Alfalfa growers who take a September cutting must accept some risk that next spring’s regrowth will be affected, says Tim Dietz, a Michigan State University (MSU) Extension agronomist.

Research from the 1980s showed that alfalfa’s spring regrowth rate is largely a function of temperature and carbohydrate root reserves when dormancy was induced the previous fall, he notes. More recent research has shown that nitrogen root reserves play a greater role in regrowth and levels increase faster than carbohydrates.

“Regardless of the form of the energy, it is important that these reserves are available in sufficient quantities in the spring,” says Dietz. “Alfalfa has little photosynthetic (leaf) area to draw from in the early spring, so it relies on energy reserves in the crowns and roots.”

When alfalfa reaches 8” in height, its roots and crowns have the minimum level of energy reserves needed for maximum spring regrowth, he says. So growers only want to harvest in September if they’re reasonably certain that the crop will have enough growing degree days (GDD) to grow 8” tall. Under normal soil moisture conditions, 500 GDD are needed. Energy reserves are also maintained if the crop doesn’t regrow more than 2” after a fall harvest. To limit growth to 2”, less than 200 GDDs should accumulate.

“Like so many actions that we take as producers, the outcome is largely dictated by the weather,” he says. “We can do an excellent job of managing our crops, but abnormal weather conditions turn the hard work and preparation into an exercise in futility. We can, however, utilize regional historical weather data to make an informed decision.”

MSU meteorologists and agronomists developed probability charts to help producers in various regions of the state decide when to harvest. The charts show the probability that more than 500 or less than 200 GDDs will accumulate after a range of fall cutting dates. They can be found on the MSU Forage Information Systems Web site.