductions were greater than 90 per cent under alfalfa, soybeans and rye (21), 70 to 80 per cent under an oat cover crop (4), and from 30 to 95 per cent in various locations of an apple tree (6). Weeds may also interfere with the uptake of nutrients from the soil. Mann and Barnes (8, 9, 10, 11) have shown that certain weeds when grown in competition with barley were able to take up nitrogen at the expense of the crop. Similarly, Blaser and Brady (3) found that many weeds were better able to absorb potash from the soil than forage grasses and legumes. During the summers of 1952 and 1953, experiments were conducted at the University of Wisconsin to determine quantitatively the effects of weed competition on vegetable crops. A study was made