The increasing cost of forages, fearing continued 2012 droughty conditions, and extending the winter season has many producers wondering about the productivity of their hay fields in 2013. While temperatures have been respectively cold this winter, the good news is that an insulating layer of snow has persisted across most of the state for an extended period of time.
Factors affecting winter survival
Photo 1. Spring alfalfa regrowth in research plots located in West Central MN. Stems/ft2 can be counted and stand performance estimated.
Overwintering success of forages is typically a combined function of climatic conditions and management decisions. Environmental factors such as snow cover, lethal temperatures, ice sheeting and fluctuating air temperatures play important roles in plant survival from one growing season to the next. These effects are heavily influenced by stand age, variety genetics, fertility programs, and harvest schedules.
- Snow cover - defined as six or more inches, can protect plants from low temperatures and the thawing and freezing cycles that can break plant dormancy. Snow cover this year has been good in many regions of the state; however, in January some region's snow melted exposing the plants.
- Lethal temperatures (typically believed to be below 15 F over a period of time) are those cold enough to freeze the crown (below ground) at a cellular level causing injury or plant death. Desiccation is the result of plants exposed to winter winds in which moisture is pulled from the crown resulting in injury or death. Fortunately, we have had few periods of low air temperatures during our periods without snow cover.
- Ice sheeting can occur when snow melts and the water later refreezes. Ice sheeting typically occurs in lower areas of the field and on poorly drained soils. Ice sheeting kills plants by allowing deep freezing of the soil and also by smothering. Significant rain events in January and February across much of the state could contribute to this problem.
- Soil moisture: Low levels of soil moisture in the fall are actually conducive to hardening of plants which enhances winter survival. Therefore, although forage growth in the fall is reduced (as in fall 2012), winter survival can be increased. Conversely, very wet soils are susceptible to flooding and ice sheeting. Both of these could be a concern in spring of 2013.
- Air temperature fluctuations: Air temperature fluctuation can affect winter survival two ways: soil heaving and breaking dormancy. Heaving typically occurs on high moisture holding soils during the spring. As freeze/thaw cycles shrink and swell the soils, tap roots are broken and plants "popped" out of the ground. Seldom do any of these factors appear across an entire field but are more localized and based on field topography. South facing slopes may be more susceptible to heaving. Air temperatures above 40 F can also cause forages to break dormancy. Actively growing plants are more susceptible to freezing if low air temperatures occur later.
Some of the significant management factors affecting winter injury include:
- Species and variety selection: Grasses and legumes differ in genetic winter hardiness. Even within species, some varieties may be more susceptible to winter injury.
- Soil fertility: nutrient deficiencies can stress plants and make them more susceptible to winter injury. The risk of winter injury is reduced when recommended levels of soil pH, P, K, and S are maintained. Conversely, high levels of N fertilizer applied to grasses can reduce their winter survival.
- Cutting management: Fall cutting that removes regrowth or causes stimulation or new growth in the fall can reduce winter survival. In addition, removal of fall stubble can reduce snow catch and subsequent insulation.
Assessing alfalfa winter survival
For years alfalfa predictive productivity was based on plants/foot2. It was understood that a minimum of three to four plants/foot2 must be present for a stand to still have good production potential. University of Wisconsin research has found that stem counts are a much more accurate method of estimating the yield potential of an alfalfa field than plant counts. Stem counts better represents total production in a given area and accounts for variability in plant performance. Producers should use stem counts to estimate current yield potential and assess root and crown health to determine future yield potential. Stand health based on stem densities per foot2 can be assessed in the following manner:
- Greater than 55 stems indicates density will not be a limiting factor,
- Between 40 and 55 stems is understood to represent some reduction in yield but probably more than adequate in years of low inventories and high value, and
- Fewer than 40 stems indicate a poor stand and consideration for termination.
Photo 2. Alfalfa taproot and crown split for visual observation of health with regrowth shoots from buds
To assess root and crown health, dig up six inches of taproot material in three to four locations of a field. Split open the taproots to determine crown and root vigor (Photo 2). Look for healthy, off-white material indicating strong, healthy plants. Discoloration and spongy material are typical of weakened crowns. The symmetry of shoots growing from the crown also contributes information about overall plant health. While evaluating stand and yield potential, we must keep in mind contributions made from grasses. Presence of grasses may be great enough to justify keeping a marginal stand in production.
While most of the information presented is review and familiar to many, it is suggested that the same assessments should be made on second crop regrowth. Watch for slow green-up, uneven stands, or additional plant mortality as winter injury and/or death can be delayed by the shock of cutting a weakened plant, resulting in additional stand losses. It has become an acceptable practice for producers to take the first cutting of a compromised stand and then take it out of production in favor of corn for silage, soybean, or an annual forage crop. The constraint is time and the shortened growing season. This can somewhat be offset by a practice utilized in South Dakota. Growers spray alfalfa with glyphosate prior to harvest, getting good kill rates while not extending time or reducing soil moisture by waiting for regrowth and burn down. Remember that a crop following alfalfa is more susceptible to drought injury.
Whenever stand diminishes to the point of needing corrective measures, take time to determine the cause. Troubleshooting problem fields and identifying the cause of stand reduction can lead to better management decisions. Adjustments to variety selection can be made if winter-kill or diseases are problems. Management practices can be modified if age, fertility, and/or harvests have led to production disappointments.
It is challenging to predict winter survival and growing season productivity because of the many factors that affect survival. For the spring of 2013, we feel there is a good chance that winter injury will be minimal. However, the validity of that prediction will be greatly influenced by the upcoming spring. Flooding and ice sheeting are still a major concern.