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Fall alfalfa harvest requires risk assessment

Dave Wilkins Published on 14 August 2012
Alfalfa frost

Hay growers have traditionally been advised not to harvest alfalfa when the cutting is likely to take place four to six weeks before the first killing frost.

Harvesting at that time can put the crop at increased risk of winterkill.

Some experts now question the usefulness of that recommendation.

New disease-resistant and winter-hardy alfalfa varieties, coupled with today’s high hay prices, have allowed growers to be much more aggressive in their fall harvest management, says Marvin Hall, forage specialist with Penn State University.

For many growers, the benefits of a fourth or fifth cutting in the fall may outweigh the risk of winterkill.

“Think of the price of hay now compared to what it used to be,” Hall says. “It becomes an economic issue.”

Hay growers – and particularly commercial growers – seem willing to take on more risk in a marketplace where premium alfalfa can easily fetch more than $250 a ton.

“With the price of hay and the new varieties of alfalfa that are out there, you aren’t increasing your risk much by taking that fall cut,” Hall says. “They have developed alfalfas now that are much more winter hardy and (disease) resistant.”

Forage experts have long recognized the correlation between carbohydrate levels in alfalfa plants and winter survival.

Harvesting alfalfa just a few weeks before the plants are killed by a hard frost doesn’t allow enough time to replenish energy reserves in the roots.

About 50 percent of the stored carbohydrates are used during the winter for plant survival, experts say. The remaining carbohydrates are available in the spring for initial herbage growth.

While winterkill is still a concern, the four-week to six-week restriction probably makes sense only for the most cautious growers, Hall says.

“If they want absolutely the minimum amount of risk possible, that is still a sound recommendation,” he says.

Many of the new alfalfa varieties now available were developed to fit today’s more aggressive harvest management practices.

That doesn’t mean growers using them should ignore the local weather forecast when thinking about making a fall alfalfa harvest.

Halls says he probably wouldn’t take a cutting within two weeks of an expected hard frost.

If in doubt, a grower is probably better off waiting until after the first hard freeze to make a fall cutting. This will lessen the risk of winterkill.

Some growers with fairly tall stands may worry that by not taking a late-fall cutting they risk “smothering” the new spring growth. They may fear that the crop will lodge and shade out plant crowns, stifling new growth.

There’s no research supporting the so-called “smothering” effect and growers probably don’t need to worry about it, Hall says.

“When that alfalfa gets frosted, the leaves fall off and there is plenty of sunlight down in there,” he says.

“I’ve seen fields that haven’t been harvested all year and then the next year come back just gangbusters,” he says. “I don’t think it’s an issue.”

Growers with alfalfa-grass fields may have more cause for concern. That’s because tender grass buds that form in the fall need to bask in a certain type of light to grow.

“If it’s shaded, that grass doesn’t get light, and those buds can die off. But alfalfa isn’t one of those plants that has that problem,” Hall says.

He recommends growers adopt prudent management practices to minimize the risk of winterkill resulting from fall harvest.

The use of disease-resistant and winter-hardy varieties is a good start.

Plant health is tremendously important and it’s one factor that growers have some control over. A crop with good carbohydrate levels and good soil fertility is at far less risk of winterkill resulting from a fall cutting.

Soil fertility is extremely important in reducing risks associated with fall harvesting.

“There is a really strong correlation between potassium level in the soil and overwintering ability,” Hall says.

“If your potassium is below recommended levels, you run a higher risk of winter damage. If it’s healthy, it’s going to be storing more carbohydrates and getting ready for winter.”

Growers should also remember that young stands are less susceptible to winter injury due to fall harvesting than older stands.

“As the stand gets older, it becomes less tolerant (of late-fall cutting). It has diseases and becomes weaker,” Hall says. “So more aggressive cutting will take the older stand out quicker than it will take a younger stand out.”

If the stand is getting toward the end of its useful life, there’s little harm in being aggressive.

“If you are contemplating rotating it out and putting a corn crop in, then by all means push it for all it’s worth,” Hall advises.

Some forage experts also recommend taking at least one cutting during the growing season at 50 percent bloom.

Doing so may reduce the quality of that cutting but it will also reduce the risk associated with fall harvest compared to making all cuttings at the late bud or first flower stage.

“If you make one of those cuttings go a little bit longer, the crop will build up a little bit more carbohydrates and maybe have enough in the roots to get you through the winter,” Hall says.

Leaving six inches of stubble in the field in the fall can also help catch snow which serves as an insulator against subfreezing winter temperatures. Growers should avoid fall-harvesting fields that have a history of frost heaving or of accumulating little snow cover.

In the end, fall harvest decisions really hinge on economic considerations and how much risk a grower is willing to take, Hall says.

“I think farmers are getting more aggressive in managing and harvesting their alfalfa,” he says. “I don’t think that (fall harvest) is going to hurt the crop as much as people think.”  FG

Hay growers have traditionally been advised not to harvest alfalfa when the cutting is likely to take place four to six weeks before the first killing frost. Photo courtsey of FG staff.