At a recent meeting Monsanto scientists described to Midwestern weed scientists and agronomists the discovery and state of knowledge about a biotype of goosegrass (Eleusine indica) resistant to glyphosate. The problem appeared in 1997 in oil palm plantations of Malaysia where as many as eight annual applications of glyphosate have been made for the past 10 years. Unlike the resistance in rigid ryegrass (the other annual grass with glyphosate resistance) in which the specific mechanism of resistance still remains a mystery, the resistance in goosegrass is already known to be due to an altered binding site. In susceptible plants, glyphosate binds with the EPSPS enzyme thereby preventing the formation of key aromatic amino acids required to build proteins needed to sustain the plant's life. In the resistant goosegrass, the EPSPS enzyme has a single amino acid substitution (proline to serine change) in the region of the EPSPS enzyme where glyphosate normally binds, thus preventing glyphosate from binding. Thus, the formation of the amino acids continues normally and plants do not die. The mechanism of glyphosate resistance in rigid ryegrass is unknown, but it is believed to be due to multi-genetic differences and is not the result of an altered binding site.

Depending upon the method of measuring the level of resistance, the resistant biotype is two to five times more tolerant of glyphosate than the susceptible strains. The impact of the resistant ryegrass in the field is quite small in both areas where it has been confirmed (Australia and California) and the problem in these areas has not spread. This is somewhat similar to the triazine resistant velvetleaf in Wisconsin: only some farms have the problem and it basically has not moved to new sites nor developed on other farms. In contrast, the glyphosate resistant goosegrass has appeared on four oil palm plantations and already infests approximately 12,500 acres. Additionally, the infestations are reportedly similar to what we would see in fields with triazine resistant lambsquarters in Wisconsin: a large and aggressive population of a single species.

Herbicide resistant biotypes of goosegrass have previously been found in several regions of the world. A biotype resistant to trifluralin (Treflan) was documented in 1973 in the southern US after many years of using this and perhaps related products in soybean and cotton production. Biotypes of goosegrass resistant to imazapyr (Arsenal) in Costa Rica, fluazifop (Fusilade) and paraquat (Gramoxone) in Malaysia have also been documented in 1989 and 1990. Goosegrass is one of the major annual grass weeds in the tropical and subtropical regions of the world. It is considered among the 20 worst weeds in the world by the authors of The World's Worse Weeds (Holm et al., 1977). Single plants can produce 40,000 seeds so an infestation could produce 2,000,000 seeds per acre. If uncontrolled, it causes economic losses in crops and orchards in the southern and western regions of the United States. It is described on page 24 of the Weeds of the North Central States book and the map indicates it is abundant in the southern regions of Illinois, Missouri, Indiana and Ohio and that it is present in most of the central and northern areas of these states.

Goosegrass exists in Wisconsin but is rarely, if ever, found in our grain and forage crops. The book, Grasses of Wisconsin, states that it is "A weed in city streets and lawns, collected in Madison and Milwaukee" (Fassett, 1951). However, the appearance of glyphosate resistance in a major world weed and to learn that this resistance is due to an altered binding site is a serious call to practice good stewardship in our use of glyphosate and glyphosate resistant crops to ensure that we prevent the appearance of more glyphosate resistant biotypes for as long as possible.

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