RED DEER – Alfalfa can take a lot of damage. Dan Undersander says it’s just a question of how much.
Undersander, an extension and research forage agronomist at the University of Wisconsin in Madison, said a plant with a heavy stem and crown can still live after being driven over by a 10 tonne tractor.
“That’s pretty good. We all know we have to drive over alfalfa to harvest it, but maybe we haven’t thought that we were damaging it or what kind of losses we’re causing.”
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Undersander has been studying how hay producers can lessen damage.
“What percentage of the field do you think you cover with traffic per cutting?
“With a 12 foot mower and 20 inch tires on your tractor, that’s 40 inches out of 12 feet, or about 25 percent. Tedd or rake it, you’re between 12 and 25 percent for the next trip over the field, depending if you merge two windrows.
“Then you bale and if you do it one windrow at a time that’s another 25 percent of the field. So 50 percent is an easy number to come to before you have the trucks hauling the bales off the field.”
Undersander’s study included plots with no traffic, traffic two days after mowing or traffic five days after mowing. Two days after the cut gives the best drying conditions for hay or for making silage.
“All of these studies have been done on dry fields. We all know when our soils are wet and you’re leaving ruts in the field, you’re causing damage to the alfalfa.”
The study was done in six states – Iowa, Kentucky, Minnesota, New York, South Dakota and Wisconsin – on a variety of soil types and climates. The first part involved looking at bulk density, a measure of the compaction in the soil.
“We did have some. It is an issue, but overall that’s not a major cause for loss for us. There’s not much difference in the bulk density from the first trip over the field, even after six or seven trips over the field. Some soils you’ll see a bit, some you won’t.”
He said the yield difference had to come from something other than soil compaction. He thinks it’s because the machinery is breaking off the stems that are coming back.
“On average, stem counts fell from about 70 stems per sq. foot to around 65, across all sites, all cuttings, all years, as we delay driving over the field up to five days.”
Undersander said all the varieties tested showed the same trend. With no traffic, there were always more stems than if they waited for five days. At two days, it was intermediate.
On average, he lost six percent yield from the next cutting after two days and 14 percent after five days.
“I suggest that every day after mowing that you delay driving over the field, is going to cost you between three and seven percent of your next cutting’s yield,” he said.
The study looked at more than 20 varieties. Some weren’t affected as much as others between two and five days, but in every case for every alfalfa variety tested, the yield was significantly less when the traffic occurred five days after mowing compared to two days after mowing. Some had a five percent yield reduction due to traffic and others had a 15 percent reduction.
Total yield was measured over a two year period. In Iowa, Kentucky and Minnesota, with traffic after five days, yields were reduced by 1.5 to three tonnes per acre, compared to no traffic.
“In New York, we didn’t see much difference between zero and two days, which shows soil compaction was not an issue. But we did see a difference at five days.”
In South Dakota, there was no difference. In Wisconsin, the longer they waited, the lower the yield.
“On average, when we looked at it in 2002 across all sites, our yields went from 4.8 tonnes to about 4.2 tonnes, so we lost about 3/4 ton. And in 2003, we lost about 3/4 of a ton.”
They also studied differences in recovery rates between varieties of alfalfa.
“Some of the varieties of alfalfa that are coming out now are said to be fast recovery. We’ve always attributed those that had a higher number for fall dormancy tend to be the less dormant ones, tend to green up quicker in the spring and tend to recover faster between cuttings. Generally, as they recover quicker, they tend to yield a little bit more,” Undersander said.
Differences in fall dormancy were also studied.
“Looking at two years of data averaged across all sites, the fast recovery varieties appeared to do a little bit worse than the slow recovery ones. So I’m not sure what the picture is on fall dormancy.
“On one hand, I think the bulk of the damage from wheel traffic is breaking off the stems that are coming back for the next cutting. Those that recover faster and come back faster are going to have stems quicker, so we’re going to break off more stems. On the other hand, they come back faster again. So we’re seeing a mixed picture there.”
In another part of the study, Undersander cut a portion of three fields in nine foot swaths and a portion of three fields in 12 foot swaths.
“Then we raked two swaths into one windrow on each portion of the field and baled or chopped. In some cases we were driving over the field with the baler or chopper every 18 feet and in some cases we were driving over the field every 24 feet.
“We weigh every wagon off every field we harvest. For two years we did this and our average yield increase was a half a tonne an acre. The only difference was a wider cutting swath and less traffic on the field. Is this important? I think so.”
Undersander said it takes about the same horsepower to run a baler at half capacity as it does to run it at full capacity. Having more material in the swath can improve harvest efficiency.
“Go to larger equipment. Going from nine feet to 12, we had more yield. In the Midwest, probably 40 percent of our alfalfa is harvested (by contractors) and they’re using 20 and 24 foot mowers and big choppers or balers. They’re only driving over maybe 15 or 20 percent of the field per cutting instead of 50 or 70 percent. Obviously you’re going to have more compaction in that small area, but it’s a smaller percentage of the field,” he said.
“This is something we have some amount of control over. This is another reason to go to wide swaths and get the hay to dry faster. This might make (hay) drying more feasible if you can pick up some extra yield.
“Alfalfa harvested for silage always yields more than alfalfa harvested for hay. This is one reason why. You’re driving over the field sooner after you mow it.”
While hay producers have to drive over their fields to harvest, Undersander said they can minimize the loss due to wheel traffic.
“The first thing is to minimize driving over the field. This seems like a simple thing, but you’d be amazed at how many people drive across their field, simply to look at it.”
- Use the smallest tractor. “While compaction is generally not the biggest issue, it still is a problem. Things like a rake don’t need a big tractor.”
- Merge windrows where possible. “For some this is feasible, for some it’s not. You’ll be driving over the field with the merger sooner after mowing. Hopefully one day after, maybe two. Then with your baler, you’re only covering 12.5 percent of the field instead of 25 percent.”
Undersander does not recommend the use of duals, unless the field is too wet and it’s necessary.
In the debate over lugs vs. smooth turf tires, “I’d recommend lugs, because you’re hitting a smaller percentage of the plants. With a track, or a turf tire, you’re hitting the whole field. The lugs just come down in points at regular spacing.”
He suggested taking the most direct route to the edge of the field. Make a road to drive on.
Undersander doesn’t recommend preservatives on a continuous basis, but said they are suitable at times.
“It may be where you’re getting to that fourth day, the forecast doesn’t look good, things might not be drying like expected. Maybe this is the time to break out that preservative and bale that hay a bit wetter.”
- Apply manure as soon after cutting as possible.
“When I talk to dairymen, a lot of them are putting manure on their alfalfa after they harvest the hay or silage. I ask them when we look at this data, ‘If you get your hay or silage put up Friday night at 6 p.m., when do you want that guy out there putting on manure?’ Most of them will say ‘6:01.’
“The point is, you don’t want to wait until Monday, because two days later will cost you six to 12 percent off your next cutting’s yield.
“As you’re thinking about the hay laying in the field, think not just about the quality you’re losing because it’s getting rained on. Think about the tonnage you’re losing from the next cutting, as well. Everything you can do to get it up faster will help your next yield.”
