Optimizing corn plant density for silage – Jon Urness, Vita Plus

By Jon Urness, Vita Plus national forage specialist
For the past 40 years, Midwest corn growers have increased plant populations in an effort to find that “happy place” where optimum yield and quality meet.  Speaking to members of the Wisconsin Custom Operators and Midwest Forage Association, Dr. Joe Lauer, University of Wisconsin-Madison corn specialist, said growers have increased seeding rates by an average of 261 plants per acre every year for both corn for grain and corn for silage.

Despite higher seed prices, Lauer said growers have gone from about 20,000 plants per acre in 1980 to more than 30,000 now, and the good news is they’re still seeing a response.  The trick is to maximize yield while optimizing economics, which will be different in corn for grain versus corn for silage.

First, looking at corn for grain, Lauer said the maximum yield seems to occur at about 39,000 plants per acre.  Considering all the costs of production, especially seed, fertilizer and other inputs, the ideal economic advantage seems to occur right around 35,000 plants per acre.  This data was collected at the University of Wisconsin Arlington Ag Research Station.  Regional results will vary, but these results are relatively comparable to other areas.

Turning to corn for silage, the trend is similar, but dynamics are a little different.  This evaluation is more complex because a quality component needs to be considered along with yield and economics.  In all cases, Lauer said optimum quality, measured as milk per ton, will occur at very low densities, as low as 18,000 plants per acre for the highest milk per ton of silage.  Fortunately, quality does not drop off rapidly as plant population increases.  Even at 55,000 plants per acre, milk per ton is still 95 percent of where it is at 18,000 plants per acre.

As mentioned previously, finding the economic optimum for corn for silage is a little more difficult than making the same determination for grain.  However, milk per acre is a good measure to figure out the best compromise between yield and quality.  By that measure, Lauer suggested that optimum plant population is right at 45,000 plants per acre when considering milk per acre alone.  Total yield per acre is best at 48,000 plants per acre, so the two thresholds are not far apart.

Lauer said ongoing research is trying to find the “sweet spot” to optimize plant population for yield, quality and economics.  Populations as high as 120,000 plants per acre are being studied at UW-Madison to see where this goes.  He added the average corn silage dry matter (DM) yield for Wisconsin is 10 tons per acre, which will vary plus or minus a ton or two between the northern and southern regions of Wisconsin.

Lastly, Lauer briefly talked about quality and yield considerations when comparing silage cut at 6 inches versus high-cut silage at 24 inches.  High-cutting corn for silage will generally increase neutral detergent fiber digestibility (NDFD) by 2 percent and starch content by 4 to 5 percent.  Milk per ton can also go up about 140 pounds with the high cut, but milk per acre can drop about 1,800 pounds.  Another important consideration for custom harvesters is how the high cut can reduce moisture by about 5 percent.  Since moisture is concentrated in the lower portion of the stalk, this could help when the harvester shows up to a farm and debates whether the crop is too wet.