For years, forage agronomists have been telling growers that, when an alfalfa stand has fewer than 55 shoots per square foot, they should expect yield reductions. And that when shoot density falls below 40/sq ft, the stand probably should be replaced.
Those figures are wrong, says Jeff Volenec, Purdue University agronomist.
“Actually, some of our highest yields over the years have been obtained with shoot numbers that have been around 30/sq ft,” he says. “It's shoot size that drives yield. Weight, or what we call mass per shoot, determines yield more so than number of shoots per square foot.”
Volenec started studying how fertility might affect forage yield on three acres of alfalfa seeded in 1997. But he was also curious about the relationships between shoot density, shoot mass, and yield. So he and his research team counted shoots (stems and leaves) in 100 plots over four cuttings per year during an eight-year period.
“Our data are very consistent. The only time we get 50 shoots per square foot or more is when we have very small shoots. When you get a lot in a square foot — and you get really small shoots — you have poor fertility,” he says.
Stands with small shoots had some potassium (K) and no phosphorus (P), or were unfertilized. “As we look at well-fertilized plots, the shoots get large — around 30/sq ft — and yields are very high.”
The highest yields, from plots fertilized with both P and K and shoot densities in the 25-39/sq ft range, were 2.92 tons/acre from first cuttings of the second and third years of production. In unfertilized plots with the same shoot densities and during first cuttings of the same production years, yields were 2.82 tons/acre.
In both unfertilized and fertilized plots, mean yield did not increase when densities increased from the 25-39/sq ft count to 40-55. A slight yield increase was seen in fertilized plots when densities increased to over 55/sq ft.
“As you fertilize the plots, shoot growth really takes off,” Volenec says. “Shoots get large and robust, and you can't physically pack as many in a square foot as you can under poor-fertility conditions.” The increase in shoot mass more than offsets the reduction in shoot numbers, so forage yield is greater.
So what does this mean for growers wondering when to replace stands?
“If a grower is fertilizing well, taking care of soil acidity, phosphorus and potassium levels, our data suggest that stands even with as few as 25-30 shoots/sq ft are fine,” Volenec says. “Farmers need to be cognizant of their fertility levels, know how their stands perform over time and know what is a reasonable yield.
“Most know what a hayfield yields year in and year out and when they're getting a few wagons short coming off the field.” That's when they should consider whether the reduced yield resulted from thin stands or from other factors, such as the weather. Then they may want to count shoots. If they're lower than 30, with increased weed growth, they may have to replace the stand.
If a stand hasn't been fertilized properly, the scenario is different, he adds. Then stands with fewer than 40 shoots/sq ft show a pretty consistent decline in forage yield. Because they are nutrient-deprived, shoots can't get bigger. Quality may increase slightly with small shoots, but yield drops dramatically.