How Clean is Your Forage?

Two changes in farming practices have resulted in increased ash concentrations in hay crop silages.

How clean is your forage? This may seem like a strange question, but I’m referring to the amount of soil, manure residues and other foreign material that is reported as ash on forage analyses.

In the northeastern U.S. ash is a concern primarily in crops that are mowed and windrowed prior to chopping or baling, not in row crops such as corn harvested for silage. That’s because summer rains rinse most of the dust and wind-blown soil from the corn plants. Such is not the case in the Central Valley of California and other areas where flood irrigation provides most of the water the corn crop receives during the summer.

We’d expect ash concentrations to be higher in flood-irrigated corn than in corn irrigated by center pivot; the latter type of irrigation more closely simulates rainfall that rinses dust and wind-blown soil from the plants. Ash concentrations aren’t usually quite as high when a forage crop is made into dry hay because of the additional mechanical operations – tedding, raking and baling – involved in hay production. For example, according to DairyOne forage lab averages, the ash content of legume-grass hay is 9.2 percent whereas for legume-grass silage it’s 10.2 percent.

Ash content is almost always higher in hay crop forages than in corn, but there are exceptions. Several years ago flooding in the Connecticut Valley resulted in corn that was up to its ears in muddy water; the flooding elevated ash levels so much that the worst-affected fields couldn’t be harvested for silage.

Two changes in farming practices have resulted in increased ash concentrations in hay crop silages. These practices are topdressed manure and the combination of disk mowers and lower stubble height. We’ve briefly addressed ash in prior columns, but it’s a serious enough problem to deserve a closer examination.

Topdressed manure

Manure application to cropland has been addressed many times but deserves another mention. Compared with a generation ago a much greater percentage of dairy manure is now stored in pits and applied in liquid or slurry form. Many farmers don’t want to wait until fall to empty these storages, wary of wet conditions that could make field application difficult if not impossible. That’s one reason dairy farmers apply some manure during the summer, not only to grass/mostly grass fields but to alfalfa fields as well. This is a good practice since many of these fields need the nutrients more than do the corn fields – as long as the timing is right. That means manure application as soon as possible, but no later than several days after harvest and before significant regrowth has occurred.

How much topdressed manure contributes to elevated ash levels depends on a number of factors including application rate, timing, crop species and the amount of rain occurring between application and harvest. Manure contains many pathogens, some of which will survive field exposure and ensiling, another factor to consider.

Mower type, mowing height

Over the past 20 years there’s been an almost complete change from sicklebar mowers to disk mowers. Generally, this has been positive since disk mowers can be operated at much higher ground speeds than sicklebars, especially if there’s a lot of grass in the stand.

But farmers also discovered that the knives on disk mowers aren’t nearly as likely to break if during mowing the mower knives scalp the soil surface or hit small rocks. As a result, forage crop stubble height on many farms is now shorter than when farmers were using sicklebar mowers. This can elevate ash concentration in two ways: First, close mowing increases the chance that the knives will scalp the soil surface, depositing soil on the just-mowed forage. Second, disk mowers create a vacuum that can suck up surface debris – just like your lawn mower, which operates in a very similar manner.

Forage analysis averages of hay crop silage show an increase in ash concentration starting about 20 years ago, as disk mowers were first becoming popular. This is not a coincidence! I’m certainly not suggesting that farmers go back to sicklebar mowers, but you should monitor the impact of mowing height by examining some just-mowed fields to see if there are areas where the disk knives scalped the soil surface.

If you’re scalping the soil, increase mowing height by an inch or so. For pure stands of alfalfa the minimum mowing height should be 2 inches, while for grass and mixed stands of grass-alfalfa, I recommend a 4-inch mowing height because nutrients for the following crop are in the bottom few inches of the grass plant.

Good ash vs. bad ash

Not all ash is a contaminant; much of the ash comes from nutrients contained in (not on) your forage. This type of nutritive or “internal” ash is good; external ash is not. Therefore, don’t just look at the total ash content of a forage sample; consider nutrient content of the plants. In typical alfalfa-grass forage, for instance, about 6 percent of ash consists of plant nutrients – calcium, potassium, phosphorus, chloride, nitrates, etc. Heavily manured fields may contain higher concentrations of nutrients (potassium in particular) so before you decide that a forage sample with a high ash concentration indicates a field problem, look at the nutrient levels. Forages with over 12 percent ash almost always mean that there was a field problem – and I’ve seen ash concentrations of more than 15 percent.

Use caution with hay rakes

Aggressive use of hay rakes can result in elevated ash levels, especially when used by someone unfamiliar with proper use of these implements. The idea is to rake the mown forage, not the soil surface! (A sure sign of trouble is when you see a cloud of dust trailing the rake.) That’s one reason I prefer a merger to a rake for hay crop silage. In any case it’s better to leave a few wisps of hay in the field than to rake so aggressively that you wind up with high ash levels in the harvested crop. Ash is a standard assay in most forage analyses, and one not to ignore.

Read more: Find the Value of Your Hay Crop Forages