Herbicide reps tend to shake their heads at farmers who try to get by with below-label herbicide rates. Because the companies usually price by the acre, rather than on volume, it's to their advantage to set rates as low as possible while still providing consistent performance. Still, there are specific situations where reduced rates work. And the prospect of reducing herbicide expenses by up to half can be mighty tempting, especially for postemergence weed control. For growers who are willing to put in some additional time and effort as well as assume some additional risk, University of Missouri agronomist Andrew Kendig offers the following tips. Just remember that you assume all the risk of poor performance when you use below-label rates. Check your label. It may be possible to spray a low rate and still stay on label.
Scout your weeds. It's still worth it to get out of the truck and take a walk through the field. Sure, it's easier to wait until weeds are over your boot tops and then raze them with a full rate of Roundup. But you might get better results (and better yields) with a strategic strike that zeros in with a lower rate of precisely the right herbicide when weeds are very small. Scout the field again before canopy closure and spot treat escapes. Most herbicide labels list lower rates for smaller weeds. If you want to go with even lower off-label rates, bear in mind that weeds taller than an inch will probably require the full rate. Half-inch-tall weeds may need only a half rate. Weeds a quarter-inch to a half-inch tall usually succumb to a quarter rate. For a quarter rate to work, you'll need to spray eight to 10 days after planting soybeans. Weeds will often be so small that they are difficult to see. Applying herbicide this early will probably require a sequential quarter-rate application in 10 to 14 days. This still amounts to only half of the full rate. Kendig recommends trying this technique on a small acreage first to convince yourself that it works.
Beware tankmix antagonism. Mixing some broadleaf and grass herbicides can create considerable reduction in performance. The effects of that reduction will be magnified at lower rates. It is especially inadvisable to mix a partial rate of broadleaf herbicide with a full rate of grass herbicide.
Plant narrow rows. Ten-inch rows will canopy and shade the soil surface up to 20 days sooner than 30-in. rows. This can allow you to treat weeds while they are small with less worry of a second flush.
Delay planting. Delayed planting allows more time for weeds to emerge and be killed with preplant weed-control practices such as burn down or tillage. The disadvantage is that delayed planting can significantly reduce yield potential.
Tillage and banding. Under good conditions, 50 to 60% weed control can be attained with cultivation. Most of the weeds missed will be those that are growing within a few inches of the crop. This can make band application equipment (commonly used on cotton and ridge till) especially useful for applying a precisely targeted herbicide. This isn't really a reduced rate; it's a full rate sprayed over a smaller area.
Herbicide selection and application. Choose your herbicide based on its effectiveness against the weeds you have. Use the right herbicide with properly calibrated equipment and the right time, and you increase your chances of acceptable weed control.