Grass control tricky in Central Valley alfalfa

Producing high quality alfalfa hay means keeping the field clean, and that can get tricky at times. Grasses propagate by seeds, stolons or rhizomes and can produce viable seeds within 30 days of clean cutting or between harvesting.

“With a lot of broadleaves, we can cut and remove them,” says Mick Canevari, UCCE farm advisor in Stockton, Calif. “Grasses just keep coming back because the growing point is at or just below the soil surface.”

Not all grasses are bad, according to Canevari. Good grasses, such as orchard grass and rye grass are often desirable, but not under all circumstances.

“Even the good grasses have the ability to compete with an alfalfa stand if they are not properly managed,” he says. “Over time they will populate and tend to push out alfalfa. But we do have a market for them and we get paid for them, so often they're a good grass.”

Grasses are especially competitive on new seedling plantings of alfalfa. Stands decline at a much faster rate when grasses are uncontrolled.

“Some grasses pose health issues to horses,” Canevari says. “Grasses can also accumulate high rates of nitrates, particularly in a dairy situation where we're using effluent on the fields. You have to watch out for high accumulations in the feed.”

The price of alfalfa varies wildly within the season and from one year to the next, but it's the wide spread in price from a high quality hay to the lower end hay that will often make or break profitability.

“There always seems to be $60-$90 differential between the high end and the low end,” he says. “The bottom line is there's money to be lost or money to be made when you produce good quality forage.”

The proper herbicide applied at the proper time is one of the keys to producing high quality forage. Fortunately, there are several newly registered herbicides to make the job easier.

Among the newer pre-emerge herbicides, Prowl is pretty close in performance to an old standard Treflan, Canevari says. “Prowl was recently registered for use in established alfalfa in late 2007-2008. Prowl is very effective controlling grasses, many broadleaf weeds and dodder when applied pre-emergent to weed germination.”

Prowl H20 is formulated to be stable on the soil surface for several weeks with little volatility or loss, according to Canevari. “Another advantage of the Prowl liquid formulation is that is can be tank-mixed with other pre- or post-emergent liquid herbicides (paraquat, Velpar, Chateau, 2,4DB, Prism, Post and Raptor) to compliment a long-term control strategy,” he says. “The rates of Prowl can be adjusted from 2 to 4 quarts per acre depending on weed type and expected weed pressure. The higher rates applied during January/February have provided excellent long season grass control into late summer cuttings.”

The most recent herbicide registration in alfalfa is Chateau. It controls both broadleaves and some grasses.

“For grass control, it is somewhat limited because it doesn't get all grasses,” Canevari says. “However, I think it's going to shine on common groundsel. I think it's going to fit in that Velpar market. Chateau can be tank-mixed with other post- and pre-emergent grass herbicides (Prowl, Prism, Poast) for a more effective and longer grass control program. Chateau has a 25-day PHI between cuttings which allows use during the summer growing period. It's one of the few products after harvest in the summertime we can use. It's got a 25-day PHI. I think it remains to be seen how it will fit in the summer months, but it's certainly a possibility.”

In the winter months, Canevari says a Chateau/Gramoxone tank-mix will likely be the best strategy. “It did provide some nice winter weed control,” he says. If you use Chateau in the summer months with a surfactant, expect to see a little bit of leaf burn, but the injury is short-lived.”

Of the post emergent herbicides available, Poast or Prism can be used between alfalfa cuttings to control summer grass problems including yellow and green foxtail, barnyard grass and perennial grasses. Well-established perennial grasses usually require multiple applications.

Canevari says that research trials have shown that Poast and Prism work best on young immature grasses before tillering stage that are vigorously growing and not drought stressed.

“Some years ago there was controversy about how effective Poast and some of these herbicides were,” Canevari says. “We knew that they were effective controlling grasses, but a lot of times they would fail. So we experimented with timing. We looked at applications before an irrigation versus applications after an irrigation. The difference between applying it in a hot dry climate where there was not much soil moisture and the leaf cuticle was not absorbing the herbicide as well meant anywhere from 10 percent to 30 percent difference in control.”

“So, if you're not satisfied with the control you're getting with some of these grass herbicides, it may be that the application is being made under stressful conditions or being applied too late in the summer.”

Raptor is a selective translocated herbicide that controls broadleaf and grass weeds. “The weed spectrum controlled is similar to Pursuit except it is far more effective on grasses — especially winter annual grasses,” Canevari says. Performance is best when weeds are small, 1-3 inches and vigorously growing.”

It's always difficult to pinpoint the perfect timing to apply a grass herbicide. “I would say that it is always better to apply a herbicide early,” Canevari says. “Generally, when we apply a herbicide, it's about a month too late.”

The take-home message, according to Canevari, is that a growing, healthy stand of alfalfa is the best form of weed control. “Avoid using nitrogen fertilizers in alfalfa, especially if you have grass weeds as a problem,” he says. “Grasses love nitrogen, and alfalfa is usually not going to benefit from nitrogen applications. Another strategy is using a taller harvest height to suppress grass germination.”

Extending harvest intervals can also help suppress grasses, as can delayed irrigations.

“I think this is something we can all be a little more cognizant of,” Canevari says. “If you're in a soil that has enough moisture that you can get regrowth after you pick up the bales and move off the field and still get a little regrowth to cover the soil surface, that will probably go a long way in helping with weed control. There are some soils that you can't do that with. They run out of moisture.”

TAGS: Alfalfa
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