Taking That Fall Cutting of Alfalfa?

With the sunny weather we have experienced this September, it has been very tempting to cut some forage for haylage or baleage. But how do we evaluate the risk this contributes to winterkill?

Alfalfa Winterkill In 2009

Alfalfa winterkill experienced across Ontario in 2009 was unprecedented. Contributing factors included saturated soils going into winter, poor winter hardening due to wet autumn conditions and an early onset of winter, very cold winter temperatures during periods of minimal snow cover, water ponding, ice sheeting, alfalfa crown and root diseases, and frost heaving. A major contributing factor was cutting alfalfa during the fall. With extended wet summer conditions and the need to make some quality forage, it is understandable why many took that risk. However, this was one year when this practice led to significant winterkill, even in areas where winterkill is less common.

Many fields that didn't winterkill have been weakened, and are at a greater risk of continued decline in plant health. Digging some alfalfa crowns and roots and doing an assessment for disease and plant health can help in making fall cutting and rotation decisions.

Critical Fall Harvest Period

In order to reduce the risk of alfalfa winterkill, it is generally recommended that alfalfa not be cut during the Critical Fall Harvest Period. This is the 6 week period (450 Growing Degree Days) before the average date of killing frost. This is the approximate time that a harvested alfalfa plant takes to regrow and store sufficient root reserves to survive the winter. Early in the period the alfalfa will use the existing root reserves for regrowth, "emptying the tank". Later in the period, the alfalfa stores photosynthesis produced carbohydrates as root reserves, "refilling the tank". Cutting in the middle of the Critical Period is usually higher risk than cutting at the beginning or end. The actual date of killing frost seldom occurs on the average date, so the beginning of the Critical Fall Harvest Period is a guideline only.

Even when winterkill does not occur, cutting during the Critical Period typically results in loss of vigour and yield the following spring. It can sometimes be difficult to observe, but still be significant. Research shows that the yield sacrificed by not harvesting during the Critical Fall Harvest Period is usually regained in first-cut yield the following year. The decision to cut should be weighed against the immediate need for forage. If you do decide to cut, consider leaving some check strips that you can use for comparison next year.

Other Contributing Risk Factors

Sufficient top growth produced during the Critical Fall Harvest Period is required to hold snow that helps insulate overwintering crowns against cold temperature damage. Insufficient top growth and snow holding capacity can also contribute to alfalfa frost heaving. Cut alfalfa initiates regrowth from crown buds and axillary buds, not the cut end of the stem, so cutting higher does not reduce usage of root reserves. However, cutting higher does allow for holding more snow as insulation.

Older stands with less disease resistance, low potassium soil tests, poor varieties and poor soil drainage all increase the risk of winterkill. Aggressive cutting schedules can also increase the risk. Cutting intervals of less than 30 days between cuts increases the risk of winterkill, while intervals over 40 days (allowing flowering), reduces the risk.

Some areas of the province, such as the Ottawa Valley, have a higher historical risk of winterkill. In situations where forage inventories are adequate, increasing the risk of winterkill by fall cutting is far less acceptable.

Late Fall Cuttings After The Critical Fall Harvest Period

Risk can be reduced (but not eliminated) by cutting towards the end of alfalfa growth, as close as possible to a killing frost. A killing frost occurs when temperatures reach -4 °C for several hours. After a killing frost, alfalfa feed value will quickly decline, as leaf loss occurs and rain leaches nutrients. Leaving at least 6 inches of stubble will help trap snow to insulate the alfalfa crowns during cold weather. Stubble will also protrude through winter ice sheeting, should that occur. Try to limit late cuttings to fields that are otherwise lower risk - well drained, good fertility and pH, etc.

Smothering?

There is always the question of smothering in heavy forage stands that are left unharvested. Heavy stands of grasses or red clover can sometimes smother over the winter because the top growth forms a dense mat. In contrast, alfalfa loses most of its leaves as soon as there is a hard frost, and the remaining stems remain upright and seldom pose any risk of smothering.

More Information

Refer to OMAFRA Factsheet 91-072 "Alfalfa Winterkill Risk Factors" at www.omafra.gov.on.ca/english/crops/facts/91-072.htm, or Figure 3 - 4 of the new Publication 811 "Agronomy Guide For Field Crops" for more information, including a map detailing the Critical Fall Harvest Period in your area.