What Silage Harvest Can Tell Us About Nitrogen Uptake

By: Bob Gunzenhauser

By far, most corn grown in the United States is for grain; a small amount is harvested for silage for ruminants like beef or dairy cattle.  Of the 90-95 million acres of corn grown in the US yearly, about 6-7 million is devoted to corn silage.

Despite this, corn silage can give us a very good indication of nitrogen uptake.

Because the whole plant is harvested, analysis of the silage can provide a crude protein content, along with moisture content.  If a yield, in wet tons per acre, is available, then through math an estimate of total nitrogen removed by the plant per acre can be determined.

Crude protein is estimated to be 6.25 times the nitrogen content of a substance on a dry (0%) basis.  Corn silage may range from 7 to 8.5% crude protein, often in the 7.5 to 8.0% range.  As we discussed in a prior article, corn can be a luxury feeder of nitrogen if excessive amounts are available.  Depending on the genetics, this extra nitrogen is often deposited in the grain, but some may still be in the stalk and leaves as well.

About 55% of a corn plant's above ground biomass at physiological maturity is concentrated in the grain.  This is called the Harvest Index and can range from 50 to 58%, but many modern genetics peak at 53 to 55%, again on a dry matter (0% moisture) basis.

Precision Technology Helps Shine a Light

Oftentimes, to get crude protein, a sample must be sent off for analysis at a laboratory.  However, with the advent of NIR (near-infrared) sensors mounted to silage harvesters, we now can obtain estimates of yield, moisture content and an array of nutrient analytes (crude protein, starch, carbohydrates) that can help us understand where the crop is generating value in the field.

I recently worked with some silage data from a Harvest Lab-equipped John Deere silage harvester on a 25-acre field.  This field surrounds a small beef feedlot and receives manure yearly.

In the county this field is located (Wayne County, Iowa), I've built a predicted parent material map based on soil sample observations and digital terrain attributes.  In this parent material map, the black areas are wind-blown loess, silty loam soils.  These are generally the better yielding soils on flats and ridges.  The green areas represent clays as paleosols, old soils that may at one time had the loess on it but are now eroded.  The blue areas represent alluvial soils, where the eroded loess silty loam soils eventually deposited at.  The few red areas in the field are indicative of predicted sand/grave zones, highly eroded old glacial till soils downslope of the clayey paleosols. 

Figure 1. Estimated Parent Soil Material.  Black = Loess silty loam.  Green = Clay paleosols.  Blue = Alluvial soils

Recognizing the spatial patterns of the soils found in this field, let's next look at the estimated wet corn silage harvest in tons per acre:

Figure 2: Estimated wet corn silage yield in tons per acre.  65% moisture 

It should be noted that the farmer/operator admits he doesn't always calibrate the Harvest Lab for yield, as he doesn't have a set of scales handy as he would a grain cart with scales for a combine yield monitor.

We can easily see a pattern; High yields in the southwest area of the field that match the black loess silty loam, and lower yields in areas with more clay paleosols.

Next, consider the estimated crude protein percent map:

Figure 3: Estimated crude protein content on 0% basis 

In this map we do see drastic spatial patterns emerging.  The clayey paleosols tend to have lower crude protein content, while the loess and alluvial areas have higher protein levels.

With these components, and estimated harvested nitrogen per acre map can be generated:

Figure 4: Estimated Harvested Nitrogen, pounds per acre.

This map provides a continuous estimation of the amount of nitrogen that was removed in the entire plant across the field by combining yield and crude protein estimates.  A similar pattern appears as before; the areas with black loess silty loam soils tend to have higher nitrogen removal (300-400+ lbs N/ac) than the clayey paleosols (150-250 lbs N/ac).  

A fair assumption can be made that the entire field was treated about the same; a uniform commercial nitrogen application was made, and blanket, uniform rates of manure were applied as best as possible.   

What Can We Learn from This?

Yield, Crude Protein, and estimated Harvest Nitrogen generally aligns within this field.  Areas that had extra nitrogen, likely mineralized nitrogen from organic matter, allowed more N to make it into the plant at key growing points.  Recognizing that these areas can mineralize more nitrogen, and vice-versa, areas that do not mineralize as much nitrogen, can help guide farmers as to where they may want to put their nitrogen dollars.

Is the low yielding area low because of fertility, drainage, or organic matter?  Could the parent material have an effect on mineralizable nitrogen?  Organic matter from soil tests taken on this field do not correspond to the parent material map patterns, but do they have different forms of organic matter?  Perhaps the combination of available OM and water holding capacity/drainage has something to do?

Precision technology like NIR analyzers on silage harvesters provide an opportunity to dive deeper into our fields' ability to mineralize nitrogen and drive not only yield but nitrogen concentration.  These tools, while not widely used (yet), may give us useful insights that we don't always get with grain harvest.