April's Ag Advice By April Borders We have recently been blessed with good precipitation and this moisture has helped our crops but it has also helped to promote winter annual weed germination and growth.
Winter annual weeds are especially troublesome because they compete with the crop throughout the growing season. Additionally, some winter annual grasses will lead to dockage and/or foreign material discounts when contaminated grain is delivered to the elevator. To help minimize these losses, growers must control the weeds in a timely manner.
Common broadleaf winter annual weeds include blue mustard, tansy mustard, tumble mustard, field pennycress, henbit and shepherd's-purse. Weeds such as henbit and pennycress generally emerge in the fall. In the spring these weeds will bolt and then flower. Unfortunately, many growers are not aware these weeds are present in their fields until they start blooming in the spring. By this time, control is difficult and most of the crop damage has already occurred. To be effective, winter annual broadleaf weeds need to be controlled early, before the plants begin to bolt or stems elongate.
Now stop for a minute and think about this: is there something to their name: winter annual weeds? Remember I said that generally they emerge in the fall. A good question to think about is: why do you want to spray in the spring when these weeds emerged in the fall (November)? Why didn't you treat them then when herbicide selection was easier? Typically you only need two herbicides and maybe one: 2,4-D and glyphosate. 2,4-D ester will do a great job of cleaning up winter annual weeds and if you have winter annual grasses, a glyphosate with 2,4-D will give good results in the fall.
Well, you are probably thinking that I am dumb talking about fall weed control in the spring but there is a purpose to it. Since these winter annuals are clearly visible now, this is a good time to make maps of where these weed areas are for later treatment. The weeds are very small in the fall and you won't see them without close inspection of your fields. With your map, you'll be ready to get those pesky winter annual weeds next fall, then when spring rolls around you will be ready to plant instead of spraying weeds.
If you are planning to spray weeds in the spring, you need to do it when these weeds are small, preferably before they start bolting. Often this opportunity only exists for just a few days. Generally after some warm weather, sunshine and spring rains, these weeds start growing in the rosette form. A few days of rosette growth and bolting starts then the race is on between the producer and the weed to see who will win. This can all happen in as little as a week so the producers don't have a lot of time.
Now how do we deal with these weeds in our crops like winter wheat? Blue mustard is perhaps the most difficult of the winter annual broadleaf weeds to control because it bolts very early. To be effective, herbicides typically need to be applied to blue mustard in late March or early April. Early April applications of 2,4-D usually provide excellent control of tansy mustard and other winter annual broadleaf, but only fair control of blue mustard. If timed correctly, 2,4-D provides low-cost and effective control of these weeds.
Wheat should have at least four tillers before applying 2,4-D or serious crop injury may occur. Adding a sulfonylurea herbicide to 2,4-D will improve control, particularly after these plants have bolted, but it may not help increase yield because the weeds have already used soil moisture and nutrients. If the sulfonylurea herbicide is used after bolting, but prior to seed production, it may reduce the amount of seed produced.
It has just been in the last couple of years that it has been possible to selectively control some of the winter annual grass weeds, such as downy brome, jointed goatgrass or feral rye, in winter wheat. Although control of these weeds is often best when herbicides are applied in the fall, some spring control is possible.
If winter annual weeds are a regular problem, change the crop rotation. Including a spring-seeded crop in the rotation with winter wheat-fallow provides an additional year in which to prevent seed production and allows the soil seed bank to gradually decrease.
For more information about weed control contact the Clay County Extension Office or stop by and pick up our 2005 weed control guides.