To spray or not to spray

– Jimmy Henning, Extension Professsor, Livestock Forage Specialist, University of Kentucky (first published in August 6th issue of Farmer’s Pride)

‘Should I spray this field?’ is such a common question, it should be easy to answer, right? Turns out, it is not. I was recently looking at an excellent orchardgrass/red clover pasture (with occasional ironweed and Queen Anne’s lace) when the producer asked me if he should spray the field. I think he was surprised when I said no. Spraying was not warranted for several reasons, but mainly because spraying to kill the problem broadleaf weeds would completely take out the clover, which was significant.

Here are some guidelines that help me formulate a weed control plan. I will be the first to admit this is a highly subjective set of guidelines or suggestions.

Weed management is more than chemical control.

Farmers have other options besides spraying herbicides. Sometimes the best approach is to use agronomic or grazing management to strengthen the forage crop and deal with the weed. Johnsongrass is a classic example of a weed that can be managed by grazing but it is very problematic in a hay field.

Mowing is another tool for weed management. Mowing annual or biennial thistles after they bolt (put up a seed head) but before they make seed is a good way to prevent the spread of these weeds. Timely mowing of cockleburs can prevent seed production as well. The UK publication AGR 207 ‘Controlling Broadleaf Weeds in Kentucky Pastures’ evaluates the effectiveness of mowing as a weed management tool for many of our problem pasture weeds.

Determining if a spray threshold has been reached

The Clover Dilemma

Controlling broadleaf weeds usually means killing the clover present, something I call the clover dilemma. How do you decide if the infestation is bad enough? How much clover does it take to withhold the herbicide and live with the weeds? Certainly it does depend on the weed and the extent of the infestation. And it depends on the type and amount of clover. A vigorous, thick stand of red clover would be worth protecting in all but the worst infestations. A stand of small, white dutch clover probably not. And remember that some new herbicide formulations will take out broadleaves without killing clover. Proclova ® is one example.

Annuals

With annual weeds, it is usually best to first try to thicken up the stand of the forage. Annuals are opportunistic; they germinate and grow when forage stands get sparse. Addressing lime, P and K needs and strategic use of nitrogen fertilizer are some of the most powerful tools to shift the advantage to the desirable forage. Implementing rotational grazing and maintaining good residual heights on the base grass will help suppress the onset of these weeds.

Managing toxic and invasive plants

Toxic and invasive weeds will often necessitate the use of herbicides. The cost/benefit ratio of using chemical control is influenced greatly by the threat of loss of livestock and the loss of value due to their presence in hay. An infestation of hemp dogbane, which contains toxic glycosides cause one farm owner to avoid using that field for horse hay and used it for cattle after he had sprayed it. The harvest interval for the herbicide he used was 14 days, which means he had to wait 14 days after spraying to cut for hay. The harvest intervals for many common forage herbicides are found in AGR 172 ‘Weed management in grass pastures, hayfields and other farmstead sites.’

Cash hay vs pasture

Some weeds can be tolerated or even be beneficial in pasture that would warrant herbicide application in a cash hay crop. For example, johnsongrass and crabgrass are highly palatable forages that benefit summer pastures but are not welcome in hay intended for high end horse markets.

Estimating the spray threshold

Quantifying the area of the pasture covered by weeds can help assess the spray threshold. Assuming these weeds are not palatable, they will reduce the expected yield on the field by the proportion of weeds present. Infestations of ironweed have been shown to reduce pasture yield by 25% or more. If the expected yield for the field is 2 tons per acre, then the ironweed infestation would ‘cost’ you 0.5 tons per acre. At $50 value per ton of forage yield, the ironweed could be said to ‘cost’ you $25 per acre, which is close to the cost of spraying.

Weed growth stage matters

Weeds are most easily controlled when they are vegetatively and actively growing. Ironweed in full flower in August is very hard to control. For that reason, late summer may be a poor time to try to control weeds even though they may be very visible at this time. For perennials like ironweed, initiate herbicide applications when plants are young and vegetative. Often that means timely mowing in mid-summer to knock them back and following up with herbicide in two or three weeks.

A replant strategy is needed

A plan to spray almost always requires a plan to replant because when the weed is gone, mother nature will insert another one. I find the various replant schedules and labels confusing. For this reason, I refer often to the label for the proper re-seeding interval.

The decision to spray herbicide on pastures and hayfields is complicated. The decision to spray is a subjective process depending on many factors, including the visual assessment of the weed pressure, the invasiveness and/or toxicity of the weed, the cost of the control measure, the forage value of the weed and its life cycle and the ability to restore the pasture stand. Don’t forget that the best first step is to thicken up the existing stand of forage. The best weed control is a thick, dense stand of the desired species in a pasture or hay field.

Happy foraging.