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Oklahoma State University Stillwater, OK |
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Oklahoma Alfalfa |
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Oklahoma State University Stillwater, OK |
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Oklahoma Alfalfa |
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Evaluate Weeds in First Cutting |
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Before mowing that first cutting of hay, get a paper and pencil and record alfalfa stem density per square foot. Also, estimate % weedy grasses, % broadleaf weeds, and % alfalfa that will be in the hay from each field. NEW STANDS If weeds (such as volunteer wheat and cheat) make up more than 10% of your forage in your fall-planted alfalfa, you should cut as soon as possible. Cutting early minimizes weed competition and maximizes forage quality. If weeds are not a problem, then cut at the first sign of alfalfa flower buds (for dairy hay) or regrowth from crown buds of alfalfa plants (for grinding hay). OLD STANDS Just before first cutting is the best time to evaluate the status of established stands. Make stem counts by counting the number of alfalfa stems in five, 1' X 2' quadrats in each field. (Divide the total by 10 to get stems/sq. ft.) If there are more than 30 stems/sq. ft., then the alfalfa stand is considered a full-stand, which seldom have weed problems if managed properly (soil pH near 7, adequate phosphorus and potassium, and insect pests are properly controlled). As stands thin below 30 stems/sq. ft., some of the area is not occupied, so weeds can grow. If you didn't spray a dormant-season herbicide this winter, estimate what percent of the first harvest forage will be weeds and write it down. If it is less than 2%, then you really don't have a weed problem. If you estimate that weeds make up 5% or more of what will be cut, then you should plan to use a dormant-season herbicide treatment next January or February because there will be twice as many weeds next year. If you applied a herbicide treatment this season, record the effectiveness by comparing sprayed areas with skipped areas. If you don't have skips, then consider leaving a unsprayed strip in each field. An unsprayed strip about 20 feet long along a fence is a good management tool. It lets you see how good your weed control was and is a way to determine alfalfa damage. First harvest is also the time to decide what you are going to do about summer weed control. If you are considering using a residual herbicide such as SINBAR, PURSUIT, or ZORIAL for summer weeds, then they need to be applied as soon as the first cutting is removed from the field. SINBAR, applied after first harvest, has given fair to excellent control of pigweeds and fair to good control of grasses, depending on the rainfall after treatment. PURSUIT applied after first harvest has given good to excellent control of pigweeds in most fields and fair to good control of grasses, depending on status of germinating grasses and rainfall after spraying. ZORIAL applied after first harvest can provide season-long control of crabgrass if it is activated by rainfall before the grasses come up or are less that 1-inch tall. If you are considering using postemergent herbicides such as BUTYRAC 200 for pigweeds and/or one of the grass herbicides (POAST PLUS or SELECT), then you need to check cut fields about 10 days after cutting and apply herbicides just before the alfalfa canopy completely shades the soil. SELECT Approved for Alfalfa SELECT received label expansion for control of weedy grasses in alfalfa, tomatoes, peanuts & dry beans on April 8,1998. We have evaluated it for several years in alfalfa, and have found it to perform similarly to POAST PLUS. Both require an oil concentrate; they need to be applied to actively growing grass; and neither has any residual activity. Thus, getting a new flush of weedy grasses may results after both of chemicals. Use rate of SELECT is listed at 6 and 8 fl. oz./acre. The Valent Co. recommends using the 6 oz. rate on small weeds in seedling alfalfa and the 8 oz. rate on grasses in established stands. We have found that timing of spraying (size of weeds, growing conditions, etc.) is more important than rate and believe that the 6 oz. rate is adequate if applied correctly. Our suggestion is to try both rates and decide for yourself.
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| Jim Stritzke Former Alfalfa Weed Control Specialist Department of Plant and Soil Sciences Oklahoma State University
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