The first selective herbicides to control annual bluegrass (Poa annua) were the arsenates, introduced in the 1930s. These products included lead arsenic and calcium arsenic, and at times they could also cause injury to the desired turf. The arsenates were removed from the market in the 1970s due to human exposure risks and environmental hazards.
The herbicide active ingredient, ethofumesate, was first registered by Fisons Corp. (Ipswich, England) in 1977 for annual grass and broadleaf weed control in sugar beet crops. After some company mergers, Nor-Am Corp. acquired ethofumesate, and George Raymond was their turf marketing manager. So, in the mid-1980s, Prograss (a.i. = ethofumesate) was specifically labeled for postemergence annual bluegrass control in turf. Back then, this was a big deal, as ethofumesate represented an alternative to the arsenates.
A research report noted: “This herbicide offers what turf managers have needed for nearly 40 years — safe and effective annual bluegrass control.” For example, during the 1980s and 90s in the Mid-Atlantic U.S., perennial ryegrass (Lolium perenne) fairways were popular and also tolerant of ethofumesate. Therefore, golfers would enjoy playing on Poa-free fairways and tees. Foliar applications of ethofumesate to annual bluegrass produces a stunting of growth, epinasty (the downward bending or drooping of a plant part), swelling of the crown, darker green color and fusing of expanding leaf blades.
One question was commonly asked at the time: Why is annual bluegrass more susceptible to ethofumesate than creeping bentgrass or perennial ryegrass? To answer this question, research was conducted under the direction of Bruce Branham, Ph.D., at the University of Illinois (Urbana, Ill.) to determine the site of uptake in annual bluegrass, and to compare ethofumesate absorption, metabolism and translocation activity among annual bluegrass, creeping bentgrass and perennial ryegrass.
Detailed plant physiology studies were conducted in the greenhouse and growth chamber. Annual bluegrass plants were transplanted into a porous ceramic clay soil amendment and grown to one-to-three-tiller stages. Ethofumesate was carefully applied to the foliage and separately to the soil to evaluate which site of uptake produces the most annual bluegrass control. Annual bluegrass, creeping bentgrass and perennial ryegrass also were grown in the same ceramic clay and treated with radiolabeled ethofumesate to track its movement into and through the plants.
Specifically with annual bluegrass, better control was observed if ethofumesate was assimilated through the leaves and not taken up by roots, because foliar absorption was a more direct route to the target site. Ethofumesate disrupts the meristems, which are sites of rapidly dividing cells responsible for growth and development. Soil-applied ethofumesate had a minimal effect on annual bluegrass.
Annual bluegrass absorbed significantly more ethofumesate than creeping bentgrass or perennial ryegrass. Over a 14-day experiment, annual bluegrass absorbed an average of 33% of applied ethofumesate, versus 11% for creeping bentgrass and 13% for perennial ryegrass. Also in annual bluegrass, ethofumesate absorption increased rapidly within the first three days of application.
Within 24 hours after ethofumesate application, all three grasses metabolized about 20% of the absorbed ethofumesate. At the end of the 14-day experiment, an average of 67% of ethofumesate was metabolized for all three grasses. Also, translocation of ethofumesate was similar among all three grasses.
This research documented that absorption is the most important factor in determining selectivity of ethofumesate for annual bluegrass control within a stand of creeping bentgrass or perennial ryegrass.
Overall, annual bluegrass absorbs three times more ethofumesate than creeping bentgrass or perennial ryegrass.
Branham recently retired from the University of Illinois after a long and successful career of research in turfgrass physiology, weed control and herbicide chemistry. Thank you, Dr. Branham, for your contributions to turfgrass science.
Source: Koehler, E.A., and B.E. Branham. 2002. Site of uptake, absorption, translocation and metabolism of ethofumesate in three turfgrass species. Weed Science 50:576-580.
Mike Fidanza, Ph.D., is a professor of plant and soil science in the Division of Science, Berks Campus, at Pennsylvania State University in Reading, Pa. He is a 23-year member of GCSAA.