Scout New Stands for Weed Problems

Scout newly-planted alfalfa about one month after emergence to determine weed problems. If all of the plants are alfalfa (have a single leaf for the first leaf, then three leaflets for all additional leaves), then you can forget about weeds in this stand until your stands start thinning from old age.   

If some of those plants are weeds, then you should evaluate the weed infestation and decide if control with a herbicide is needed. Plan to control thick stands of weedy grass and competitive weeds such as tansy mustard in seedling stands since one or two competitive weeds/sq. ft. can cause significant reductions in alfalfa yield. In addition, several competitive weeds/sq. ft. can even cause stand failure if they outgrow the alfalfa and shade it during warm wet weather next spring.   

It is usually profitable to produce weed-free alfalfa if selling alfalfa for dairy hay or using it for milking dairy animals. Top price is paid for weed-free, high-quality alfalfa. The discount for having 5 to 15% weeds is normally $15 to $23/ton. Since most producers have at least this many weeds in their first cutting of hay, herbicide treatment cost would essentially always be covered by increased hay value at first cutting. There are also the added benefits of hay being easier to sell and having increased alfalfa yield at second harvest.   

If you are planning on feeding hay to beef cattle, then newly-planted alfalfa needs to be scouted about one month after emergence to determine if controlling weeds would be profitable. The following are guidelines to help with these decisions.   

If more than one competitive broadleaf weed is found per square foot, then you should consider spraying before the end of November. This is especially important if weeds are growing faster than the alfalfa seedlings. Competitive broadleaf weeds include mustards, cutleaf eveningprimrose, and prickly lettuce.   

If broadleaf weeds are primarily henbit or chickweed, it usually does not pay to spray. First, they are not highly competitive and second, ButyracŪ is not that effective on them.   

If weedy grasses (wheat or cheat) are two or less/sq. ft., it would usually not pay to spray. However, if weeds make up more than 50% of the fall growth, it would pay to spray. If more than six grass plants/sq. ft. are found, and they are tillering and becoming the dominant forage, then it is important to control them with herbicide. Once alfalfa is shaded, it stops growing and stand loss can result in May if cutting is delayed due to warm wet weather.   

ButyracŪ 200 mixed with Poast PlusŪ is the only good option for controlling a mixture of emerged broadleaf and grassy weeds in fall-planted alfalfa. However, it is critical that this herbicide mixture be applied in the fall and not in the spring to avoid damage to alfalfa. Estimated herbicide cost (2 pt/A ButyracŪ & 1.5 pt/A PoastŪ) is $17.15.   

If there is an annual ryegrass problem, it can be controlled with Poast PlusŪ in the spring. Control of most other cool-season grasses with Poast PlusŪ in the spring will be poor.   

Spray Actively Growing Weeds in Fall: You should spray postemergence herbicides (such as ButyracŪ 200 and Poast PlusŪ) in the fall when: a) weeds are actively growing, b) there is good soil moisture, and c) temperature is warm, 60o or above.   

Keep insects controlled: Seedling stands can be severely damaged by even moderate infestations of some insect pest (grasshoppers, cutworms, and spotted aphids), so it is critical to keep seedling fields scouted and insects controlled. See OSU Extension Facts F-7150 (Alfalfa Forage insect Control) and F-7184 (Alfalfa Aphids in Oklahoma) for details.   

Consider Spraying Field Borders: In many cases weeds are only a problem around the borders of the field. Scout fields of fall planted alfalfa and if weeds are only in the borders, then just spray the borders in October or early November.