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Fall alfalfa management can make or break survival

For good winter survival and quick green-up next spring, that last cutting needs to be early enough for the alfalfa to be able to regrow and replenish root reserves, or be so late that it does not regrow much at all, reports Dan Undersander, UW-Madison forage specialist.

Undersander was part of a webinar sponsored last week by Professional Dairy Producers of Wisconsin (PDPW).

Undersander says producers tend to worry about alfalfa and winterkill, while actually of more concern is winter injury to this forage. In other words, the alfalfa is slower out of the starting gate and first-cutting yield is not as big as it could be come spring.

Alfalfa gets ready for winter by hardening or changing to its winter structure, says Undersander. How effectively the crop hardens depends on genetics and fall weather conditions.

The fatty acid composition of the alfalfa root changes during hardening due to the preferential accumulation of polyunsaturated fatty acids. The preferential deposition of linoleic and linolenic fatty acids causes an increase in the average number of unsaturated bonds in the fatty acids. Lipid metabolism of the roots is altered during hardening from saturated fatty acid to unsaturated.

More fall-dormant cultivars harden faster than less-dormant ones, i.e., 3s and 4s, versus 5s and 6s, respectively. Many farmers lean toward less-dormant types as they tend to recover faster after cutting and yield more, yet they also do not get ready for winter quite as quickly either.

Hardening starts when the crown temperature gets to 60 degrees Fahrenheit, with hardening happening fastest between 40 and 50 degrees Fahrenheit. Hardening is actually increased by fluctuating temperatures of daytime highs around 50 degrees Fahrenheit and lows near freezing.

Long warm falls that end suddenly with a cold snap can predispose alfalfa to potential winterkill due to lack of hardening. The good news is that while in older varieties, fall dormancy and winter survival traits were highly related, today’s newer varieties are winter hardy at higher fall dormancy values.

During an untimely winter warm-spell, de-hardening can threaten alfalfa. The threat is when temperatures are more than 60 degrees and the soil temperature at 2 to 4 inches is more than 40 degrees Fahrenheit.

As the industry moves toward planting alfalfa with a fall dormancy rating of 5, versus a 4 rating, there is more risk of the crop breaking dormancy during a week of unseasonably warm weather during winter.

Fall cutting management influences alfalfa tap root reserves and winter survival.

The traditionally recommended no cut window for Wisconsin has been from Sept. 1 to a killing frost; however, it’s now known that the timing of the last cutting is not really related to a killing frost, but rather determined by growing degree days (GDDs).

GDD equals (daily maximum temperature minus daily minimum temperature) minus 5, all divided by 2.

Alfalfa needs 500 GDDs (base 41 degrees) until a killing frost (25 degrees) after the last cutting of the season in order to regrow sufficiently for good winter survival and yield next year.

In other words, date is not as important as temperatures following the last cutting and how much the alfalfa regrows. This means Wisconsin growers can cut late as long as it looks like 500 GDDs will accumulate and not hurt winter survival.

On the flipside, growers also can safely take a late cutting if it looks like little regrowth will occur, in other words, if it looks like 200 GDDs or less will occur. The alfalfa will not use up significant amounts of root carbohydrates by regrowing.

It’s important to remember that growers do not need to wait for a killing frost to make that final cutting, says Undersander.

They must only wait until its cool enough so little-to-no regrowth occurs, or, as noted, it’s warm enough so that there is sufficient regrowth prior to that killing frost.

In summary, farmers should aim for more than 500 GDDs for good growth and recovery, or less than 200 GDDs, for little reserve usage and regrowth.

He also mentions that leaving residue improves overwintering, as it provides some insulation for tender crowns and helps hold the snow, too, which offers further insulation.

At Eau Claire, Marshfield and Plymouth, 100 percent, 97 percent and 93 percent of the time, respectively, 500 GDDs are accumulated after Sept. 1, based on 42 years of tracking the data.

The probability of 500 GDDs accumulating before a 25-degree frost falls to 60 to 70 percent 1 week later. Thus, for farms at those locations, not harvesting after Sept. 1 is the safe alternative, but oftentimes, being a week later was not detrimental.

The last half of September is the riskiest in those areas, with low probability of either more than 500 GDDs or less than 200 GDDs accumulating. Waiting until mid-October is often safer at those locations, whether or not a frost has occurred.

As for southern Wisconsin, based on UW research at its Lancaster and Beloit trials, 500 GDDs or more always accumulate after Sept. 1. While the probability remains 100 percent for 500 GDDs or more at Beloit, it falls to 74 percent at Lancaster by Sept. 8.

The middle of September through mid-October is the riskiest time to cut alfalfa in southern Wisconsin, based on 42 years of weather monitoring.

Because alfalfa forage quality changes little during September, Undersander says harvesting versus delaying harvest should be based on the likelihood of winter injury or survival, if the stand is to be kept.

Growers need to select winterhardy varieties for less chance of winter and less yield loss at first cutting due to winter injury, i.e., buds from the fall killed and spring growth delayed and reduced.

Alfalfa with a winter survival score of 2 or less is very winter hardy, and at Arlington and Lancaster, also came in with higher season-long yield in research trials. Couple a winter survival score of 2 with a fall dormancy of 4 for southern Wisconsin, where decent snow cover can be fickle.

Alfalfa growing on low pH soil also has greater risk of winterkill or injury. Undersander recommends fall fertilizing with potassium and sulfur to increase alfalfa’s chances during the winter. This application can be done into October, as long as the ground is not frozen. He adds that 50 to 55 pounds of potash are removed with every ton of harvested forage.

Finally, there is lots of interest in alfalfa-grass mixtures for feeding cows. Undersander looks at fall management of those mixes. If a stand is thin, growers can seed tall fescue or meadow fescue into it late summer, early August is best for doing so.

If a stand has more than 50 percent alfalfa, manage it as you would straight alfalfa, he advises. If the stand is less than 50 percent alfalfa, manage it as if it were a grass stand. The recommendation for grass stands is to cut late in the fall to have less than 6 inches of growth going into winter.

That is so the grass does not mat down, creating snow mold and spots that die out.

(Source – http://agronomy.wisc.edu/2014/09/04/fall-alfalfa-management-can-make-or-break-survival-says-dan-undersander/)

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