Fall harvest management of alfalfa after a dry summer
Marvin Hall, Forage Specialist, Penn State University
During September and early October, weather conditions are usually favorable for alfalfa growth. Increased emphasis on production coupled with high hay prices are causing alfalfa producers to consider harvesting during this period. When making the decision to take a fall harvest or not it is important to understand the risks and possible economic benefit of harvesting during this period.
Importance of fall to alfalfa survival
During the fall, winterhardy alfalfa plants develop cold resistance in response to shortening day lengths and cool temperatures. This process, known as hardening, involves reduced herbage production and increased carbohydrate storage in the roots. Carbohydrates are produced in green alfalfa herbage. In the spring and after cutting, carbohydrates are removed from the root until 6 to 8 inches of herbage is present, then carbohydrates begin to accumulate again.
Winter survival of the alfalfa plants and spring regrowth are dependent on adequate carbohydrate storage before cold temperatures kill the herbage in the fall. About 50 percent of the stored carbohydrates are used during the winter for plant survival. The remaining carbohydrates are available in the spring for initial herbage growth. Cutting alfalfa in the fall may cause plants to regrow and expend some carbohydrates in the process. Subsequent regrowth often is insufficient to restore root carbohydrate levels by the first killing frost, which increases the risk of alfalfa winterkill.
Alfalfa stubble left in the field has the potential to catch and hold snow which insulates the crown and roots from extremely low air temperatures. Stubble may also serve to reduce the incidence of freezing and thawing, and disrupt ice sheets which may smother alfalfa plants.
Should I harvest alfalfa in the fall?
Since harvesting during the hardening period disrupts carbohydrate accumulation in the roots, there is increased risk of stand loss associated with fall harvesting. This led to the traditional recommendation not to harvest alfalfa 4 to 6 weeks before the first killing frost. However, advances in alfalfa breeding and overall alfalfa management allow greater flexibility in fall harvest management than traditionally recommended.
The real question is an economic gamble. Is the forage in the field worth the risk of reducing stand persistence? Alfalfa that has been drought- stressed during the summer has had extended periods between harvests and has had an opportunity to build up root carbohydrate reserves over a year with normal rainfall. It is still acknowledged that fall harvesting can stress the alfalfa plant and increase the potential for winter injury, but management practices and planning (as outlined below) can minimize this potential.
1. Young stands are less susceptible to winter injury due to fall harvesting than older stands.
2. Length of harvest interval during the growing season is more important than date of fall harvest in determining alfalfa stand survival. Taking at least one harvest during the growing season at 50 percent bloom reduces the risk of fall harvest compared to taking all harvests at late bud or first flower.
3. Maintaining soil fertility levels is extremely important in reducing risks associated with fall harvesting.
4. Use of disease-resistant and winterhardy alfalfa varieties will greatly decrease the risk of winter injury due to fall harvesting.
5. Environmental conditions will moderate or aggravate the effects of fall harvesting. These include:
a. Temperatures of 5 to 15°F will injure alfalfa crowns and roots. Fortunately, the soil and snow generally insulates the crown and root from dangerously low temperature.
b. Amount of snow cover determines the amount of crown and root protection from cold temperatures. Snow cover greater than 6 inches is usually adequate to insulate the crowns and roots from low air temperatures.
c. Soil moisture is important because water is a better conductor of heat than is air. Wet soils cool faster than dry soils, which may lead to lower soil temperatures. In addition, wet soils will freeze and thaw more extensively, which increases the potential for frost heaving.
d. Summer drought stress of alfalfa will reduce the risk of harvesting during the 4- to 6-week dormancy period.
Summary
Although fall harvesting increases the risk to long-term stand persistence compared to not fall harvesting, it should be considered a management alternative to allow flexibility in alfalfa harvest scheduling. Fall harvesting of alfalfa that has been drought-stressed will cause less stand thinning than fall harvesting alfalfa that has not been drought-stressed.
Risks to stand persistence can be reduced through management, use of alfalfa varieties with sufficient levels of disease resistance and winterhardiness and high levels of soil fertility (especially potassium). Practices such as leaving 6 inches of stubble in the fall or leaving unharvested strips in the field will also help by increasing the potential for snow cover. HG
—Excerpts from Penn State Field Crop News, Vol. 7, No. 25
