Mowing grass pastures and hay fields every 30-45 days and proper fertility practices can often adequately manage weeds. However occasionally significant problems occur which require chemical control using herbicides. As wet as this spring was, there may be more problem areas than usual.
Fall is the best time to use herbicides to control perennial broadleaf weeds in forages. The reason is that late in the season, plant metabolism changes from promoting growth to preparing to survive the winter. To do this the plant begins to transport nutrients stored in the leaves and stems to the root system. This provides an opportunity for effective weed control using herbicides as chemicals sprayed on the leaves are also transported to the roots, improving the chance of eliminating the weed. For this reason, applying herbicides between mid-September and late October is generally the most effective timing for control of perennial weeds.
There are many products available for controlling broadleaf weeds. Correctly identifying the weed or weeds present is a key to making this selection. Pesticide labels will state which weeds a product may be used against as well as provide additional information on proper application procedures. Remember to always read and follow all label directions.
Controlling broadleaf weeds in grass pastures and hay fields is simpler than controlling grasses as you are able to use a selective herbicide which acts only against broadleaves. In grass, most herbicides which control grassy weeds will also eliminate desirable plants. If weeds have completely taken over an area of a field then spot spraying with a non-selective product such as glyphosate followed by seeding may be most effective. If a substantial population of desirable plants exists then the use of proper fertility and mowing to prevent seed development can give your desirable species a competitive advantage over weeds.
The same principle applies for a grass-legume field. Applying a grass herbicide will likely injure desirable grasses while using a broadleaf weed control product will damage legumes. Spot applications and re-seeding or improved fertility may be the best strategies. Keep in mind that you will have difficulty establishing alfalfa in an area recently planted to alfalfa due to allelopathic effects.
If you are planning to maintain fence rows and field borders you may encounter some woody perennial weeds. Multiflora rose, brambles, black locust and others including my personal nemesis, mulberry, are common in these areas. If you are selectively clearing these rather than applying a brush killer to the entire area, then using a cut stump treatment is often the most effective method. Cut the woody plant as near to the ground as you can and immediately apply herbicide to the stump. This causes the product to be carried into the root system which will reduce or eliminate suckering. There are a variety of products available for this however glyphosate at over 20% active ingredient often provides very good results. Triclopyr products with over 8% active ingredient also work well.
For additional information, including recommended herbicides, see Extension publication WS16, “2017 Weed Control Guide for Ohio, Indiana, and Illinois,” pages 154-175. This is available online at: https://cpb-us-w2.wpmucdn.com/u.osu.edu/dist/7/3461/files/2016/12/2017-Weed-Control-Guide-tnkpfi.pdf. Since this is two years old, be sure to check product labels carefully before using. A 2019 version of this publication is available but you must purchase it from Ohio State at: https://extensionpubs.osu.edu/2019-weed-control-guide-for-ohio-indiana-and-illinois-pdf/.