It’s tough to control weeds in any alfalfa field, but add a little weather into the equation and it gets even trickier. That’s what intermountain growers are up against.
“Since the alfalfa is dormant for a much longer period of time, we can have a real serious problem with winter annual weeds,” says Steve Orloff, Siskiyou County University of California Cooperative Extension farm advisor. “As a result, there are a number of issues unique to the intermountain area.”
Weed control in alfalfa can be a significant cost, according to Karen Klonsky, UC Davis economist. “It’s about 7 percent of all production costs,” she told her audience at the recent California Weed Science Society meeting. “The importance in regard to quality is a huge issue. If you don’t control the weeds in alfalfa, sometimes you can’t even sell the hay.”
Application timing for weed control is critical in established alfalfa fields, particularly in the intermountain regions, according to Orloff. “In our environment, we really can’t apply herbicides in the middle of the winter because the ground may be frozen. The question becomes is it better to apply herbicides in the late fall (November), early winter (January/February) or are you better off making an application in the spring? The concern with the latter timing is that alfalfa is coming out of dormancy at that time and there can be considerable injury.”
Orloff looked at three timings and different herbicides on various weed species — fall (late November), winter (January/February), and mid- to late March. Shepherd’s-purse control was best achieved with Velpar at the January application, he says. Mid-March was the next timing with the November application lagging behind. Similar results were noted with Sencor.
“I think what may be happening with the November application is that we’re getting rainfall and snow and some of that herbicide may be leaching out below the root zone of some of the weeds so we’re not getting good control,” he says.
Karmex also provided better control with the January application than it did with the November application, according to Orloff. “Gramoxone applied alone did not give very good control of shepherd’s-purse, but when mixed with Karmex improved control quite a bit. The situation was similar with Sencor and Gramoxone. Raptor worked well as a rescue treatment if you had to make a later spring application.”
For foxtail control, the overall situation was similar. What was very apparent was the potential for a lot more injury with the spring application timing.
“The mid-March applications caused significantly more injury compared to the other two timings,” Orloff says. “It’s common for growers in our area to be forced into a late March application because we have a narrow window. We’ll have a fair amount of windy weather and storms. We think that timing causes a lot of damage to the alfalfa — visual injury that is carried over into yields.”
Regardless of timing, it’s clearly important to treat for weeds in alfalfa, not only for increased yields, but also for forage quality.
“Typically what we want to see is an ADF of around 27 percent or lower,” Orloff says. “With untreated alfalfa, the ADF was up there around 30. In contrast, all of our weed control treatments significantly reduced the weed populations, so we came in at less than 27 percent ADF or what would be considered dairy quality alfalfa.”
Summer annual grass and broadleaf control in established alfalfa stands is another problem for intermountain growers. “We have a big problem in some intermountain areas in some valleys with green foxtail, and we also have a problem with pigweed,” Orloff says. “Treflan at 20 pounds per acre has been a standard over the years but it’s hard to justify in the intermountain areas where we only have two cuttings. Typically, it’s our second and third cuttings that would be infested so it’s too expensive to justify the cost sometimes. Also with Treflan being a granular, it’s an entirely separate application. You can’t tank mix it with your winter herbicide program.”
Last year’s registration of Prowl H2O in established alfalfa has helped the situation immensely, according to Orloff. “It has a very similar weed spectrum to Treflan except that it appears to be more effective on dodder and slightly less effective on the summer grasses,” he says. “It has greater surface stability than Treflan does and doesn’t need to be incorporated with rainfall quite as soon after application. It can also be tank mixed with a number of winter herbicides, so the theory is that you can get season long weed control with a single herbicide application.”
In trials, Prowl H2O mixed with either Velpar, Sencor or Gramoxone boosted mustard control to near perfect, as well as performing quite well as a stand-alone treatment. Prowl H2O in combination with Velpar also provided the best green foxtail control.
A tank mix of Prowl H20 and Velpar also provided the best pigweed control in the trial. “We’re starting to have a big problem with pigweed in established alfalfa,” Orloff says. “Before, we had a problem in seedling fields, but now we’re seeing more of an issue in established fields, especially if the grower irrigates soon after cutting and the pigweed gets established. Prowl H2O really helps that situation. Velpar alone simply does not last long enough to give us control of these summer broadleaf weeds, and it’s a similar situation with Sencor.”
Weed control in mixed stands of alfalfa and orchard grasses is a challenge. Since buyers don’t need the high alfalfa ratio for the horse market, growers are starting to leave alfalfa stands in the field longer resulting in more weeds.
“That’s problematic because the herbicide has to be safe to both the alfalfa and the orchard grass and there are very few herbicides that are labeled for that mixture,” Orloff says.
He looked at two different application timings of the available materials — fall and late February. What he found was a bit of a surprise.
“I thought that Gramoxone being a contact material would work the best under that situation,” he says. “Since orchard grass is a perennial I thought it would rebound pretty quickly, but Gramoxone alone or applied in combination was very injurious to orchard grass. Our best results were with Sencor in the fall. It worked very well, and Sencor in the spring looked pretty good also.”
Weed control in grass hayfields is another area of increasing interest for intermountain growers. “There’s a big interest in grass hay, especially for the horse market,” Orloff says. “We have a lot of straight orchard and also some tall fescue fields that are being sold to the horse market. As these stands go longer and longer we’re getting thinner plant populations. Winter weeds encroach into these grass stands, and broadleaves as well as grassy weeds are getting to be a problem.”
Weed control during grass establishment is very important. There are very limited materials to control grassy weeds after that point. Orloff recommends growers simply avoid planting fields that already have perennial grass weeds such as foxtail barley.
“There is just no way to control that weed selectively in a grass field.”
Other strategies for grassy weed control include pre-irrigating and treating the weeds with Roundup prior to planting. Winter seedbed preparation in combination with a spring Roundup application can be helpful.
“After the grasses emerge, treat broadleaf weeds as soon as possible to prevent early season competition,” he says. “It’s important to treat weeds early. If you don’t use a herbicide during that stand establishment period, you can lose the stand completely.”