COMPETITION AND CROP LOSS DUE TO VELVETLEAF William C. Akey Section of Biology, University of California Davis, CA 95616

Velvetleaf ( theophrasti Medik.) is a major weed of , cotton, and sorghum in the eastern United States and hundreds of millions of dollars are spent each year to control it. According to a 1985 survey, velvetleaf was considered to be the major weed of soybean in 9 out of 14 north central states.

Velvetleaf, like other weed species, interferes with crop growth through competition for the growth factors water, nutrients and light. Changes in crop morphology and yield losses associated with velvetleaf competition have been well documented for a few crops. However, the mechanism(s) by which velvetleaf reduces crop yields have received less attention. Studies specifically designed to test the ability of velvetleaf to compete for nutrients have not been reported to date. Velvetleaf shows an opportunistic response to nutrient availability, being able to use nutrients supplied at different times during the growing season for reproduction. Whether competition for nutrients would occur between velvetleaf and a particular crop might then partly depend on how well the crop is able to capture and utilize nutrients which become available during the growing season. Soybean, for example, may not be as opportunistic as velvetleaf in acquiring nutrients. Foliar fertilization of soybean during early pod filling does not produce consistent or dependable yield increases. From tissue analyses of leaves of velvetleaf and soybean grown together, Dekker and Meggitt (1983) concluded that competition for soil nutrients was not involved in observed reductions in growth of . Velvetleaf is able to compensate for low nutrient availability, such as might occur in poor soils or when competing with crops, by increasing root production. Velvetleaf growing in unfertilized soil produced twice as many roots as those growing in fertilized soil (Stafford, 1989). In situations where availability of nutrients is insufficient to meet the demands of both crop and weeds, velvetleaf should be a good competitor for nutrients. With normal rainfall or under well-watered conditions, competition for water does not appear to play a role in interference of velvetleaf with crop growth, based on the few field studies that have been conducted. However, in a dry year, velvetleaf competition for limited soil water may reduce availability of water to the crop and increase crop water stress. For example, under dry conditions photosynthesis and transpiration of both velvetleaf and soybean were reduced when they were grown together compared to when each species was grown alone (Munger et al., 1987). Several researchers have examined water relations and gas exchange parameters of velvetleaf and soybean under controlled environment conditions. However, there are no consistent patterns in the reported results and the relevance of these studies to field situations is not established. Velvetleaf was more affected by water stress than cotton under growth room conditions, but water stress did not affect the relative competitive ability of velvetleaf or its impact on cotton growth (Patterson and Highsmith, 1989). Competition for water between velvetleaf and crops may be affected by their competition for light (Salisbury and Chandler, 1993.).

207 With adequate water, velvetleaf normally uses about twice as much water as cotton. Under dry conditions, water use by velvetleaf and cotton are both reduced by half. Velvetleaf reduces water stress by dropping its lower leaves and by leaf wilting, thus reducing its leaf area and making it less able to compete for light. Under well watered conditions, if cotton is shaded by velvetleaf-the usual situation in crop fields when velvetleaf emerges with the crop-water use by cotton is decreased by half while water use by velvetleaf remains high. However, under dry conditions, shading by velvetleaf does not significantly change water use by cotton. This implies that cotton may be a better competitor for water than velvetleaf under dry conditions.

In modern cropping systems where adequate nutrients and water are supplied, light may often be the only resource that becomes limiting to crop growth. Much of velvetleafs interference with crop growth appears to be due to competition for light. Velvetleaf possesses a number of attributes that lend it a strong potential to compete for light. A rapid early growth rate, high leaf area expansion rate and strong investment of resources into stem growth rapidly produce a tall plant with a large leaf area available to intercept light. These features combined with a leaf senescence and branching pattern that concentrates leaf area near the top of the plant above the crop canopy, enhance the ability of velvetleaf to compete for light. Individual velvetleaf plants have an area of influence of about 0.4 m. Therefore, 1 to 2 velvetleaf plants/m2 would be sufficient to form a full canopy above shorter crop plants that would intercept 44 to 56% of the available light. Another factor which may influence the ability of velvetleaf to compete for light is the phenomenon of solar-tracking. Solar-tracking involves the active movement of leaf blades during the day to maximize light interception. Solar-tracking behavior in velvetleaf increases the amount of light the plants have available and the amount of carbon fixed through photosynthesis, especially early and late in the day. Solar-tracking also improves the water use efficiency of velvetleaf and increases the shading of shorter crop species.

In addition to competing with crops for growth factors, velvetleaf may also interfere with crop growth through allelopathy. Allelopathy involves the production of chemical compounds by one plant that directly or indirectly inhibit the growth of other plants. Velvetleaf seeds produce phenolic compounds that have been shown to inhibit seed germination and/or seedling growth in several species including radish, cress, cabbage, tomato and soybean. These same compounds are also responsible for reduced microbial activity in the vicinity of dormant velvetleaf seeds and this may contribute to their longevity in the soil. Thus, allelopathic chemicals produced by velvetleaf seeds may be advantageous to velvetleaf both by reducing germination and growth of competing plants and by inhibiting seed decomposers. Leaf extracts of velvetleaf containing phenolics, inhibited soybean seedling growth by interfering with water relations, chlorophyll production and root nodulation. Exudates of glandular hairs of velvetleaf, which also contain phenolics, do not appear to have allelopathic activity under field conditions. Incorporation of velvetleaf plants prior to planting or during crop cultivation may contribute to crop yield losses. Bhowmik and Doll (1982) found that residues of velvetleaf incorporated into the soil reduced soybean yields by 14%. Residues of older velvetleaf plants appear to be more toxic than those of seedling velvetleaf.

208 Data on yield losses due to velvetleaf in California are generally unavailable. In other states, severe yield losses due to velvetleaf infestations have been reported for corn, sorghum, soybean and cotton . High yield losses from velvetleaf competition could be expected in many of the vegetable crops grown in California. Yield losses from velvetleaf competition will generally be greatest when velvetleaf emerges simultaneously with the crop. Velvetleafplants which emerge 2 to 4 weeks after crop emergence will normally result in substantially less crop loss. The duration of velvetleaf competition will also influence expected yield losses. Generally, a lower density ofvelvetleaf can be tolerated for a longer period before a significant yield loss will occur. Velvetleafis quite competitive in soybean and yield losses range from about 13 to 31% for velvetleaf densities of 1.6 to 2.5 plantsfm2, respectively. Similar yield losses might be expected for dry beans. Sugarbeets are more sensitive to velvetleaf competition than soybean. The minimum density of velvetleaf required to reduce sugarbeet yields in Colorado was 3 to 4 plants every 10 m of row. In California, the minimum density may be even lower. Yield losses in cotton can be quite severe even from relatively low densities of velvetleaf. Full season competition from two velvetleaf plants every three meters of row resulted in 25% cotton yield loss. On the other hand, yield losses in corn from velvetleaf competition may be less than with other crops, perhaps because corn is much taller than most crops.

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