It’s almost September and time to decide when or if to take the last cutting of alfalfa. Start by considering if there’s enough forage already in storage to get the cows through the winter. Then take a look at your fields and decide if there’s enough alfalfa in the field to make it worth the time to harvest. And, if so, what are the risks to next year’s crop?

When to take your last silage cutting depends on your goals and individual field conditions. The rule of thumb is to not harvest alfalfa within six weeks of the first killing frost. Timing depends on your location, but typically frost starts hitting the Midwest in October, and the first hard frost occurs in early November. So avoid cutting after mid-September.

Silage growers should assess their needs going into harvest to determine how big of a risk they are willing to take versus what they will achieve with a later cut.

“I tend to visual the alfalfa season as a fuel tank in a car,” said Marvin Hall, professor of forage management at Penn State University. “If you cut after six weeks prior to the first killing frost, it lowers the fuel tank and impacts the sugars and carbohydrates that are stored in the plant, sending it into winter with lower carbohydrate storage, or a lower fuel tank, than what’s needed to get through the winter. As spring approaches, plants are weak due to low energy levels, making it difficult to begin vigorous growth.”

If choosing to cut late, Hall advises growers to increase the time period between their cuttings to minimize risk.

“The longer the plant has to rebuild carbohydrates, the better off it will be. If cutting is every 28 days, it doesn't get the fuel tank refilled each time before you cut it again,” he said. “By September or October, you’re going to go into the winter low. Low carbohydrates storage is generally associated with poor overwintering. So, the more frequently you cut, the greater your risk will be going into the winter.”

Soil fertility can interfere with plant growth and slow down the process of rebuilding carbohydrates. Also, moving into winter, day length begins to shorten and evening temperatures become cooler. The plant senses those changes and starts to change physiologically.

“In fall, varieties start to slow down vegetative growth, so the plant doesn't grow as tall. It may still have about as many leaves as a tall plant would have, but it’s carrying out photosynthesis and storing a lot of sugars in the roots and the crown, where it’s going to be used over winter,” he said. “Those sugars also help keep the plant from freezing. Plants that have adequately prepared for winter can withstand temperatures down to about 5°F.”

Soil moisture content also can reduce your risk. Lower-lying soils where moisture stays high will slow down the winter hardening or fall dormancy process, so that plants are less likely to survive winter and build up carbohydrates. Ensuring a high enough potassium level is also important for overwintering alfalfa.

The type of winter you experience is another risk factor. Snow cover provides great insulation, so it may be –20°F above a few inches of snow, but the soil surface and roots are still well protected. Open fields without snow cover, or parts of the country that don't get snow cover, but see fluctuations in temperature in the middle of winter and spring, can be very damaging to plants.