by Mark Sulc, Peter Thomison, Bill Weiss, Ohio State University Extension
Corn development has been progressing at a rapid pace with the recent warm temperatures. Early planted corn is already being harvested for silage in some parts of Ohio. So it is time to check the whole plant moisture content now, if you haven’t done so already.
Ensiling corn at the proper dry matter content provides high quality preservation resulting in good animal performance and lower feed costs. Harvesting corn too wet (low dry matter content) results in souring and seepage of the silage and reduction in animal intake. Harvesting too dry (high dry matter content) promotes mold development because the silage cannot be adequately packed to exclude oxygen. Harvesting too dry also results in lower energy concentrations and reduced protein digestibility.
Harvest Moisture Guidelines
Corn silage preserved between 30 and 38% dry matter (62 to 70% moisture) generally provides good silage fermentation and animal performance. The optimal dry matter content varies with type of storage structure (Table 1).
Table 1. Optimal dry matter contents for different storage structures.
type of structure optimal % dry matter
Horizontal bunkers 30 to 35
Bags 30 to 38
Upright, top unloading 33 to 38
Upright, bottom unloading 35 to 40*
*The higher DM concentration for bottom unloading silos is a compromise between forage quality and unloader requirements
Kernel stage not a reliable guide for timing silage harvest
Dry matter content of whole plant corn varies with maturity. Research has demonstrated that the position of the kernel milk-line is not a reliable indicator for determining harvest timing. Geographic location, planting date, hybrid selection, and weather conditions affect the relationship between kernel milk-line position and whole plant dry matter content. In a Wisconsin study, 82% of the hybrids tested exhibited a poor relationship between kernel milk-line stage and whole-plant % dry matter. In Ohio we have seen considerable variation in plant dry matter content within a given kernel milk-line stage.
Determining Silage Moisture
The only reliable method of determining the optimal time to harvest corn silage is to sample the crop and directly measure the % dry matter of whole plants. This information combined with average whole plant dry-down rates can be used to roughly predict the proper time to harvest corn silage.
How to Sample Fields
Collect about 5 representative plants from the entire field. The plants should be representative from an area with representative plant population and not from edge rows. Collect separate samples from areas that may have different dry down rates, such as swales and knolls. The moisture concentrations of plants can vary within a field (plants will be wetter in low lying area and drier on knolls) and this should be considered when collecting your sample plants.
Put plants in a plastic bag, keep them cool, and chop as quickly as possible. The plants should be uniformly chopped (using a cleaver, machete, chipper shredder, or silage chopper) and then mixed thoroughly to obtain a sample with representative grain to stover ratios for dry matter determination. Some farmers prefer sampling only 2 or 3 plants without any additional sub-sampling to reduce the chances of a non-representative grain to stover ratio that can affect the results. In this 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, taking to a lab or using a vortex dryer. For more details on these and other methods, see the following links:
Make sure the sample does not 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.
When to Begin Field Sampling
We know that kernel milk stage is not reliable for determining the actual harvest date, but it is a useful indicator of when to sample fields to measure plant dry matter. Corn in Ohio should be first sampled to measure dry matter at full dent stage (100% milk, no kernel milkline) for conventional tower or bunker silos, and at 1/4 milkline (milkline one-fourth down the kernel, 75% milk remaining) for sealed (oxygen-limited) tower silos. It is important to begin sampling early as a precaution against variation in dry down.
Predicting the Harvest Date
Once whole-plant % dry matter is determined, an average dry down rate of 0.5% unit per day can be used to estimate the number of days until the optimal harvest moisture. For example, if a given field measures 30% dry matter at the early sampling date, and the target harvest dry matter is 35%, then the field must gain an additional 5% units of dry matter, thus requiring an estimated 10 days (5% units divided by 0.5 unit change per day).
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 like we’ve been experiencing can accelerate dry down rates to 0.8 to 1.0 % unit per day. Fields should be monitored closely and more frequently under these conditions. In general, corn silage that is slightly too dry is worse than corn silage that is slightly too wet. Therefore, starting harvest a little early is usually better than waiting too long.