AGP - Euphorbia heterophylla
 

EUPHORBIA HETEROPHYLLA L. 

 

 

 

 

Family: Euphorbiaceae

Common names: Lechosa

Euphorbia heterophylla is an annual plant with milky latex in all plant parts. It grows 30 to 100 cm tall and has simple or branched hollow stems with angular ribs. The leaves have variable shapes (as the species name suggests) both within and between populations. The lower leaves are alternate and the upper leaves are opposite and often have a whitish or bright red base. This coloration explains the English common name of wild poinsettia for this plant. The fruits are three-lobed and explode when ripe, dispersing the seeds 1 m or more from the parent plant.

In tropical areas freshly harvested seeds are non-dormant, but in warm temperate regions they are dormant. This is an apparent adaptation to these environments as plants that started growing at the end of the season in subtropical areas would be killed by winter temperatures. Seeds germinate over an extended period in the field and plants grow very rapidly and thus are very competitive with crops. They can completely cover a soybean crop within 2 to 3 weeks after emergence.

Plants may flower as early as 30 days after emergence. Pollination is by insects attracted to flowers by large amounts of nectar produced by glands on the flowers. Ripe seeds are formed 20 to 25 days later. Single plants may have 100 seeds at a given time and can produce over 4500 seeds during a season. Seeds germinate readily in alternating temperatures of 25 to 35oC. Emergence is greatest when seeds are 0 to 8 cm deep (40 to 47%) and drops to 22% at 10 cm, 12% at 12 cm and zero at 14 cm. This ability to emerge from such depths adds to the competitive ability and difficulty in control of E. heterophylla.

E. heterophylla originated in tropical America, but is now distributed in most of the tropical and subtropical areas of the world. Plants can be found from sea level to nearly 1400 m elevation, but it is only a serious weed in warm climates. It is found in cultivated crops, vegetables, pastures and wastelands and is particularly troublesome in soybeans, cowpeas, maize and sugarcane. It occurs as a weed of cowpeas in Nigeria; of cassava in Ghana; of citrus, avocado and mangos in Florida (U.S.A.); of soybeans in Nigeria, Brazil and U.S.A.; of cocoa, tea and upland rice in Sumatra; and of cotton in Israel. This illustrates its ability to grow and compete in many environments and cropping systems.  

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.

In most crops mechanical and manual control measures are effective if done on a timely basis several times a season. Soil disturbance creates conditions favorable for other seeds to germinate and since the weed is only slightly affected by shading, they can continue growing and competing. The integration of mechanical, manual, cultural and herbicide use into well planned management systems is the best approach to E. heterophylla control.

 

 

Countries: Cameroon, Cuba, Congo (Democratic), Dominican Republic, Eritrea, Togo

 

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