Controlling Summer Annual Grasses in Forages

 

 

Summer annual grasses are tough weeds to deal with, especially in perennial systems like pasture or hay fields. They can take advantage of the smallest opportunity to invade and, once established, are hard to control. Summer annual grasses also have a larger impact at reducing hay quality compared to broadleaf weeds. Proper management requires the right timing and patience.

 

Species like foxtail, sandbur and crabgrass are annuals that often emerge after perennials, grow fast and quickly set seed. This life cycle can make control difficult. In many circumstances, cultural practices like timing of hay harvest or grazing management may offer the most cost, and control, effective option. If herbicide control is decided upon, there are a few options to choose from.

 

 

In Roundup Ready® alfalfa stands, treatment is pretty straight forward, by using an alfalfa approved glyphosate product. Label guidelines do recommend treating before weeds exceed 6” in height. It is important to make applications before the alfalfa canopy begins interfering with spray coverage.

 

Straight alfalfa stands may use grass selective products like Select® or Poast®. Recommended weed height will vary depending on product, target and rate, so be sure to follow the label recommendations.

 

Pursuit® and Warrant® may be an option to consider for added broadleaf control. Warrant® is labeled as a pre-emergent option only and will have limited impact on established grasses, while Pursuit® is labeled for both pre- and post-emergent applications. The Pursuit® label does not list sandbur as a controlled species in alfalfa. As with Roundup®, canopy cover can interfere with spray distribution, so applications following harvest are recommended.

 

Another option may be Gramoxone®. Paraquat herbicides like Gramoxone® are non-selective, burn down products, so any green plant material will be damaged. However, if used immediately following harvest when alfalfa regrowth is limited, you may get control of annual species with a minimal yield reduction. 

 

 

Prowl H20® is labeled for alfalfa and both perennial cool and warm-season grass hay. Therefore, Prowl H20® can be used in mixed, alfalfa/grass hay fields. Prowl H20® should be applied similar to Warrant®, pre-weed-emergence in established hay fields. Pursuit® can also be used in mixed alfalfa/grass hay fields, but the label warns that significant stunting to the perennial grass can occur. Potentially, stunting of the perennial grass may serve as a further opportunity for weed encroachment or reduce hay tonnage. The use of Pursuit® should be carefully considered in the mixed hay fields.

 

Timing herbicide applications in alfalfa can be tricky. No product will provide season-long weed control, so it is important to consider the timing of weed germination and haying restrictions before making an herbicide application. All alfalfa herbicides will provide better weed control if applied when both the hay crop and weeds are small. Generally, the best time to apply is after hay harvest, as soon as bales are removed and before significant hay regrowth occurs. Applying herbicides immediately after hay removal also will help meet haying restrictions (Table 1).

 

Table 1: Alfalfa post application harvest restriction (days) for several herbicide options.

If you plan to make an application following alfalfa, cutting, make sure the target weeds are at the appropriate soil temperature. Just like our seeded crops, annual grass weeds need certain sustained soil temperatures to begin germination (Table 2). You can monitor soil temperatures through CropWatch, or by placing a thermometer in your own field.

 

As with any pre-emergent herbicide, yearlong control may require more than one application. However, a second application may come into conflict with maximum annual application amounts, so be sure to check with the pesticide label before reapplying. 

 

Summer annual grasses in forage crops are not the easiest weed to deal with, but with the right product, a bit of patience and proper timing, it doesn’t have to be a problem we can’t control.