Alfalfa growers will need to make a decision if they should take another cutting of alfalfa, and if so, when. The recommendation in the newly revised 15th edition of the Ohio Agronomy Guide is to complete the last regular harvest of alfalfa by Sept. 7 in northern Ohio, Sept. 12 in central Ohio and by Sept. 15 in southern Ohio. At this point, undoubtedly some alfalfa growers are saying that they have taken a last cutting at the end of September or early October without any harm to the stand. True though that be, the fact is that the last or fall harvest of alfalfa is a question of risk management. Sticking to the Ohio Agronomy Guide recommendations provides the least risk of an alfalfa stand suffering damage due to low root reserves. Later fall cutting dates increase the risk for stand damage.
All perennial forage plants including alfalfa use the fall period to build up carbohydrate reserves that keep the plant alive over the winter, provide sugars to keep the plant from freezing, and provide the energy needed to start spring growth. Photosynthesis is the process that produces the needed carbohydrate reserves. Photosynthesis requires green, healthy leaves to collect energy from the sun. The rate of photosynthesis in those leaves in the fall remains high. The issue with fall cutting is vegetative growth and recovery after a harvest cut is slow. There may not be time to regrow leaves and replenish the energy expended to grow them before a frost or cold weather ends the growing season. The worst time to take a last cutting is in the late September to later October period.
Growers also need to decide if they need the forage a last cutting will provide. If the last cutting is extra, just some insurance, the better decision may be to let the stand rest. Realistically evaluate the amount of growth out there. Is there enough forage to justify the cost of harvest equipment and labor? In terms of risk management there are a number of factors besides the last cutting date that need to be considered when trying to determine if and when to take a last cutting. Those factors include overall stand health, variety, disease resistance, insect stress on the stand during the summer, age of stand, cutting schedule/frequency, fertility, soil drainage and winter weather.
A vigorous, healthy stand is more tolerant of fall cutting than a stressed and weakened stand. This past year, many alfalfa fields were harvested under wet soil conditions. Ruts in the field and/or areas of soil compaction can reduce plant vigor. Damage to plant crowns from harvest equipment traffic decreases stand health.
What alfalfa variety is in the field? Are you choosing improved genetic varieties that have been selected to perform under more intense cutting schedules? Alfalfa varieties with high disease resistance and good levels of winter hardiness will be more tolerant of a fall cutting. Adequate fertility, especially soil potassium levels, and a soil pH near 6.8 will improve plant health and increase tolerance to fall cutting. Stands under three years of age are more tolerant of fall cuttings than older stands where root and crown diseases are setting in.
The cutting frequency during the growing season can affect the energy status of the plant going into the fall. Frequent cutting (30-day intervals or less) results in the plant never reaching full energy reserve status during the growing season. A fifth cutting taken in the fall carries more risk than taking a fourth or third cutting during the fall.
Another factor is soil drainage. Alfalfa stands on well-drained soils tolerate later fall cuttings better than alfalfa on moderately or poorly drained soils. Removing the top growth of alfalfa plants on heavy soils and poorly drained soils going into the winter increases the risk of damage from spring frost heaving. Winter weather is another risk factor. Plants with adequate carbohydrate reserves can withstand temperatures down to about 5 degrees F at the crown before damage occurs. For colder temperatures, plants are dependent upon snow cover for insulation and protection from damaging cold. How reliable is that snow cover? Will it be there if temperatures get to zero or lower? Open winters increase the risk of damage to late harvested stands due to the potentially lower carbohydrate reserves and less top growth.
Finally, if a last cutting is taken in that late September through late October period, leave some un-cut comparison strips in the field. Observe what happens in those strips vs. the cut portions of the field in terms of spring green up, vigor of new growth, number of stems initiated per crown and first cutting yield.