Reposted from the "Weeders of the West" blog with author's permission.
Marcelo Moretti summary of research on glufosinate best practices in hops (direct link to article).
-Brad
Best Practices for Using Glufosinate in Hops
Marcelo L. Moretti, Oregon State University, Department of Horticulture
Glufosinate, marketed as Rely 280 and sold by BASF, is a postemergence herbicide in WSSA group 10, a glutamine synthetase inhibitor. Rely 280 is a recent registration in hops, and studies were conducted to evaluate weed efficacy and crop tolerance following the label recommendations.
Label recommendations and weed control
Rely 280 can be applied to mature hops in most US states when the entire label and supplemental (EPA Supplemental label No. 7969-44) are in possession, except California. The rate ranges from 32 to 55 fl oz/A, and applications must be made when hop bines reach at least 6 feet tall. However, it is best to wait until the bines are 8 feet tall; see next section for more details.
Up to three applications per season are allowed; the maximum rate per application is 55 fl oz/A. If banding, do not concentrate the broadcast rate on the band. The entire Rely label has a section indicating how to calculate the proper rate for band applications (Equation 1).
It is essential to calibrate the sprayer to ensure the proper rate is applied since over-application of glufosinate leads to significant crop damage and illegal residue (Figure 1).
Our experience with glufosinate in hops
We evaluated Rely 280 in the spring of 2024 at four different hop fields grown with the cultivars Strata, Citra, and Mosaic. Hop plants were treated with three different rates (20, 32, and 55 fl oz/A) compared to carfentrazone (Aim at 2 fl oz/A). Treatments targeted the lower 3 feet of the plant base. Applications were made at three different times spaced a week apart starting June 6, 2024, when hop bines were on average 5.5 ft (Citra), 6.0 ft (Mosaic), and over 10 ft tall (Strata). The critical detail is that bine height varied among and within plants, as is expected for the crop.
Herbicide injuries were initially subtle one week after treatment and very localized to a few parts of the hop plant, with crop injury levels below 5% in all sites. After 14 days, injuries became more noticeable, but injuries were restricted to short bines (<6 ft) and located primarily in the bine-growing area, not in the treated area (Figure 1). It is likely that glufosinate moved in the plant and accumulated in the growing points. The injury started as chlorosis, then increased to tissue necrosis after two weeks, and culminated in bine mortality in some cultivars. The higher injury and mortality were observed in Mosaic and Citra cultivars, likely because they were smaller at the first treatment than Strata. Glufosinate causes more injury than carfentrazone, and glufosinate injury increases with the herbicide rate.
By delaying the herbicide application by one week and allowing hop plants to grow taller than 6 ft, injury and mortality declined to the levels observed in the nontreated plots. No differences among herbicide and herbicide rates were detected in treatments applied on June 14 and June 20.
Glufosinate provided better basal shoot control (52-81%) than carfentrazone 14 days after treatment. Basal shoot control was greater with glufosinate at 32 and 56 fl oz than 20 fl oz (52%) applied at the same time in Strata. Please remember that the 20 fl oz application is not recommended by the label. Applications made on June 6 provided greater basal shoot control than in the late application (June 20). Similarly, weed control levels declined as the application was delayed. For instance, Canada thistle control was excellent at 35 days after application with glufosinate at 56 fl oz, when applied on June 6. However, the same treatment controlled 60% of the Canada thistle when applied on June 20 (Figure 2). The lower control resulted from poor coverage in the late application because thistle plants were over 4 ft tall, and the growing points were never exposed to the herbicide application.
Figure 2. Canada thistle is recovering from a glufosinate application done on June 20.
Concluding remarks
Glufosinate is an effective herbicide for managing hop shoots and weeds in the growing season. However, crop injury and bine mortality can occur if treatments are done when the bine is close to 6 ft tall. We suggest applying glufosinate when the bines are at least 8 ft tall. Lower rates (Rely 20 fl oz/A) did not improve crop tolerance but significantly reduced basal shoot and weed control efficacy. Therefore, one should use Rely 280 at 32 to 55 fl oz/A with appropriate adjuvant for best results. This study promoted herbicide injury by exposing a larger portion of the hop bines (lower 3 ft) to the spray solution. This was done intentionally to evaluate a worst-case scenario for crop safety and improve weed control efficacy. Growers should minimize the bine exposure early in the season, but it can be done once hop bines are 8 feet or taller.
Acknowledgment
This work was funded by the Hop Research Council. We thank the hop growers who made this research possible.