Navigating Winter Injury in Alfalfa Stands: Assessment, Management, and Interseeding Strategies

February 28, 2024 5:25 PM
Blog Post

High February temperatures have prompted many alfalfa fields across the state to initiate greening up over the past couple of weeks (Figure 1). However, temperatures then plummeted (9°F) earlier this week, likely resulting in the demise of the new growth. But the more pressing question is whether this has led to stand damage or crown injury. Furthermore, this winter has witnessed numerous warm and extremely cold days. As spring approaches, evaluating your alfalfa stand for winter injury and potential yields becomes crucial to inform rotation and interseeding decisions.

An alfalfa stand in southeast Iowa starting to green up
Figure 1: An alfalfa stand in SE Iowa starting to green up. The photo was taken on February 26.

Stands that are more prone to winter injury include older stands, which are particularly vulnerable, stands with a low pH (< 6.5), and those with low fertility. Maintaining a neutral pH of around 6.6-7, ensuring high fertility—especially potassium levels—in well-drained soils, and cultivating winter-hardy varieties can help mitigate the risk of winter injury in the future.

Symptoms indicating winter injury encompass a slow and uneven greening up. An initial assessment involves comparing your fields with neighboring ones; if neighboring fields are greening while yours lags behind, conducting a stand health evaluation is warranted. Uneven greening, where some crown buds perish while others endure, leads to stands with irregular height and distribution. Additionally, root damage serves as a reliable indicator of winter injury. Inspecting root health by excavating and examining taproots can reveal significant indicators. Healthy taproots exhibit a white, firm appearance, while those affected by winter injury may appear grayish, water-soaked, or even blackened. It is important to cut the root in half to notice the discoloration (Figure 2).

Figure 2-A: Healthy alfalfa root (below).

Healthy root.

Figure 2-B: Unhealthy alfalfa root (below).

Unhealthy root
Figure 2: A) Shows the cross-section of a healthy alfalfa root. B) Depicts a cross-section of an alfalfa root with moderate root damage.

Upon identifying winter injury, it's crucial to assess the field's productivity for the upcoming year. Fields affected by winter injury should undergo potential yield evaluation to determine if they merit continued production and, if so, do they need to be managed differently to optimize survival chances. Evaluating yield potential entails counting the number of living stems per square foot. Over 55 stems suggest no hindrance to yield potential, indicating a good stand. Conversely, fewer than 40 stems may necessitate stand replacement or interseeding (Table 1). Stand health assessment can also be gauged by the number of plants per square foot, with the target being 4-5 plants in established stands, although stem count has been shown by the University of Wisconsin to be a more precise predictor of yield potential.

Stand assesment with related actions.

Adjusting cutting schedules for winter-injured stands involves extending cutting periods to allow plants more time to replenish carbohydrates to the roots, thereby enhancing survival prospects. Whether extending maturity for each cut or solely the initial one depends on the severity of the observed winter injury. The more severe the injury, the more advantageous extended cutting periods for each cut become. Furthermore, raising cutting height when extending maturity is vital to accommodate potential new growth during the extended cutting period leading into flowering. Ensuring adequate fertility and weed control is also crucial to mitigate competition and aid in the recovery of injured stands.

Additionally, if stands are less than desired, another option for farmers is overseeding or interseeding to thicken an existing stand. With alfalfa, it is not recommended to reseed or interseed alfalfa into an existing stand that is two years or older because of autotoxicity with alfalfa. Rather, it would be recommended to interseed with another legume like red clover or cool-season grass, such as orchardgrass, or perennial ryegrass. The decision of what to use can be determined by your goals for the stand. If you only need to extend the stand for a year or two but need to maintain high quality and yield, red clover is a good option. However, if your goal is to extend the stand life for multiple years, adding a perennial cool-season grass is a better option. If you plan to interseed, this should be done by the first half of May as seeding later becomes riskier due to less frequent rains to help sustain young seedlings as they establish themselves.

 

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Rebecca Vittetoe Field Agronomist in EC Iowa

Rebecca Vittetoe is an extension field agronomist in east central Iowa. Educational programs are available for farmers, agribusiness, pesticide applicators, and certified crop advisors.

Areas of expertise include agronomy, field crop production and management of corn, soybeans, and...