Weeds after Drought

Weeds after Drought

The dry weather this summer has made it very difficult to get a good alfalfa seedbed prepared, so if you have planted alfalfa this fall, expect some erratic emergence of alfalfa. This means that it will be very important to protect what plants do come up from insect and weed pest. So, start scouting your newly planted alfalfa as soon as it rains enough to get it germinated. To do this correctly, get out of your pickup, put on your glasses, get down on your hands and knees, and count the number of weeds per square foot.

Most weeds emerge at the same time that alfalfa does, and they will be actively growing by mid October. Weeds that emerge after the initial flush of plants usually will not develop into competitive plants in a good stand of alfalfa, but in a thin stand, they can become a problem. For best herbicide results, spray weeds some time in October or early November after most of the
weeds have emerged and while they are actively growing (good soil moisture and shirt sleeve day weather). Expect excellent weed control and good alfalfa production if you do it correctly.

Fall applications of Poast Plus at 1½ pt/acre has been a good choice for controlling volunteer wheat seedlings and other grasses in fall planted alfalfa.

Select 2 EC received label clearance for use on alfalfa for control of weedy grasses on April 6, 1998. Volunteer cereals, some of the bromus species, and ryegrass are listed as controlled with 6 fl. oz/acre when plants are 26 inches tall. We have one experiment in 1996 where we compared cool-season grass control with Select 2 EC and Poast Plus. Control of wheat and ryegrass in this study was good to excellent with both herbicides, but control of downy brome was only fair. Control of cheat and rescuegrass was good to excellent with Poast Plus and only fair with Select 2 EC. I would recommend only trying Select 2 EC on limited acres until we learn more about how it is going to control the cool-season grasses in fall planted alfalfa.

For control of most broadleaf weeds (including seedling dock), use of Butyrac (2,4DB) in the fall is the best choice. However, it is very important to spray weeds in the fall while they are small and in the rosette growth stage. Many of the weeds are not controlled with spring applications of Butyrac. Shepherd's purse and treacle mustard are seldom controlled with spring spraying
of Butyrac, and producers are usually very disappointed with spring spray results. That's why it is so important to examine newly sown fields in early October and get them sprayed this fall.

Some guidelines to use based on Weed Status determined by Scouting include:

Weed Status Recommendation

Only henbit & chickweed............................................................Don't Spray, See 1
>1 mustard & primrose/ft².....................................................Spray Butyrac, See 2
1-2 cheat & wheat plants/ft².........................................................It depends, See 3
3 cheat & wheat plants/ft²....................................................................Spray, See 4
Dock present...........................................................................Spray Butyrac, See 5

1. Henbit and chickweeds are not highly competitive, so they do not reduce alfalfa yields, and they usually mature and dry up before alfalfa is harvested. They are also not effectively controlled with Butyrac.

2. Mustards & cutleaf evening primrose (broadleaf plants) can be competitive, so alfalfa yield is usually reduced when these weeds compete with alfalfa. Stands can be thinned by these weeds when the weeds outgrow and shade out alfalfa plants. Use of 2 pints of Butyrac in October or November. Trying to control these weeds in the spring with Butyrac just does not work.

3. This level of cheat and wheat weeds will result in grassy hay, but it will not damage alfalfa stands. With less than 30 lbs/A of N in soil, you can expect 20% grass in hay. With more than 30 lbs/A of N in soil, you can expect 50% grass in hay at first harvest. If you can sell or use grassy hay for beef cattle, then don't spray. If you want to sell weed-free alfalfa hay, you should spray.

4. Use Poast Plus or Select 2 EC this fall. Three or more cheat or wheat plants/ft² will be very competitive, and can result in hay that is nearly all grass when soil nitrogen is above 30 lbs/A. In addition, this much grassy weeds can damage your new alfalfa stands, especially if the weather results in delayed harvesting in the spring.

5. If you have had a dock problem in your fields in the past or find any dock seedlings, you probably need to spray 3 pints of Butyrac per acre on your new stand this fall to control seedling dock. The only time that dock can be selectively controlled in alfalfa is at establishment while the dock is still small. Next spring is too late. Also, good control of disturbed old dock plants has been reported with 3 pints of Butyrac in seedling alfalfa.

Other comments:

A problem that many producers face is that weed infestation varies within a field, and they can not decide if it will pay to control them. For example, often there are more weeds around the edges of the field. One option would be to spray a couple of rounds around field edges where weeds are thickest. However, my experience is that once a producer does this, he is so satisfied with the results that he wishes that the whole field had been sprayed.

For detailed information on herbicide choices and rates, visit with your County Extension Educator for Agriculture. He/she has Fact Sheets and the Extension Agents' Handbook that gives specific information about pest control, including weeds. Also, do not forget to read the label carefully and follow the instructions closely.

Jim Stritzke
Former Alfalfa Weed Control Specialist
Department of Plant and Soil Sciences
Oklahoma State University

	Oklahoma State University Stillwater, OK
Oklahoma Alfalfa Oklahoma Cooperative Extension Service Oklahoma Agricultural Experiment Station