Higher prices for wheat, corn and grain sorghum have made crop rotation a more appealing option to many Southwest farmers who see benefits from alternating crops without sacrificing profit potential. But with rotation come possible challenges with weed control residue management and plant-back restrictions following some herbicide applications.

Texas AgriLife Research and Extension agronomists Wayne Keeling, Todd Baughman and Brent Bean explored the advantages and challenges of crop rotation during a panel discussion at the recent West Texas Agricultural Chemical Association annual conference in Lubbock.

“Residue management, weed control and herbicide restrictions will be important factors in crop rotation,” said Peter Dotray, weed scientist and panel moderator.

“Rotation is certainly nothing new,” Keeling said, “but different phases in farm programs and market factors have affected rotation programs. We’ve been looking at rotation and conservation tillage systems since the late 1980s and early 1990s.” Keeling has worked extensively with cotton and grain sorghum. Part of the effort included harvesting sorghum and then turning under the residue. “But we also looked at conserving residue to help get the real benefits from rotation and conservation tillage.”

Keeling said early problems with reduced tillage centered on weed control. “Roundup Ready helped and with better prices for grain, reduced tillage became even more important.”

He said residue management and weed control are key elements in reduced tillage systems. Planning herbicide use to allow for subsequent crops also plays a role, he said.

Baughman works with cotton, wheat and other grain crops in the Texas Rolling Plains. “Wheat is an important commodity, especially dryland wheat considering current prices for wheat and cotton. He agrees with Keeling that before Roundup Ready, farmers lost significant yield to weed competition. “Also, we can’t make a cotton, wheat and fallow system work in the Rolling Plains. “Leaving a field out for a year does not pay. We don’t get enough rainfall to recharge the soil enough in the fallow year to boost the next cotton crop.”

He said herbicide selection is a critical aspect of rotation plans, “especially with a long residual herbicide. Think ahead to make cropping (and herbicide) decisions.”

Keeling said center pivot irrigation increases value of rotation. “In dryland situations we can get two crops in three years. At AGCARES (a research site near Lamesa) we get a crop every year.”

He’s looked at several rotation systems, including planting into wheat residue. “That worked well,” he said. “Sorghum residue management is more difficult.”

He’s tried two-row sorghum planting and then planting cotton between the rows with minimum tillage. “We did as little tillage as possible.”

Bean works out of Amarillo and said wheat behind corn “usually produces no herbicide residual problem. But we can run into disease problems, such as head scab in wheat. We are suggesting fungicide seed treatments when planting no-till following corn, sorghum or wheat.

“It is also a good idea to increase wheat seeding rate 20 percent when planting into any heavy crop residue. Nitrogen rates may also need to be increased by 20 percent because of tie-up of nitrogen in the crop residue.”

Bean said area farmers are producing more corn silage for dairies. They cut silage earlier than they would grain, and follow with wheat. “The plant stubble left in the field may still be green when they plant wheat,” he said.

Curl mites survive on the green tissue and move into the wheat. “Curl mites transmit wheat streak mosaic and other viruses to the wheat. Farmers need to give that green material time to dry down before they plant wheat,” he said.

Burying residue also may help control the mite. “Also, delay planting two weeks after cutting silage. That’s still earlier than they could plant behind a corn for grain crop. The first week of October is the best planting date for wheat for grain. We don’t want to delay past the third week of October.”

Bean said curl mites may come out of CRP land into wheat. “Farmers might mow CRP back 100 to 200 yards to reduce the mite population. They might also leave a 100 to 200 yard strip unplanted and plant it later (after CRP vegetation dies down).”

Bean said one wheat variety, Tam 112, has some tolerance to wheat streak mosaic. “However, the disease can still devastate yield if the infection is high enough.”

“The wheat curl mite is not as mobile as other mites,” Baughman said. It’s also very small. “If you think you see it with the naked eye or a small hand lens, you’re looking at something else.” Baughman said a green bridge into the field typically helps curl mites get into wheat.

