by Ev Thomas and Bill Mahanna
Thomas is retired from the William H. Miner Agricultural Research Institute and president of Oak Point Agronomics Ltd.; Mahanna is with Pioneer, a DuPont Business, and is an adjunct professor at Iowa State University.
Beginning about 10 years ago, considerable field interest surfaced regarding harvesting corn silage at higher chop heights, primarily in an effort to improve neutral detergent fiber digestibility (NDFD). The original assumption by many, including some university researchers, was that chopping corn 6 to 12 inches higher than normal would result in greater energy content and higher NDFD. The higher energy content occurs because the fixed amount of grain is concentrated in a smaller volume of stover (leaves, stalk, husks, and tassels).
What we learned was that starch levels did indeed rise by high chopping . . . as much as 2 to 3 percentage points. When chop height was raised by 12 inches (from 6 to 18 inches, for instance), NDFD also rose by 2 or 4 percentage points, depending upon the specific hybrid and growing season.
Some say it's 6 to 8 inches, but this can vary significantly throughout the country. In some areas, such as California, it is a common practice to chop as low as 2 to 3 inches; whereas in other regions such as the Northeast, chopper operators harvest higher so as not to risk damaging equipment by hitting stones. There is certainly less potential gain in quality by raising chop height if normal chop height is already high (greater than 8 to 10 inches). It also does not make sense to reduce silage yields by high chopping brown midrib (BMR) hybrids because BMR stalks are already very high in fiber digestibility.
There is nothing magical about high-chop corn silage; it's simply a trade-off between yield and quality. The impact on yield depends to some extent on the yield potential of the hybrid, but in general, expect yield (35 percent DM basis) of the stover to drop by about 300 pounds per acre for every inch of higher chop height.
A Penn State summary of 11 chop height trials found that yield declined by an average of 7 percent when chop height was raised by 12 inches. Predicted milk production per ton of corn silage rose by 5 percent, while milk production per acre declined by only 2 percent. The impact on milk production depends on many factors; in one Penn State trial with high corn silage yields, both predicted milk per ton and per acre were enhanced by high chopping (6 inches versus 18 inches).
Not all hybrids behave the same when high-chopped - there appears to be a significant hybrid-by-environment interaction meaning that hybrids will respond differently to high chopping depending upon growing conditions. One approach to determining the environmental impact is to hand harvest 5 to 10 representative plants at normal chop height and at high chop height at about one to two weeks prior to harvest and have the samples analyzed for NDF digestibility to see if high chopping is worth the yield loss.
High-chop corn can be a practical management tool to boost corn silage NDFD, especially if you know that the hay or haylage already in storage is low in fiber digestibility. It can also be used by farmers with more corn than needed for silage (at normal chop heights) but no economical way to harvest the crop for grain.
One advantage of chop height decisions is that they don't have to be made until you pull into the field to harvest. If you know that you'll have more than enough to fill your silos at normal chop length, you could raise chop height in an effort to make your crop fit into your storage structures and provide your nutritionist with higher quality corn silage.
Finally, don't high chop unless you have enough corn silage to fill an entire silo. Your dairy nutrition consultant has enough problems without trying to deal with the quality differences between normal and high-chop corn. In fact, it might be good to discuss high-chop decisions with your nutritionist before you harvest.
This article appeared in the August 10, 2011 issue of Hoard's Dairyman on page 505.