Issue No. 5, Article 8/April 22, 2005
Challenges for Alfalfa Growers
Problems are being reported in alfalfa fields throughout the state, from the southern counties to the northern areas.
A quick survey of some counties in southwestern Illinois indicated that alfalfa weevils have wintered well and are poised to cause alfalfa growers some concern. Growers in that area need to be scouting fields now. In northern Illinois, scouting should probably begin within the next couple of weeks.
Alfalfa weevils overwinter as adults and begin to lay eggs as early as November; depending on the weather, they can lay eggs throughout the winter. Generally, the weevils become active in late March to early April. After the larvae have hatched, they feed on the inside leaves of the alfalfa plant, gradually moving to foliage on the lower portions of the plants. Early injury shows up as pinholes in the leaf terminals. Leaves are skeletonized as the larvae increase in size. Adults generally feed on leaf margins. Injured leaves dry out quickly, giving the field a whitish cast.
When scouting for alfalfa weevil, follow a U-shaped pattern, taking samples from representative areas of the field. Collect at least 30 stems along the U-shaped swath through the field. Shake the stems over a collection bucket to dislodge and count the weevils. When sample counts average three or more weevils per stem and 25% to 50% of the leaf tips are being skeletonized, a management decision must be made soon.
More detailed management recommendations can be found in the Field Crop Scouting Manual, Fifth Edition. The manual can be ordered through local Extension offices or online at U of I Publications Plus.
Another problem for northern Illinois alfalfa growers has been winterkill. The extent of this injury varies from field to field and within fields. Factors that contribute to winterkill are lack of snow cover, lack of crop residue, lack of winter-hardiness, low soil fertility, and low soil pH.
When alfalfa crowns are exposed to temperatures below 15 degrees, plants may be killed. Two other contributing factors are ice sheeting and stress on the plant caused by cutting during the "no harvest period" of September 1 to mid-October. Very low temperatures following ice sheeting can result in rapid killing of plants because cold temperatures are easily transferred through ice to plants. Ice sheeting causes suffocation, and plants can start to die in about a week. One way to reduce damage from ice sheeting is to leave 6 to 8 inches of stubble in the fall. The stubble will also increase the chances of "catching" snow, which acts as an insulator.
Plants suffering from winterkill have soft and fibrous crowns and taproots, and many times a distinct brown line is visible across the taproot 2 to 3 inches below the soil surface. In assessing alfalfa stands, it is important to dig some plants (include the top 6 inches of the root) and examine the crown for size, symmetry, and number of shoots. Split the crown and taproot lengthwise and check for the degree of discoloration, which is indicative of disease and/or injury.
In addition to assessing crown and root health, evaluation should also include plant or stem counts. Stem counts would be the preferred method (they relate better to potential yield), but alfalfa top growth needs to average at least 6 inches. The optimum is 55 stems per square foot. Consider replacing the stand if there are 39 stems or fewer per square foot. Plant counts per square foot can be used prior to alfalfa being 6 inches tall. Density less than 8, 5, 4, and 3 plants per square foot for 1-year-, 2-year-, 3-year-, and 4-year-old stands, respectively, will not yield well.
What are some forage options to seed into winterkilled alfalfa? Choices depend on the desired forage quality, yield, length of stand needed, and how the crop will be harvested (bale, silage, or grazing). Some possibilities include the following:
- Reseeding. If the stand is 1 year old or less, alfalfa may be reseeded. However, overseeding with additional alfalfa if the stand is over 1 year of age is not recommended because of the likelihood of autotoxicity. Red clover is not affected by the autotoxicity of alfalfa, so it can be used to "thicken" an alfalfa stand.
- Small grain (oat, spring triticale, barley) seeded solo or in a field pea mixture. Harvest should be based on the maturity stage of the small grain.
- Annual (or Italian) ryegrass (select rust resistant varieties) with or without clover. Depending on conditions next winter, ryegrass may not survive and grow in spring 2006.
- Sorghum-sudangrass hybrids (consider the brown midrib type), pearl millet, or sudangrass hybrids. All of these, except pearl millet, produce prussic acid.
- Corn silage
- Orchardgrass (select rust resistant varieties) and red clover
Each of these options has advantages and disadvantages. More information is provided in the Illinois Agronomy Handbook.--Mark Hoard and Jim Morrison