By Jarrod Hardke, Rice Extension Agronomist; Jason Norsworthy, Professor – Weed Scientist; and Tom Barber, Professor – Extension Weed Scientest
Loyant is a Group 4 (auxin type) herbicide – basically it affects plant hormones and how the plant grows. When you are working with an auxin herbicide, environmental conditions before and after application can play a huge role in its performance.
Loyant provides excellent control of a wide range of broadleaf weeds and annual sedges including ALS-resistant rice flatsedge. It has also provided control of barnyardgrass in our research plots when used in a diversified herbicide program utilizing overlap of multiple residuals up front. This herbicide can be a great complement to our rice weed control programs, but there are still questions, many of which arose from the 2018 growing season where there was extensive use of the herbicide. Here’s a review of what we know so far.
In previous years, we sometimes observed injury following Loyant application. We periodically observed transient leaf roll in medium-grain conventional varieties as well as hybrids, but these symptoms never persisted throughout the season. There was very little indication that rice could be injured by Loyant to the extent of causing yield loss based on trials conducted prior to 2018.
In 2016, we conducted a cultivar tolerance trial where we noted significant injury and subsequent yield loss with RT CLXL745 and RT XP753 (Fig. 1). In this trial, both cultivars yielded less than the untreated check regardless of Loyant rate or number of applications. CL151, CL163, Jupiter, and Roy J were also in the trial with variable levels of visual injury that resulted in no yield loss. Significant lodging across the plot area caused variability in yield resulting in inconsistent results. In addition, no other researchers in the Mid-South reported any issues. Another trial was established in 2016 to determine if injury could be reduced based on changing adjuvants but results were inconclusive, and no yield loss was observed.
Fig. 1. Grain yield and crop injury of rice cultivars treated with preflood applications of Loyant at 1 pt/A compared to a non-treated control at the Rice Research & Extension Center near Stuttgart, AR in 2016.
In 2017, tolerance trial work evaluating CL111, CL153, CLXL745, RT 7311 CL, and Titan showed some crop injury (up to 25%), but no yield loss when applications were made at the 2- to 3-leaf (lf) stage or 2- to 3-lf followed by 4- to 5-lf (Fig. 2). An additional study evaluating a range of rates and timings also showed injury ratings below 20% and no significant reduction in yield for CL111, CLXL745, and Diamond (Fig. 3).
Fig. 2. Grain yield of rice cultivars treated with preflood applications of Loyant at 1 pt/A (2-3 lf rice) or Loyant at 1 pt/A (2-3 lf rice) followed by Loyant at 1 pt/A (4-6 lf rice) compared to a non-treated control at the Rice Research & Extension Center near Stuttgart, AR in 2017.
Fig. 3. Grain yield of rice cultivars treated with preflood applications of Loyant at various rates and timings at the Rice Research & Extension Center near Stuttgart, AR in 2017.
In 2018, similar results were observed with injury less than 20% and no yield loss in tolerance trials for CL111, CLXL745, and Diamond (Fig. 4). However, a separate trial was conducted in 2018 with CL111, CL272, and CLXL745 to determine if significant injury could be influenced by increased rates and time between application timings. Rates of Loyant were either 1 or 2 pt/A with sequential applications made at 4, 12, 15, and 20 days apart. Injury to CLXL745 was often higher when the Loyant rate was increased from 1 to 2 pt/A at Stuttgart (RREC) (Fig. 5). Also at RREC, the closer the applications were (4 DAT), the more overall injury was observed. With the labeled rate of Loyant (1 pt/A), applications made 4 days apart resulted in >50% injury. We did not observe the same levels of injury at Pine Tree (PTRS) that we did at Stuttgart (Fig. 5). At Pine Tree, while not significantly different, yields were lower than the untreated check for all Loyant treatments (Fig. 6). Similar yield trends were observed at Stuttgart with most applications resulting in significant yield loss on CLXL745 (Fig. 6). Please note that the label requires 14 days between Loyant applications.
Fig. 4. Grain yield of rice cultivars treated with applications of Loyant at various rates and timings at the Rice Research & Extension Center near Stuttgart, AR in 2018.
Fig. 5. Injury ratings of RT CLXL745 treated with sequential applications of Loyant at either 1 pt/A or 2 pt/A compared to a non-treated control at the Rice Research & Extension Center near Stuttgart, AR and the Pine Tree Research Station near Colt, AR in 2018.
Fig. 6. Grain yield of RT CLXL745 treated with sequential applications of Loyant at either 1 pt/A or 2 pt/A compared to a non-treated control at the Rice Research & Extension Center near Stuttgart, AR and the Pine Tree Research Station near Colt, AR in 2018.
Additional trials were planted late in 2018 utilizing cultivars based on levels of injury observed in grower fields. Injury was significant especially when applied 7 days preflood onto dry soil for all cultivars evaluated (Fig. 7). Treatments applied at timings closer to flood resulted in overall less visual injury for all cultivars. Post-flood applications caused slight injury at a later evaluation timing. Yield results were variable dependent on cultivar and application time but no significance or trends were apparent (Fig. 8)-
Fig. 7. Injury ratings of rice cultivars treated with Loyant at different timings and soil moisture conditions compared to a non-treated control at the Rice Research & Extension Center near Stuttgart, AR in 2018.
Fig. 8. Grain yield of rice cultivars treated with Loyant at different timings and soil moisture conditions compared to a non-treated control at the Rice Research & Extension Center near Stuttgart, AR in 2018.
