Farming Magazine - May, 2008


Forages: What Time of Day Should You Mow Hay Crops?

By Everett D. Thomas

As I speak at meetings across the northeastern United States and eastern Canada, one of the frequent questions I get involves the best time to mow hay crops, especially alfalfa and alfalfa-grass mixtures. This is in part because of several press articles citing the advantages of mowing forages in the afternoon. What we know about plant physiology supports this recommendation, but there’s often a difference between theory and practice when it comes to on-farm applications.

It’s all about the sugars

Forages go through a diurnal (daily) cycle. During the day plants accumulate sugars, the concentration of sugars usually reaching their peak late in the afternoon. Sunny days result in higher sugar concentrations. During the night the plants respire, losing sugars. Plant sugar levels are usually lowest in the morning just before the sun comes up. The idea of mowing in the afternoon certainly isn’t new; 20 years ago farmers in southwestern United States were using refractometers (similar to what maple producers use to check the sugar content of sap) to determine the sugar content of alfalfa harvested for hay. More recent research in the Southwest, with alfalfa hay mowed in the morning versus the afternoon, sure seems conclusive: compared to the same crop mowed in the morning, alfalfa mowed in the afternoon had significantly greater sugar content and higher fiber digestibility, and when fed to dairy cows had better palatability and resulted in higher milk production. That’s a pretty powerful combination!

West vs. east

The first thing to consider when applying this practice in the Northeast is the difference in sunlight intensity between east and west. Anyone who has spent time in the arid Southwest can attest to the intensity of the sunlight there compared to here in the humid Northeast. More sunlight reaching the plant means a greater sugar buildup. However, even though the difference in sugar content might not be as great, research in the East has shown the same general trend—lowest sugar early in the day, highest late in the day, with little additional increase after about 5 p.m. We did two years of research at Miner Institute involving two alfalfa-grass fields, with two quadrants in each field mowed in the morning and the other two in the afternoon. Sure enough, alfalfa mowed in the afternoon had significantly higher sugar content than that mowed in the morning. Therefore, there’s at least the potential for farmers to take advantage of diurnal changes in sugar content—and maybe in higher forage quality.

Hay vs. hay crop silage

The research done in the Southwest focused on dry hay production, while much of the alfalfa and alfalfa-grass harvested in the Northeast is harvested as medium-moisture hay crop silage—also called haylage. By spreading out windrows to increase drying time, our farmers are often able to mow in the morning and chop at 35 percent dry matter that same day. However, mowing in the afternoon makes same-day chopping just about impossible; in our research we mowed at 3 p.m. and the crop was still less than 30 percent DM at sunset. By the time we chopped late the following morning, the sugar content had declined to about the same as for the previous morning’s mowing. We ensiled the crop and followed it through the fermentation period, and weren’t at all surprised to find no meaningful difference in fermentation or silage quality due to time of mowing.

However, we aren’t the only ones who have looked at morning versus afternoon mowing of alfalfa harvested for silage; one recently reported study got somewhat better results than we did in maintaining high sugar concentrations between afternoon mowing and chopping. This isn’t surprising considering the possible differences in temperature, humidity, crop yield, etc. between the two trials.

Theory vs. practice

Once alfalfa starts to reach the full bud stage it’s a race against time to get the crop harvested before quality declines past what’s needed for good milk production. If we only mow in the afternoon we’ve reduced by about 50 percent the amount of forage we can harvest in a day, something few farmers would (or should) be willing to do. On many large farms, mowing is already the bottleneck in the entire hay crop silage program: once the forage approaches chopping stage, windrows are often doubled and tripled since today’s self-propelled choppers can easily handle massive windrows. In fact, many farmers find that forage particle size is more uniform when windrows are large enough to keep the cutterhead “busy” rather than having small windrows go flying through. Moving chopped silage from field to silo is also quick since most farms have trucks and packing equipment sized for corn silage, a crop that’s harvested at a higher tons-per-hour rate than is hay crop silage. For these reasons, some large farms use two mowers just to keep up with the chopper.

At Miner Institute we start mowing early in the morning, even if the dew is still on the forage, as long as we have a good “hay day” in the offing. On sunny days, by stretching out windrows to about two-thirds the mower width, the forage we mow in the morning can be chopped that afternoon or early evening. But, we don’t stop mowing at noon; we mow well into the afternoon, and usually get enough wilting by sundown to somewhat reduce the overnight respiration losses. Then we can start chopping by mid-morning the following day. Once alfalfa advances past full bud (and once grass advances past the boot stage) the daily decreases in forage quality as the crop matures almost certainly outweigh any differences between morning and afternoon mowing. There’s been very little research done on morning versus afternoon mowing of grasses, most of the work having been done with straight alfalfa or alfalfa-grass with alfalfa the predominant crop. Therefore, we can only assume that the diurnal changes in sugar in grasses are somewhat comparable to those in alfalfa.

Take advantage of the sun’s influence on forage quality, but don’t let it take complete control your hay crop harvest schedule. Timely harvest—the day of the month, not the time of day—is still the most important determinant of forage quality.

The author is an agronomist and vice president for agricultural programs at William H. Miner Agricultural Research Institute in Chazy, N.Y.