(Shared by Marvin Hall, professor of forage management, Penn State Extension)
Not harvesting alfalfa 4-6 weeks (critical period) before the first killing frost has been a long standing recommendation. That is still very safe advice! However, the dry weather some areas experienced in August may moderate that recommendation. Perhaps the risk of a cutting during that "critical period" is outweighed by potential increased profits.
Below are some things to consider when deciding to take an alfalfa harvest during the "critical period". Assessing the risks can be helpful in making the best decision.
Age of stand: Older alfalfa stands are more likely to winter kill or suffer winter injury following a fall harvest than younger alfalfa stands.
Variety: Alfalfa varieties with moderate resistance to several diseases and sufficient winter hardiness have greater tolerance to stress from fall harvesting than less disease resistant or winter hardy varieties.
Soil pH and fertility: Adequate soil pH and fertility minimizes the risk of fall harvesting by allowing alfalfa plants to develop properly and be healthier.
Soil drainage: Alfalfa on well drained soils is less likely to suffer winter injury than alfalfa on poorly drained soils.
Harvest frequency: Alfalfa harvest schedules which do not allow the alfalfa plant to flower once during the season, predisposes the plant to winter injury.
Dry conditions in August: Dry weather, especially in August, causes alfalfa to store excess root energy reserves, making it more winter hardy.
Fall cutting height: Leaving six to eight inches of stubble when taking a fall harvest will reduce the risk of winter injury.