The hybrids have seemed the most sensitive to Loyant injury, with testing mainly on RT CLXL745 and RT XP753. The medium-grain varieties Jupiter and Titan have been variable in their response, but several fields with injury were observed on-farm this year. Most of the long-grain varieties have shown little injury or yield loss potential other than Diamond, which was injured severely in late testing this year and was also observed to have greater injury on grower fields. Previous work with Clearfield varieties like CL111, CL151, or CL153 and conventional varieties such as Roy J resulted in very little injury and very few grower calls were received on these varieties.
Plant stress appears to play the greatest role in inducing injury. If conditions for the few weeks following application are mild, injury potential may be reduced. However, if plants are already stressed, or if they quickly become that way after application, the likelihood of injury occurring tends to increase, especially on hybrid cultivars, medium grains, and Diamond. The most problematic part is not being able to say, “this is when you’ll have injury so don’t spray then.” At this point we can only say to avoid stress to rice if Loyant is being applied. For this reason our recommendation will be to avoid Loyant applications on these cultivars unless it is absolutely necessary, at least until we fully understand the causes for increased injury.
Barnyardgrass control with Loyant in some fields was less than that expected based on our previous research where we had utilized multiple residual herbicides in front of a preflood Loyant application on small barnyardgrass. More than 20 weed control trials were conducted in Arkansas evaluating barnyardgrass control prior to launch of Loyant in 2018. These trials were conducted just like all the rest for which we generate herbicide ratings. The lack of Loyant control in some of the grower fields we walked was unexpected. It should also be noted that there were plenty of instances where it worked as advertised, most of these were applications made with sufficient moisture present in the field. In our plot work this year as in past years, barnyardgrass was controlled in a program approach with multiple residuals regardless of pre-flood herbicide applied and when an early post-emergence was removed from the program, control with Loyant was often less than acceptable.
Other causes for reduced grass control in 2018 may have been the hot and dry conditions that were common during application. A full dose of Loyant is needed to control barnyardgrass and previous research by Dr. Ryan Miller, previously a graduate student, showed that uptake and translocation of Loyant could be impacted by environmental conditions at or following application with dry conditions lessening control. Additionally, some growers put their pre-emergence herbicides out and waited until preflood for Loyant to rescue the rice crop from large barnyardgrass. This is not a plan for success with Loyant or any other barnyardgrass herbicide. Similar to our research plots, on-farm success with Loyant this year included multiple residuals and an early post-emergence application prior to Loyant preflood. In a few instances, barnyardgrass still escaped timely applications of Loyant when used as part of a sound weed control program, and escapes from these fields are currently being evaluated in greenhouse trials. In research conducted prior to launch of Loyant, there were no differences in tolerance of barnyardgrass populations to the herbicide, even though many of the populations evaluated were resistant to other herbicides including quinclorac.
Trials were conducted prior to 2018 to evaluate tank-mix options with Loyant and several products were identified as not being suitable mixtures. For this reason, the label stated that certain herbicides should not be applied with Loyant. Because of label confusion, it is likely that some of the failures observed this year were due to improper tank mixing. Remember that the tank-mix options are very restrictive and product specific – consult the website and label to be sure you are using approved tank-mix partners.
Did Application Play a Role in Reduced Control?
The Federal label for Loyant lists 10 GPA (gallons per acre) as a minimum for a reason. We have plenty of research showing the rapid decline in efficacy as you decrease water volume. The data in Fig. 9 provides a snapshot. If applied at 3-5 GPA, the occurrence of misses should not be shocking. Some respond, “I used 10 GPA by ground and it still missed grass!” Fair point – but think about how dry it was for the ground rig to get that application out. The extreme dry conditions mentioned above likely had more to do with it. The volume requirement is on the label, and data clearly indicate the need for this volume requirement.
Fig. 9. Percent weed control from Loyant applications made at spray volumes of 5, 10, or 20 gallons per acre (GPA) in trials conducted at the Rice Research & Extension Center near Stuttgart, AR in 2014-2015 (Miller 2017 dissertation).
Injury to Soybean
We knew soybean were sensitive to Loyant (Fig. 10). This was stated in field days and production meetings. Today, we are not aware of documented yield loss from soybean injured by off-target movement of Loyant. That is a positive, but injury to a crop will not be tolerated by most growers. While definitive data are lacking for buffers when applying near soybean, extensive field observations in 2018 lead us to suggest general application buffers of 0.25 miles downwind by ground and 1.0 miles downwind by air. Our research from this year has shown that injury is not caused by volatility, but rather physical drift, and keep in mind that soybean plants are super sensitive.
Fig. 10. Loyant injury to soybean.
Like most herbicides, Loyant has a great fit in some situations, but it is not a herbicide that should be applied to all rice acres. It can be part of a sound weed control program in rice, but it is not a foundational grass herbicide like clomazone. It must be used as part of a program that utilizes multiple residual herbicides in front of a timely application of Loyant. For best results, avoid applying it to hybrids, medium grains, or Diamond unless absolutely necessary. Use the labeled rate and 10 GPA water with good soil moisture conditions and flood in less than 7 days. If this is unachievable, then it is recommended to NOT use Loyant for grass control. If rice is stressed, particularly by hot, dry conditions, consider alternatives to Loyant. We hope this information will be helpful in making plans for 2019. Let us know if you have any additional questions.