These days, Dan Undersander anticipates several forage-management questions from growers at fall meetings. At last week’s Feed & Nutrition Conference, hosted by the Professional Dairy Producers of Wisconsin, he got them all.

1) Should I interseed into thinning alfalfa seedlings?

“I would recommend that you simply disk it once and reseed the next spring. Or you could have done it this fall if we had had some rain,” said the University of Wisconsin Extension forage specialist. He doesn’t like interseeding into new stands – not because of autotoxicity, because that’s not a problem before alfalfa stands are well established, he said. If a grower does interseed into a young stand, some plants will already be growing and then new seedlings emerge. “You never get a good, thick stand. Whereas, if you disk it once and start over, you can have a good, higher-yielding stand.”

2) When should I take the last cutting off this fall?

If you’re confused about when to take hay off fields in fall without damaging your alfalfa stand, look at the calendar – but count growing-degree days (GDD) rather than going strictly by calendar date, said Undersander.

“We have always had a no-cut window in this (Upper Midwestern) region that, for example, a farmer shouldn’t cut after Sept. 1 until frost or thereabouts. But every time I’ve talked about that, someone stands up and says, ‘Well, I cut after Sept. 15 and it came through the winter just fine.’

“Sometimes it will; no question. Actually, 25% of the time it will. And 75% of the time it won’t, so how lucky do you feel?”

Undersander said alfalfa should be cut early enough in the fall to build up 500 GDDs, allowing it to regrow and replenish carbohydrates and proteins. Or it should be cut so late – to stay below 200 GDDs – that the alfalfa won’t regrow and use root carbohydrates needed for the next spring’s growth. A GDD is a measure of the amount of heat that accumulates above a specified base temperature during a 24-hour period.

“It’s not about a magic date,” Undersander added, “it’s about what the weather’s going to be like and whether or not the plant’s going to regrow.”

He compiled 42 years of weather data gathered at five Wisconsin sites to show the probability of the two fall-cutting options. He reminds that a 25°F temperature is needed for a killing frost.

At the Lancaster and Beloit locations, 500 GDD always accumulated after Sept 1, but by Sept. 8, the probability of accumulating that many GDDs dropped to 74% at Lancaster.

The middle of September through the middle of October was the riskiest time to cut alfalfa in southern Wisconsin in the last 42 years, his research shows.

At Eau Claire, Marshfield and Plymouth, growers could cut around Sept. 1 with a probability of 500 GDDs ranging from 100% to 97% to 93%, respectively. The 500-GDD probability dropped to 60-70% a week later. The last half of that month, the probability of more than 500 GDDs or fewer than 200 GDDs drops farther, and waiting until mid-October was safest whether or not a frost occurred.

“I would suggest that you’re ahead to play the odds and cut under a little better harvesting conditions than to wait for that 25° killing frost. But you have to do what you’re comfortable with.” For more on Undersander’s research, visit here.

3) What about grasses? When should I cut them?

“If you have more than 50% grass in a stand, you definitely want to cut that pasture or grassy hayfield,” the Extension specialist said. “Grasses tend to lay down and make a mat, and alfalfa tends to stay standing. Under those mats, there is a disease called snow mold that grows right at that freezing-thawing temperature range, right at 32°. The mold will kill spots ranging from 1’ to 10’ in diameter across fields and pastures.

“We recommend for all our grassy fields, that they be cut short – down to 3-6” – prior to going into winter. If you have more than 6” there, I’d think about grazing or cutting it,” he said.

4) How should I handle a field after an early killing frost followed by more warm weather?

“If the stems actually die back to the crown and you have that warm weather, then they’re going to come back no matter what you do, so you might as well cut if there’s enough tonnage to justify it. If they didn’t freeze all the way back to the crown, then they will continue growing and I would leave the stand. I recommend, when you have an early frost like that, that you watch for a couple of days and see if it just burned the tips or if it froze all the way down.”