We are receiving reports and have personally observed fields of alfalfa injured by the excessive soil wetness the first half of this growing season. Generally, waterlogging injury appears as stunting with a general yellowing of the entire plant, a result of nitrogen deficiency due to inactive N-fixation in waterlogged soils. Anaerobic respiration occurring in the plant under low soil oxygen also produces compounds that are toxic to the plant.
You should evaluate your stands in the next few weeks by counting the number of live plants per square foot, based on the age of the stand. The following table is taken from an Iowa State University article:
Plants per Square Foot
|Stand Age||Good||Marginal||Consider Reseeding|
|Year after seeding||+12||8-12||Less than 8|
|2 years||+8||5-6||Less than 5|
|3 years||+6||4-5||Less than 4|
|4 years and older||+4||3-4||Less than 3|
While plant counts are useful, to get a true determination of stand health, plants must be dug up so that crown and root tissue can be evaluated. To do this you must split the crowns/roots. The inside should be a creamy white color. If it is yellowish brown to chocolate brown color, the tissue is damaged or dying. If more than 50% of the roots show these symptoms, reduce your stand counts.
You will likely find some roots and plants that are completely dead, while on others the crown tissue will appear healthy but with roots that are rotting off 4-6 inches deep. We have also seen plant death in wheel tracks where harvesting operations occurred on wet soils, as illustrated in the photo (Photo courtesy of R. Lewandowski).
So what should be done about these stands? The answer of course depends of the degree of permanent damage and how much of the field was affected. Stand recovery over the coming weeks will help you decide whether the field is worth saving, needs nothing but some TLC, or if interseeding other species will help extend its useful life. Last week I discussed alternative forage options for producing supplemental forage. This article discusses management of alfalfa stands to help them recover and interseeding options to enhance forage yield of thinned stands.
With drying in the past week, we have observed remarkable recovery in some fields. Plants are gaining height and the normal green color is returning, indicating that N-fixation is operating normally once again. Plants with healthy crowns and intact upper root tissue will recover as soils drain, even if the taproot was rotted off a few inches below the crown. Assuming no serious pathogens are present, those plants will eventually produce a branched root system from the point where the taproot rotted off, which will function adequately as long as soil moisture is not limiting. However, when moderate drought stress develops in the future, those plants will stop growing sooner compared with plants having deep, healthy taproots.
You will probably see spatial patterns of poor growth under drought stress that reflect the stress patterns you observed during the waterlogging stress (i.e. severe waterlogged areas will have shorter growth under drought stress because of lost taproots). If the variety is susceptible to Phytophthora root rot, Fusarium root rot, Fusarium wilt, or other soil pathogens and if those pathogens are present in the soil, plant death will continue to occur and will become even more evident if drought stress develops in the next couple of months.
DO NOT cut alfalfa stands that are severely stressed in an attempt to make them grow back more quickly. The opposite could happen, because cutting and energy use for regrowth is an additional stress to the plant. Wait for plants to gain back a healthy green color with new shoots and new leaf growth before harvesting. Wait at least until the normal cutting interval has passed. If the normal cutting interval is well passed, then wait for the stand to begin to recover and cut when soil conditions can support equipment without damaging plant crowns. Remove harvested forage from the field as quickly as feasible. Consider harvesting silage or balage to reduce the time the cut forage lies on the field.
Check the stand for potato leafhopper numbers and lower the action threshold in stressed alfalfa stands to half the normal, as shown in the factsheet found at http://ohioline.osu.edu/ent-fact/pdf/ENT_33_14.pdf. It is likely that leafhopper feeding is occurring, which is an additional stress on the stand.
Alfalfa growers are also wondering if anything can be seeded into existing alfalfa stands to increase yield this year and next. First of all, seeding alfalfa back into alfalfa is not recommended, unless it is a 2015 spring seeding. Autotoxic compounds are released from older alfalfa plants that inhibit germination and growth of seedling alfalfa.
The options for interseeding into existing alfalfa stands in late summer are red clover, Italian/annual ryegrass, perennial forage grasses, and oat or spring triticale. The success of interseeding depends greatly on the existing stand density (existing competition) and soil conditions for seedling emergence and growth.
Red clover and other legumes will produce higher quality forage than grass interseedinged into alfalfa, but grasses are likely to actually improve yields more than red clover. Italian/annual ryegrasses have strong seedling vigor and varieties exist that are more likely to survive the winter and contribute to forage yield next year (see last week's article on supplemental forage options, in CORN 2015-22). Ryegrasses are more difficult to cut with sickle bar mowers and they dry more slowly than other grasses, so they are a better option for those with disk mowers and balage or silage harvesting capability.
Orchardgrass and even low-endophyte tall fescue can be interseeded into alfalfa; however, they are slower to establish, especially in late summer seedings. They will not contribute to yield this year, but assuming they establish this autumn, they will contribute to yield next year and beyond. Perennial ryegrass in northern Ohio and festuloliums establish quickly, but the same cautions apply concerning slower drying rates and difficulty with sickle bars as with Italian/annual ryegrass.
Oat, spring triticale, and field peas can be interseeded in August to contribute to forage yield this coming autumn. Winter triticale, winter wheat, winter rye, or barley can be interseeded in autumn to contribute to first harvest yield next spring.
Interseedings should be completed right after the existing stand is harvested, to minimize the competition from the existing stand during the germination phase. Seeding rates should be from half to normal full stand rates for the species being used, depending on the density of the existing stand (use higher rates in thinner stands). Density and vigor of the existing stand will determine whether interseeded species will establish at all. If new seedlings are shaded completely, their establishment will be poor.
The University of Wisconsin forage website has a good article on interseeding into existing alfalfa stands, available athttp://fyi.uwex.edu/forage/files/2014/01/Thickening-Alfalfa-StandsFOF.pdf. The Wisconsin article discusses interseedings made in the spring into winter-injured alfalfa. Keep that in perspective, because not all options discussed apply to late summer interseeding (e.g. seeding oat followed by sorghum-sudangrass). Seeding costs in the article reflect 2003 prices.