It’s that time of year when manure storages are getting full and farmers are looking for places to spread. Research has shown that manure can safely be applied to alfalfa and alfalfa-grass fields, but there’s a difference between “safe” for the plants versus what happens when this forage is harvested, ensiled and fed.

The heat and acidity produced by fermentation kills many pathogenic bacteria that are present in dairy manure, but Clostridia is one of the potentially harmful pathogens that can survive these conditions. Therefore, if you’re going to apply manure to forage fields during the growing season it’s important to do it right. Following are a few suggestions.

Timing Is Everything

Research both in the Midwest and at Miner Institute (Chazy, New York) has shown that it’s important to apply manure very soon after harvesting alfalfa, before significant regrowth has started. In a Midwest study, manure was applied immediately after harvest versus. delays of one and two weeks with a “no-manure” control for comparison, while at Miner Institute the manure was applied three and seven days after harvest and compared to an unmanure control. At Miner Institute, the amount of damage (primarily from the tires on the tractor and manure spreader) was barely noticeable when the manure was applied three days after harvest, but was very apparent with a seven-day delay.

Since alfalfa regrows quickly after harvest, every day of delay between harvest and manure application makes a difference. But manure application right after harvest is important not only to minimize plant damage from field traffic (a 5,000-gallon tanker load of slurry manure weighs 20 tons plus the weight of the spreader) but also from the impact that delayed manure applications can have on the forage quality of the next harvest.

There are several variables that can impact this including the amount and severity of rainfall between harvests, but since you can’t predict the weather it’s better to be safe than sorry. In the Midwest research showed that even though the silage appeared to be well-preserved with no undesirable odors that are characteristic of clostridial fermentation, Clostridia bacterial counts were higher when manure application was delayed by one and two weeks after harvest.

Dry Matter And Disk Mowers

Clostridial fermentation is more common in overly wet silages, so if you topdress forage crops with manure it’s especially important to allow the crop to dry in the field after mowing – preferably in wide windrows – until it’s at least 35 percent dry matter (DM). If you applied manure after the alfalfa had visible regrowth, allowing the crop to field dry to 40 percent DM may be advisable because some manure solids may have remained on the alfalfa leaflets. (I’m less concerned about silo packing and fermentation problems with 40 percent dry matter (DM) alfalfa than with 40 percent DM grass.)

Avoiding very low dry matter silage is also important if you grow straight grass. After mowing, alfalfa regrows from crown buds while grass regrows from the cut stems. Therefore, topdressed manure can adhere to grass stems, and, depending on the weather conditions during growth of the succeeding crop, the manure solids may still be there at harvest.

Almost all commercial dairy farms now use disk mowers instead of sicklebar mowers. Disk mowers and mower-conditioners can be operated at faster ground speeds, especially if there’s a significant amount of grass in the stand, and farmers have learned that they can leave a shorter stubble because the disk units are quite rugged compared to sicklebar knife sections. However, the combination of low stubble height and the vacuuming action of disk mowers may result in some surface debris deposited onto the windrows of a just-mowed crop.

In the Miner Institute research (which used a disk mower-conditioner), there was significantly more ash in the harvested alfalfa following manure application, with higher ash content where manure application was delayed by seven versus three days. In fields that have been topdressed with manure, it’s fair to assume that some of this ash and other crop debris may contain manure pathogens: Some manure pathogens are killed by sunlight but others will survive on the soil surface for a long time. These may include the pathogen that causes Johne’s Disease, a serious disease of dairy cattle. Increasing stubble height may decrease this problem since the mower knives aren’t as close to the soil surface, also using straight versus curved disk mower knives.

The Best Defense Is A Good Offense

Many farmers topdress liquid and slurry manure on their hay fields each and every year, sometimes making repeated applications, and never seem to have any problems with clostridial silage or any of the other problems related to manure contamination of silages. That’s probably because the rest of their silage production program is up to snuff including harvest timing, stubble height, chopping at the proper dry matter content, and good silo management both during and after filling. It also includes the use of a silage inoculant.

Countless research trials have found that silage inoculants supporting the production of lactic acid improve the chances of homolactic (“good”) fermentation. I don’t have a favorite inoculant, for two reasons: First, some products appear to work better for a particular forage species (corn silage versus alfalfa silage, for instance); and second, there are a number of high quality, research-proven silage inoculants that, if used properly, will do the job, and at a reasonable cost.

Finally, to some comments on manure application rates, I haven’t recommended application rates in this column, in part because there’s a wide range in solids content (percent dry matter) in the slurry and liquid manures found on farms. Generally we refer to manure that has higher solids content as “slurry manure” while manure containing a significant amount of water in addition to urine and feces is known as “liquid manure.”

The nutrient content of a particular tanker load of manure also depends on how well the manure pit was agitated, with the manure in the bottom of the pit – or that removed last – usually being considerably higher in nutrient concentration. The best way to use manure as a fertilizer is to have it analyzed and then apply it at a rate to meet the needs of the crop. Research has shown that the nutrients in manure have about the same plant availability as the nutrients in commercial fertilizer.


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