Evaluating and Managing Alfalfa Stands for Winter Injury

Dr. Heather Darby UVM Extension Agronomist

 Introduction

Many conditions occurring in the fall, winter or spring can have an impact on the winter survival or injury of perennial forage stands, particularly alfalfa. Therefore it is important to assess stands early in the spring and explore your options for managing winter injury.

How to Diagnose Winter Injury

First and foremost it is most important to determine if your fields were impacted by the winter weather. The most obvious sign of winter injury are stands that are slow to green up in the spring. If other fields in your area are starting to grow and yours are still brown, those stands should be checked for injury or death. In addition to “slow green-up,” fields with uneven growth patterns may also indicate damage.

winter_kill_alfalfa3

The best way to diagnose damage is to examine the plant roots in a suspect field. To do this, walk diagonally across a field and at regular intervals (every 4 to 5 paces) dig up a shovelful of plants (4 to 6 inches deep) and examine their roots. The roots of each plant should be firm and the interior color should be white or cream colored. If the roots are soft and the interior yellow to brownish in color it most likely was winter-killed. winter_kill_alfalfa4For alfalfa, the majority of healthy crown buds should be white or pink and firm throughout the bud. It is important to try and inspect as many plants as possible to determine the percentage of the stand and/or areas of the field that are injured.

 

Options for Fields Moderately Affected by Winter Injury

Winter-injured stands will require different management practices than healthy stands if they are to stay in production. If winter injury is evident consider the following:

Allow alfalfa plants to mature longer than normal before cutting. This will help the plants rebuild needed energy for future production. For severely-impacted stands, allow plants to go to full bloom before taking a first cut and to the early flowering stage for subsequent harvests. Increasing the cutting height may also help stands recover (Cosgrove and Undersander, 2003). New shoots will be developing at the base of the injured plants and it is important to not remove these shoots, which would result in further detriment. Lastly, do not cut winter-injured stands late in the fall; this will allow them to build up more reserves before winter.

If a significant loss of alfalfa was seen in a predominant grass stand, then you could manage it for grass. This will work best if the grass species are dominated by tall-growing species such as reed canarygrass, orchardgrass, and/or timothy. If the grass is less than 10 inches tall, it may still be economical to apply 50 pounds of nitrogen per acre to boost yield and protein. If the grass stand consists mainly of a lower-yielding forage such as “June” grass (bluegrass) you may want to consider replanting.

If the alfalfa stand was only partially injured (25 to 50 %), interseeding with a quickgerminating forage crop in thin spots could provide additional production (Stauffer, 2009). Interseeding can be done with a grain drill. No-till application into existing sods will provide the best results. In the event a drill is unavailable broadcast seeding is an option but generally results in a less uniform stand. Species that could be considered for interseeding include clover (4-6 lbs/acre), orchardgrass (8-10 lbs/acre) or perennial ryegrass (8-10 lbs/acre). Remember that perennial ryegrass maybe a short-term option since it does not overwinter well in all areas of the Northeast. Alfalfa should not be reseeded into the stand due to autotoxicity issues. When dealing with winter-injured stands, it is particularly important to adequately fertilize and control for weed competition.

Options for Fields Severely Affected by Winter Injury

If your stand was over 50% winterkilled, you may want to consider replanting. Depending on your needs, there are several forage choices.

A small grain/field pea mixture could be a good choice if the forage is needed in early- to mid- summer. Early-planted small grains (60 lb/acre) such as oats, barley, or triticale with the addition of field peas (50 lb/acre) will be ready for harvest between late June and mid-July. Research from the University of Wisconsin has reported yields between 2.5 and 3.0 tons/acre and Relative Forage Quality (RFQ) of approximately 100-125 (Undersander, 2003). Small grain/pea mixes should be harvested when the small grain is at late boot stage.

Corn silage will be the best choice for optimizing full-season forage production. If corn silage is planted by the end of June it will normally outyield most other forages; however you risk lower quality forage. At these later dates (mid-June to early July) you may want to consider planting a summer annual. A few options include sorghum-sudangrass hybrids, sorghum -sudangrass hybrids enhanced with the Brown Mid Rib (BMR) gene, forage sorghum, sudangrass, and forage millets. Recommended seeding rates for these species vary considerably by forage type and range from 25 to 60 lbs per acre. These forages should be harvested when they reach approximately 30 inches. It is important to note that these crops need high temperatures to yield well and may not be the best choice if growers are experiencing cool to average temperatures. Studies conducted at the University of Wisconsin have reported summer annual yields between 2 and 6 tons/acre and RFV between 90 and 100 (Undersander and Lane, 2001).

There has been a lot of interest in growing sorghum-sudangrass enhanced with the BMR gene. The BMR gene has a characteristic of reduced lignin content, and hence tends to be highly digestible. The seed should be drilled ¼ to ½ inch deep at a rate of 50 lb/acre and fertilized the same as corn. Harvesting should take place after the grass is 30 inches tall or just before heading (Undersander and Lane, 2001). If planting is done by the first half of June, a second harvest can probably be made in September. Reports from Cornell University show that the forage quality of BMR sorghum-sudangrass can have a digestible neutral detergent fiber concentration (dNDF) of almost 70%and crude protein (CP) of up to 18% (Ketterings et al., 2002).

References and Citations


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