Older alfalfa stands can produce high yields — when growers manage them carefully, say Purdue University experts.

But how long can a grower keep a stand? “It depends,” says forage agronomist Keith Johnson, “on the stresses a crop has or hasn't experienced. In my opinion, there is no predetermined formula on the day of seeding that tells one this is the year to rotate to another crop.”

“One ought to know what type of yield is needed to justify keeping a stand an extra year by working through an economic analysis of crop value and expenses,” Johnson suggests. “It's darn costly to establish a crop of alfalfa and we've got producers who are still producing 6 tons of hay in year four. They sure as heck are not going to plow it up just because it is the end of year three.”

Johnson was referring to University of Wisconsin research showing the economic value of keeping stands only for the establishment year plus two. (See “Short-Lived Alfalfa Delivers More Profit,” March issue, page 6.) He doesn't imply that the analysis of that specific set of data might be incorrect; he just cautions that longer, well-managed stands can also be profitable.

Eight years of Purdue research have shown that stands stay productive longer with the right amounts of potassium (K) and phosphorus (P).

“To maintain a stand of alfalfa for a long time, balanced nutrition is important,” says agronomist Jeff Volenec. Volenec studied plots provided zero, 22, 44 and 66 lbs/acre of P in combination with zero, 90, 180, 270 or 360 lbs/acre of K.

The soil the alfalfa was seeded into was low in both nutrients, he says. “The first, second and third production years, when we added good rates of P and K, we were able to increase yields 30-40%. In their seventh year of production, those well-fertilized plots are still doing nearly as good as the unfertilized plots did in their best year. So we're still getting reasonably good yields if we keep fertility high.”

Some plots were getting one nutrient and not the other, he says. “That's when plant counts really diminished quickly and yields went down. In years five to seven, yield and plant persistence were both lower in plots provided only P or K when compared to plots that were left unfertilized for the entire seven years of the study.”

Volenec recommends that growers test their soils and fertilize appropriately so alfalfa stands receive the right amounts of both P and K.

Another way to keep alfalfa stands productive is to harvest them at the appropriate time, Johnson says.

“I had a phone call from an agronomist asking how to get rid of summer-annual grasses in a three-year-old alfalfa stand. The 30-second answer is you apply Poast or Select herbicide at a certain rate and that's the end of the conversation,” he says.

But Johnson was interested to learn why those weeds appeared in the first place. He learned the crop was being harvested routinely every 25 days.

“The timeframe ought to be 28 to 35 days. That poor crop didn't have enough time to get vim and vigor to ward off those summer-annual grasses.”

Beyond low soil fertility and short harvest intervals, other stress factors reduce stand life, Johnson says, including:

  • Low soil pH. Alfalfa needs a soil pH of greater than 6.5 throughout the stand life, not just the establishment year.

  • Wet soil. Alfalfa cannot tolerate poor soil drainage conditions.

  • Insect damage. Scout frequently for damage-causing insects and use an integrated pest management program for their control.

  • Improper variety selection. Select varieties that are appropriate for your region and have resistance to disease problems that may be encountered on the soil type where they're grown.

  • Autumn harvest management. Reduce the risk of winter injury by harvesting the final cutting of the season at least one month prior to a killing freeze.