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Nevin Lawrence is an associate professor and weed management specialist at the University of Nebraska-Lincoln. (Journal photo by Amy Bickel.)

Effective weed control starts with a healthy hayfield.

Weeds can be prevalent in newly seeded alfalfa without proper management. Weeds can influence yield, quality and standability, said Nevin Lawrence, an associate professor and weed management specialist at the University of Nebraska-Lincoln. 

Most of the crop/weed competition happens in the seedling stage

“That’s your biggest risk for losing yield,” Lawrence said. “It has to do with light quality. We are starting to learn more and more about how weeds interfere with crop yield, and it has less to do with competition for resources and more to do with light quality.”

When seeds are about to germinate, they can sense sunlight. The seeds can tell if there is nothing growing above ground. 

However, when there is a crop or another weed above that seed, certain wavelengths of light go through or are reflected off the plant. The seed can sense that and will wait to germinate. 

“Having an established crop is your biggest prevention,” he said. 

Fall is another prolific time for weeds.

Following the last cutting of the season, weeds can grow all winter and be well established once the alfalfa comes out of dormancy.

Weeds also surface between cuttings if there is rain or irrigation.

Also, as an alfalfa crop ages, the stand becomes thinner, which increases potential for encroaching weeds.

Weed management in a new alfalfa stand should begin well in advance of seeding, he added. 

Why control weeds?

Under certain circumstances, weeds can reduce yield, Lawrence said. 

However, before spraying weeds, make sure it makes economic sense. Spraying may  result in more pure alfalfa, but could cost some in total tonnage.

Weeds can also reduce quality.  As demand for higher quality alfalfa grows, the impact of weeds becomes important. One Nebraska study showed the first cutting of alfalfa without weed control had a relative feed value of 170, while another field with herbicide applied had an RFV of 200. 

However, the second and third cuttings evened out with similar RFV scores.

It also depends on the weed. Perennials like hoary alyssum, curly dock and yellow rocket can have a bigger impact on quality; lambsquarters and pigweed can accumulate nitrates in the feed which, if high enough, may cause some toxicity issues depending on the end market, Lawrence said.

Hay movement can spread weeds. After the wildfires last year in Montana, ranchers from across the nation sent hay. 

However, “almost every sample had a weed in there that hadn’t been introduced to Montana before,” he said. 

Weed control

Mowing is often recommended for broadleaf control in the establishment year. 

Another possibility is to harvest early to prevent weeds from going to seed.

“That’s difficult to time and it creates more opportunities for weed invasion,” he said.

When considering using herbicides, think about what stage the weed is in, Lawrence said. 

“It is important to know what your target weeds are, what stage you want to put the herbicide down on and what your (herbicide) options are.”

Amy Bickel can be reached at 620-860-9433 or abickel@hpj.com.

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