Industrial Hemp Tolerance to Early-POST Herbicides

Industrial Hemp Tolerance to Early-POST Herbicides

As Nebraska just legalized industrial hemp in 2019, during the 2020 season we have conducted a total of five studies (four field studies at Mead and one in a greenhouse in Lincoln) to test hemp tolerance to various herbicides that are commonly used in corn and soybean. This is the third article (in a series of five), that outlines the results of a study that tested hemp tolerance to herbicides applied early-POST to about one foot tall industrial hemp.

The treatment list included total of 11 herbicide combinations that were based on two, primarily grass-type, herbicides (Dual II Magnum, Assure II) used in combination with three, primarily broadleaf, herbicides (Moxy 2E, Stinger and Hornet). There were no weed control ratings presented. For individual weed response ratings to tested herbicides please check the corn and soybean sections of the 2020 Guide for Weed, Disease and Insect Management in Nebraska.

Industrial hemp (variety X-59 for fiber) was planted on June 8, 2020 in 30” rows, 0.5” deep and at about 20 thousand seeds per acre (10lbs / acre). Herbicides (Table 1) were sprayed on 10-12” tall hemp. Study was arranged in the randomized compete block design with 3 replications with 30 ft plots. Hemp injury ratings were conducted at 6, 13 and 21 days after treatment (DAT) utilizing the scale from 0-100 (0=no injury; 100=dead plant). The study was terminated after last rating.

Table 1. Hemp injury ratings at 6, 13 and 21 day after treatment (DAT) to 11 herbicides.
Herbicide (rate/acre) 6 DAT 13 DAT 21DAT
1. Dual II Magnum (1.67pt) + Assure II (6.8) 5 0 0
2. Dual II Magnum + Moxy 2E (1.5pt) 38 23 10
3. Dual II Magnum + Stinger (6 oz) 8 5 3
4. Assure II + Moxy 2E 47 33 18
5. Assure II + Moxy 2E+ Dual II Magnum 37 18 13
6. Assure II + Stinger 6 2 0
7. Assure II + Stinger + Dual II Magnum 7 0 0
8. Hornet (2.oz) 69 97 100
9. Hornet + Assure II 70 93 97
10. Hornet + Dual II Magnum 70 93 97
11. Hornet + Assure II + Dual II Magnum 68 95 97

Results

Hemp exhibited variable tolerance levels to tested herbicides. Visual injury levels ranged from almost no injury (~5%) to temporary injury (~20%), and in few cases, the severe injury or death (60%-100%).

Slight hemp injury was evident in plots with Dual II Magnum, Assure II and Stinger (Table 1). All three products caused temporary hemp injury which lasted about two weeks and was evident in the form of leaf yellowing. For example, tank-mix of Dual II Magnum with Assure or Stinger caused 5-8% visual injury at 6 DAT, which was similar to 6-7% from a tank-mix of Assure with Stinger and Dual II Magnum. However, by the 13 or 21 DAT, all injuries disappeared, suggesting that these three products have potential for use in hemp after crop emergence (post-emergent).

Heavy hemp injury (>30%) was caused by tank-mixes with Moxy 2E, which was as much as 47% at 6 DAT, while the injuries were reduced to about 20-30% by 13 DAT and ~ 10% by 21 DAT (Photo 2), indicating the plant recovery. We do not know if these injuries resulted in any yields or biomass reduction on the season end as the study was terminated after last rating.

Severe hemp injury (60-10%) was caused by any tank-mixed containing Hornet, which caused about 70% injury at 6 DAT (Photo 2), which became much worse (97%) by 13 DAT and crop death (100%) by 21 DAT (Table 1).

Conclusion

Hemp exhibited relatively good tolerance to several products that can be used postemergence in industrial hemp. Assure II, Dual II Magnum and Stinger caused almost no hemp injury, thus they should be safe to use. Moxy 2E also has some potential for use. The temporary injury from Moxy 2E could be considered acceptable since it lasted only for about two weeks. Hornet caused unacceptable injuries, thus should not be used in hemp post-emergent. It is important to note that the above conclusions are based only on one year of data, this study will be repeated in 2021 season.

Temporary injury at 21 DAT by Moxy 2E applied early-POST
Photo 1. Temporary injury at 21 DAT by Moxy 2E applied early-POST.
Severe hemp injury at 7 DAT from Hornet applied early-Post
Photo 2. Severe hemp injury at 7 DAT from Hornet applied early-Post.