With corn silage harvest approaching in most of Ohio, it’s time to check your crop’s moisture content if you haven’t already, say Ohio State University forage specialists. But they emphasize that the position of the kernel milk line is not a reliable indicator for determining when to harvest.

Geographic location, planting date, hybrid selection and weather conditions affect the relationship between kernel milk-line position and whole-plant dry matter content, they say. A Wisconsin study revealed a poor relationship between kernel milk line and whole-plant dry matter in 82% of the hybrids tested, and in Ohio they’ve seen considerable variation in plant dry matter within a given kernel milk-line stage.

The only reliable method of determining the optimal time to harvest corn silage is to sample the crop and directly measure the percent dry matter of plants, say the Ohio State experts. This information, combined with average whole-plant dry-down rates, can be used to roughly predict the proper time to harvest corn silage, they add.

Collect about five representative plants from each field. Collect separate samples from areas that may have different dry-down rates, such as swales and knolls. Put the plants in a plastic bag, keep them cool and chop as quickly as possible. They should be uniformly chopped (using a cleaver, machete, chipper shredder or silage chopper) and then mixed thoroughly to obtain a sample with a representative grain-to-stover ratio. Some farmers prefer sampling only two or three plants without any additional sub-sampling to reduce the chances of a non-representative grain-to-stover ratio that can affect the results. In that case, choosing representative plants is even more critical.

Determine the dry matter content by drying the plant material using a Koster oven tester, microwave, convection oven, vortex dryer or taking it to a lab. For more details on these and other methods, see the following links:
http://www.extension.org/pages/Dry_Matter_Determination
http://ohioline.osu.edu/agf-fact/0004.html
http://www.abe.psu.edu/extension/factsheets/i/I101.pdf
Make sure the sample doesn’t dry down and keep it cool until the dry matter determination is performed. The accuracy of the dry matter value will be largely determined by the care taken in sampling, drying and weighing the samples. Whole kernels and cob pieces can be difficult to dry completely without burning the leaf tissue.

While kernel milk stage is not reliable for determining the actual harvest date, it’s a useful indicator of when to sample fields to measure plant dry matter. In Ohio, corn should be first sampled at full dent stage (100% milk, no kernel milk line) for conventional tower or bunker silos, and at one-fourth milk line for sealed (oxygen-limited) tower silos. It’s important to begin sampling early as a precaution against variation in dry-down.

Once whole-plant percent dry matter is determined, an average dry-down rate of 0.5 percentage unit per day can be used to estimate the number of days until the optimal harvest moisture. For example, if a field measures 30% dry matter at the early sampling date, and the target harvest dry matter is 35%, the field must gain an additional five percentage units of dry matter, thus requiring an estimated 10 days.

This procedure provides only a rough estimate for the harvest date. Many factors affect dry-down rate, including hybrid, planting date, general health of the crop, landscape position, soil type and weather conditions. Early planted fields and hot and dry conditions can accelerate dry-down rates to 0.8-1.0 percentage unit per day. Fields should be monitored closely and more frequently under those conditions. In general, corn silage that’s slightly too dry is worse than silage that’s slightly too wet. Therefore, starting harvest a little early is usually better than waiting too long.