Rapid Evaluation of Alfalfa Frost Injury

Canadex Field Crops, Alfalfa Culture 121.20, Agriculture Canada, Ottawa K1A OC7

Darryl G Stout and Barbara Brooke

Range Research Station, Agric. Canada, Kamloops, B.C.

John Hall

Research Station, Agric. Canada, Vancouver, B.C.

Ted Moore

B.C. Ministry of Agric. and Food, Kamloops, B.C.

We developed a field method to evaluate low-temperature winter injury to alfalfa (Medicago sativa L.). It can be used early in the spring as soon as the soil thaws, before much aerial growth occurs and before equipment can be used on the field, to allow time to make reseeding plans in the case of significant low-temperature winter injury.

Our method requires random sampling of alfalfa plants. To begin, pick a destination point on the far side of the field, walk toward the identified location. At regular intervals, such as every two or five paces, dig up a shovelful of soil where you stand. Inspect each alfalfa plant in the shovelful of soil as outlined below. Continue until you have inspected about 100 plants. The more plants you inspect, the more reliable will be the evaluation of injury.

Inspect and rate each plant for three characteristics – bud vigor, interior root color and resistance of the root bark to peeling. These characteristics are illustrated in the figure and the rating system is outlined in Table 1.

Follow this procedure to evaluate winter injury in your field:

• Using a method that will provide random samples, such as that given above, dig up an alfalfa plant.
• Inspect the plant and rate the three characteristics for injury. Locate the rating combination in Table 2 that best describes the plant’s condition, then put a check mark in column A beside that combination. Continue until about 100 plants have been inspected.
• Add up the check marks in each row in column A and put the total in column B.
• Add up all the number in column B by the survival probability for a plant with that combination of ratings (as shown in column C) and put the value in column D.
• Add up all the numbers in column D to get the total number of plants that are expected to live (L).
• Estimate the percent survival by dividing the number of plants expected to live (L) by the total number of roots sampled (S) and multiply by 100 (L/S X 100).

We developed this method of evaluating low-temperature winter injury using data from plots were injury ranged from 15-35%. It can only be used following winters where injury does occur. For example, in a mild winter where injury does not occur, one would expect nearly 100% survival. However, if you applied this method, all plants would receive the highest rating but it would be concluded that only 79% of the plants would survive. The remaining 21% represent healthy looking plants that end up dying when the winters are severe. In years when injury is not as severe as it was when we developed this method, it will be expected that the procedure would still predict the survival of visibly injured plants accurately. However, it might underestimate survival in plants with no visible signs of injury.

Table 1. Criteria for evaluating and rating alfalfa root vigor

1. Determine bark resistance to peeling by scratching the root surface with a thumb nail
2. Separate the symptoms of disease from those of winter injury on the tap root. For example, the dark brown streaks extending down from the crown caused by root rot are usually sufficiently decayed to distinguish them from winter injury lesions. Color often varies greatly with the distance from the soil surface; give a root that is firm and white at 15 cm deep a low rating if soft and discolor in the top 3 cm. Healthy lower root tissue is of no advantage if the upper root and crown are dead; alfalfa does not generally produce aerial shoots from root tissue.

Table 2. Calculation of expected survival rate based on a rating system using three characteristics (To demonstrate the calculation, we put theoretical values (checks) into column A.)

1 On a scale of 1-4, 1 = low and 4 = high

2 On a scale of 1-3, 1 = low and 3 = high

3 On a scale of 1-3, 1 = poor and 3 = good