Weed Control for Established Alfalfa Stands  

Sinbar^® and Velpar® applied in January or February are currently being used by many producers to control both cool-season broadleaf weeds (like mustards) and winter grasses (like downy brome). Velpar® has been more effective than Sinbar^® for controlling rescuegrass and prickly lettuce. Also, Velpar® is more water soluble, and can affect results. Velpar® is activated with about 1/2" of rain compared to about 1" needed to activate Sinbar^® ; so, Velpar® may be preferable west of I-35. If too much rain is received, however, effectiveness of Velpar® may be decreased because it leaches deeper into the soil. Consequently, Sinbar^® is normally the choice in central and eastern Oklahoma where there is a greater probability of larger rains. Label directions for both products should be followed closely, as both have use restrictions on sandy soils.   

It is important to apply Sinbar^® and Velpar® herbicides before alfalfa begins active growth to avoid herbicide damage to alfalfa. If applied after alfalfa begins active growth there can be enough herbicide taken in through the leaves to cause yellowing of plants. This discoloration will last for a week or so, then the plants will turn greener and continue normal growth. This can cause some yield reduction but does not kill the plants. Severe plant damage and plant kill that some have noted with Sinbar^® and Velpar® is usually due to improper mixing in the spray tank.   

Since Sinbar^® and some of the other products are wettable powders, special efforts of keeping them mixed in the tank are required. Failure to keep herbicides in suspension in the tank will result in alfalfa injury in the field where you start spraying, since herbicide is concentrated in the bottom of the tank. Then poor weed control will be obtained in the rest of the field.   

Pursuit® does an excellent job on some mustards when applied in March, but it is not effective on the cool-season weedy grasses. Some suppression of wheat, cheat and ryegrass is possible with Pursuit® applied in March; but expect no control of downy brome.   

The bottom line is that you should apply herbicides such as Sinbar^® or Velpar® in January or February to control cool-season weeds in your alfalfa if you had more than 5% weeds in your first cutting of hay last year. Control of cool-season weeds with Sinbar^® and Velpar® herbicides is both predictable and cost-effective. In addition to having fewer weeds in your hay, it can also extend the productive life of your stand by 1 to 2 years.   

SUMMER WEEDS:    

Do not try to control summer weeds such as pigweeds, crabgrass and foxtails by increasing rates of dormant applications of Sinbar^® and Velpar®. It does not work. By May and June, when these summer weeds germinate, the herbicide will be gone. However, good summer weed control has been possible with dormant application of Treflan ^TM  E.C. applied and incorporated in February. The main disadvantage of this treatment is that most producers do not want to do the incorporation.