If there is a golden rule for alfalfa harvest, it's this: Don't do it while the plants prepare for winter.
That means allowing alfalfa to regrow for four to six weeks before the average first frost date, says Dennis Cash. If necessary, he adds, take a final cutting after the crop goes dormant.
“In Montana, that translates to: Don't cut or graze alfalfa between early August and mid-October,” says Cash, Montana State University extension forage specialist.
Late-season cutting can interrupt the plant's winter hardening process and increase the risk of alfalfa dying or suffering winter injury, says Cash.
According to him, it's a recommendation first presented 40 years ago by former University of Wisconsin professor Dale Smith in his textbook, Forage Management in the North. Smith cited research from the 1930s indicating the importance of not harvesting alfalfa from Sept. 1 to mid-October.
Do these guidelines hold true for new alfalfa varieties? Definitely, says Cash, who has conducted cutting-schedule research with new varieties since 2000.
“We have basically reproved the concept,” he says.
Cash reports that, in a three-year irrigated trial, half of the alfalfa plots were managed “normally” with three cuttings — two by Aug. 1 and a third in mid-October. The remaining plots were managed “intensively” with four cuttings. They had the same three cutting dates as the other plots, plus an additional harvest Sept. 1.
In the third year, 2003, the average yield of 18 varieties was 4.37 tons/acre in the normal plots and 2.68 tons in the intensive plots — a 39% difference. Harvest costs make the fourth cutting an even bigger loss.
“It's not that the intensively managed trial was cut an extra time — it's the timing of when it was cut,” says Cash. “In our region, that critical fall hardening period is a poor time to cut alfalfa — even if that's your first cut of the season — because of the impact on plant viability.”
Farther south, alfalfa cutting research in western Colorado by Tri River extension agent Bob Hammon is yielding similar results.
An entomologist by training, Hammon initially studied the effect of stem nematodes on alfalfa stand persistence. In reviewing research trials, he learned that late-season alfalfa cutting often has the same impact on plants as nematodes: irregular growth, stumping, swollen internodes, and plant death.
Concurring with Cash, Hammon says: “Timing of the final cutting has an impact on stand health.” If stem nematodes are also a concern, it can double the stress on a plant.
“After cutting, plant regrowth up to a certain point is not fueled by photosynthesis, but by the carbohydrate reserves in the roots,” he says.
Thus, if alfalfa is cut too late in the fall, root reserves are depleted for regrowth without enough time to store adequate carbohydrates in the roots for spring.
Should producers give up that fourth cutting? Hammon's preliminary research indicates that skipping it can increase the following year's first-cutting yield by 20-30%. Delaying it for a few weeks is another option.
“After Nov. 1 in our region, plants are dormant,” says Hammon. “Then a producer can come back and make a fourth cutting or use it for livestock grazing without depleting root carbohydrate reserves.”
He and Cash both say these late-season cutting guidelines are for growers who want to maximize alfalfa stand life.
“If you're a cash producer who is replanting stands every three years, these guidelines may not be economical,” Hammon says.