Using Reduced Herbicide Rates

Effective wild oat control can be obtained in spring wheat and barley, but key management factors must be kept in mind

By Beverly R. Durgan, Weed Scientist, University of Minnesota

Herbicides are registered at application rates that are adequate for good weed control under a wide variety of conditions. When conditions are favorable for herbicide activity, however, lower application rates (below labeled rates) will often provide good weed control at a reduced price.

The following are factors to consider when applying reduced (below labeled rates).

1. One important point to keep in mind before trying reduced herbicide rates is that when using a herbicide below the manufacturer’s labeled rate, growers assume the liability for performance. Thus, it is important to apply reduced rates accurately and timely. The risk of weed control failure increases as rates are reduced.

2. Proper application timing is very important when applying reduced herbicide rates. Postemergence wild oat herbicides require application to wild oats and crops at precise leaf stages. Leaf number on wild oats is determined by counting the leaves on the main stem and disregarding the tillers. The youngest leaf is counted as a full leaf only when another leaf becomes visible. Lower leaves, which may have died from various stresses, such as frost or wind damage, should also be counted in the total leaf number. An accurate leaf count is important for optimum wild oat control.

3. Using reduced-rate herbicides effectively demands that growers calibrate their equipment precisely. There is less margin for application errors with reduced rates.

4. There are a number of tradeoffs for the advantages any one wild oat herbicide might offer. When using reduced herbicide rates, herbicides should be applied to the smallest labeled wild oat leaf stage. However, if the herbicide is applied too early, odds are greater that a late flush of wild oats will require a second herbicide application, or that some wild oats might escape treatment.

Research has been conducted for several years at the Northwest Research and Outreach Center in Crookston, Minn., to evaluate reduced rates of postemergence wild oat herbicides. Herbicides were applied at the lowest labeled rate, ¾  and ½ of the labeled rate in spring wheat and barley. The following tables summarize the results from 2000 and 2001.

Results of this research have shown that with accurate rates and proper timing of application, wild oats can be controlled with the most effective herbicides at rates as low as one-half of the normal use rate. In wild oat populations greater than about 40 plants per square foot, the full rate or nearly full rate, was more consistent than reduced rates, but in low to moderate infestation levels below 40 plants per square foot, the reduced herbicide rates performed very well. However, wild oat control with reduced herbicide rates can vary from year to year.  This is mostly due to environmental conditions and the wild oat population.

Reduced rates of Puma and Discover gave good to excellent wild oat control in 2001 and 2002.  However, Discover has given the most consistent wild oat control at the reduced rate of ¾ and ½ of the labeled rate.

Adoption of reduced herbicide rates for wild oat control in spring wheat and barley could lead to a savings of $7 to $13 per acre in chemical costs with grain yields equal to using full herbicide rates.  However, reduced rates should not be used if:

1. Wild oats are under stress due to adverse environmental conditions.

2. Wild oat infestations are greater than 40 plants/ft.

Wild Oat Control in Spring Wheat with Reduced Herbicide Rates at Crookston, MN - 2001   Rate % Wheat Injury % Wioa Control Yield Treatment* (product/A) 6/21 7/9 7/9 7/22 (bu/A) Assert+NIS+COC 1 pt (1X) 5 0 98 94 58 Assert+NIS=COC 0.75pt 5 0 92 90 64 Assert+NIS+COC 0.50pt 2 0 92 89 62 Puma 0.67pt (1X) 5 0 100 100 61 Puma 0.50 pt 2 0 96 96 54 Puma 0.33 pt 0 0 93 93 62 Discover + DSV 3.2 oz (1X) 0 3 100 100 63 Discover + DSV 2.4 oz 7 0 98 97 63 Discover + DSV 1.6 oz 0 0 95 95 63 Weedy check --- 0 0 --- --- 47 LSD (0.05)   nx 2 4 6 ns *All treatments were tank mixed with Buctril at 1pt/A.

Reduced Rates on Foxtail
Reduced rates can also work with foxtail - however, the grower needs to check the label -- as most post-emergence grass herbicides are labeled at lower rates than wild oats. Also, yellow foxtail is more difficult to control than green - so growers should use caution when reducing rates on yellow foxtail.

Are Low Rates Legal?
A herbicide user can legally choose a rate lower than listed on the herbicide rate unless the label specifically prohibits low rates, according to the NDSU Extension Service. However, the company has no obligation to support herbicides when the application rate was less than labeled rates.  Herbicide users should not expect a company representative to provide any  assistance if weed control is less than expected from a rate of herbicide that is less than the labeled rate. Thus, the user assumes all risk and liability of unacceptable weed control when less than labeled rates are used. Users should also follow practices such as rotating crops and rotating herbicides with different modes of action to minimize the potential of herbicide resistance.