By Chad Lee, Extension Agronomist; Donna Amaral-Phillips, Extension Dairy Nutritionist; Nick Roy, University of Kentucky

The wet August and healthy corn crop are great for tonnage, but will present challenges for determining when to harvest the corn crop for silage. Whole plant moisture is the most important factor for deciding when to harvest corn. Ideal whole plant moistures are 65-70% for bunker silos, 62-65% for uprights and 62-68% for silo bags. 

Moisture at harvest determines how well the chopped crop will pack, which directly impacts the quality of silage when fed-out. Silage harvested too wet will undergo an unwanted fermentation and could limit feed intake and hurt the health of dairy cattle. Plant growth stage is another consideration, but it secondary to whole plant moisture. 

In a “normal year,” August is hot and dry such that corn drydown in the field is relatively easy to predict. This is not a normal August or a normal season to estimate plant drydown. Based on the way August has progressed, we would expect whole plant drydown to be slower than normal. In addition, different hybrids seem to have different rates of drydown. They also have differences in leaf greenness relative to kernel maturity. 

We would guess that most corn will be past blacklayer before it reaches proper whole plant moisture this year. With the expectation of corn being more mature at harvest this year, properly adjusting kernel processors will be critical to high quality silage. 

Simply looking at milkline or blacklayer is an inaccurate method for determining whole plant moisture. Of all the methods used to estimate whole plant moisture, the Koster tester is probably the most accurate on-farm device. A microwave can be effective, but it may worth the money to have a microwave in the barn to make these measurements. You can send fresh samples to some labs for NIR spectroscopy readings. Regardless of which method used, an accurate sample from the field and a chop size representative of your chopper are necessary to get a representative reading. 

Pulling a corn sample from the outer edge of the field is a bad idea. Getting samples from inside the fields and running those samples through the chopper is probably the best way to get representative samples. Having multiple samples will provide a better estimate of the whole plant moisture in the field. 

The following guidelines are adapted from our colleague, Joe Lauer, at University of Wisconsin:

1. Sample three to five plants in a row that are in the field, not at the edge, and representative.
2. Put the sample in a plastic bag. 
3. Keep the plants cool. (Very important.)
4. Chop as quickly as possible.
5. Measure whole plant moisture by drying on-farm with a Koster tester or microwave, or by sending to a lab for NIR spectroscopy. 

Testing some samples over several days will allow you to make some estimates on how quickly the corn plants are drying. You can use that rate of drydown to better estimate when to begin harvest.