Volunteer wheat may cause problems with wheat streak mosaic virus. “Controlling volunteer wheat is the key,” he said. “We had a dry July this year so we had not had a lot of volunteer wheat emerge up to that point. We want to make sure to control late-emerging volunteer wheat prior to planting.” “We’ve seen some bad wheat streak mosaic problems (in the Rolling Plains),” Baughman said. “The key is to control vegetation between crops. Broadleaf and grassy weeds may survive through dry summers (in dryland fields).”

He said leaving these weeds will sap moisture and growers will lose some of the advantage of reduced tillage systems.

Winter weeds may be a challenge in conservation tillage, too, Keeling said. “In conventional tillage systems we would harvest and work the ground and would have no problems from harvest until planting.

“With no-till, weeds such as marestail and kochia will emerge during the winter. We have to get in early, February or early March, with 2,4-D and control them while they are small. We also need to use a residual herbicide to keep weeds down until planting time. If we let them get big they are harder to control.”

They also discussed volunteer crops as emerging weed problems.

“Volunteer wheat control is not difficult,” Bean said. “Roundup is effective but the price is up and we may need three applications to control as many as three flushes of volunteer wheat prior to planting.

“We don’t have a good residual herbicide that will control volunteer wheat on fields where we’ll plant wheat again in a few weeks, so for the most part we are relying on Roundup for now.”

“Roundup is our best option for volunteer wheat,” Baughman said. “But farmers will also deal with other weeds so they should not back off on rates.” He said volunteer peanuts pose problems for a cotton rotation. “We have to rotate peanuts to break a disease cycle,” he said. “Otherwise, we’ll have to make a significant number of fungicide applications.”

He said multiple applications of Roundup may be necessary for volunteer peanut control. “Roundup and Ignite can be applied to bigger plants but will need several applications. We also expect multiple flushes. We have no good residual herbicide to control volunteer peanuts that is also labeled for cotton.”

Keeling said volunteer Roundup Ready cotton also poses new weed problems. Using effective herbicides with a hooded sprayer is one option. He said Aim, Ignite and Buctril work early in the season, at the three-to-four-leaf stage. “Later in the season they are less effective.” Gramoxone is another possibility, he said. “We still have a lot to learn.”

He said tillage may be the answer. “Running sweeps or a well-timed cultivation may be one of the best things we can do for volunteer Roundup Ready cotton,” he said. Protecting as much residue as possible is still important.

Bean said hooded sprayer application may be the best bet for volunteer Roundup Ready corn. “We have no good answers yet.”

Keeling said a phenoxy herbicide may offer a good burn-down option for winter weeds. “We’ve used 2,4-D up to 30 days prior to planting cotton with no problem,” he said. “Later applications increase chances of cotton injury.”

He said Banvel is not an option because of limited rainfall in the area.

Baughman said phenoxy materials should be used early, from the beginning to the middle of February.

Rain makes a big difference in winter weed emergence, Keeling said. “So farmers need to be prepared for whatever the season presents. As Southwest farmers pay more attention to potential crop rotation systems they also need to concentrate more on the herbicides they use. “Some of our more effective residual herbicides have plant-back restrictions,” Baughman said. “Herbicide selections may limit rotation.”

He said cotton farmers who keep yellow herbicides in their weed management program help limit potential for herbicide resistance and also help prevent some weeds from causing trouble in subsequent crops. “Leaving out a yellow herbicide in cotton means increased potential for more rescuegrass in wheat.”

Keeling said a milo and cotton rotation may benefit from the re-registration of Milo Pro herbicide. “We have lot of products with short to medium residual activity and we should stay away from those with long residual.”

He said farmers can rotate back to cotton eight months after applying Milo Pro to sorghum in season.

“Staple in cotton followed by a corn or grain sorghum crop could cause problems,” Bean said. “We have no good way around that, so farmers have to think ahead on rotation plans. Staple to wheat is less of a problem.”

Keeling said over-reliance on Roundup in the Southeast has precipitated some glyphosate resistance problems. “Here, we’ve kept yellow herbicides in the mix. That helps. With Roundup Ready Flex, we may be tempted to quit using yellow herbicides, but that would be a mistake.” “Anyone wondering how valuable the yellow herbicides are should look at Wayne’s plots where he didn’t use it,” Baughman said. “They’ll see the benefits.”

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