E. heterophylla infests over 25% of the soybean fields in Brazil. Densities of 75 plants/m2 reduced yield only 12%. Yield losses were much greater in the U.S.A. where 8 plants/m2 competing for 8 weeks, 12 weeks, and full season lowered yields 19, 21 and 33%, respectively. Fifty plants/m2 often resulted in crop failure. Six weeks free of E. heterophylla competition is usually adequate for maximum soybean yield. However, the sticky latex in the sap of late- emerging or non-competitive levels of this weed contaminates the grain with dirt and trash during harvest and may raise its moisture content.
Yields of a semi-prostrate and an erect variety of cowpea in Nigeria were reduced 25 and 53% respectively, when 10 E. heterophylla plants/m2 competed with the crop all season. Plants that emerged 20 days or more after planting had no effect on yield, but uncontrolled plants that emerge with the crop completely shade the crop in 6 weeks. This is the reason it is so competitive in most crops: its rapid early growth allows it to form a canopy over crops unless kept in check. Plants can emerge all season, and control of late emerging plants is also important to prevent a build-up of seeds in the soil.
Several herbicides control E. heterophylla, including 2,4-D, aciflurofen, fomesafen, oxyflurofen, bentazon, lactofen, imazethapyr, imazaquin, and chlorimuron. Triazine herbicides such as atrazine and metribuzin can give acceptable control for several weeks (especially when rains are abundant following application) but seldom for the full season. The substituted ureas such as linuron and diuron have no effect on this weed.
Extensive competition studies by weed scientists at North Carolina State University (U.S.A.) led to the development of a computer program (HERB) that predicts the impact of specific densities of uncontrolled E. heterophylla on soybean yield. With only 5 plants/9.5 m2, the predicted yield loss is 8%. The program then computes the value of the lost crop and determines the returns for various control strategies for expected yield and crop value based on control costs at current weed sizes and soil moisture levels. For 5 plants/9.5 m2 that are 5 to 10 cm tall and in a field with good soil moisture, three herbicides would give returns that exceed the costs.