Corn must be ensiled at the proper moisture to get fermentation for preservation. But, determining when to harvest corn at the right whole plant moisture is difficult. Each storage structure properly ensiles at slightly different moisture optimums. Harvesting corn too wet for the storage structure will result in reduced yield, souring and seepage of the ensilage, and low intake by dairy cows. Harvesting too dry reduces yield, can cause mold to develop, and lowers digestibility, protein and vitamins A and E.

The dilemma is finding an easily measured indicator that accurately correlates with moisture. One indicator often suggested is whole plant "greenness." But, there is little relationship between greenness and moisture. The amount of greenness is subjective and differs among agronomists.

Kernel milkline may be the most widely used indicator for determining when to harvest corn for silage. One advantage of the kernel milkline is that it indicates the rate at which the crop is drying down. On average, kernel milkline works fairly well (regression line in Figure 1). But, like greenness, there seems to be little relationship between kernel milkline and moisture due to the wide variability at of hybrids at specific milklines. For example, at 50% kernel milk, some hybrids are too dry, while others are too wet for proper ensiling (min = 50% moisture, max = 74% moisture).

These observations are also borne out by farmer experience. Many farmers have reported that harvesting at the recommended 50% kernel milkline is too wet and silos seep. In the recent past few years, corn silage harvest has often been delayed resulting in poor dairy cow performance when the corn silage was fed because the kernels were too mature and passed through the rumen.

How can we use kernel milkline as an indicator for whole plant moisture? The evidence suggests that we can’t. But, we might be able to use it as a "trigger" in that once a corn hybrid is at certain kernel milkline stages, farmers should begin testing the field for whole plant moisture. The triggers vary by silage storage structure (Table 1). The kernel milkline triggers correlate with moistures too wet for proper ensiling (more than 95% of the hybrid*environment cases evaluated). Once the moisture of a hybrid in that field is known, an average moisture drydown rate of 0.5% per day can be used to predict when to begin harvesting. Furthermore, the milkline can continue to be monitored to observe whether the rate of moisture loss is changing dramatically.

Table 1. Using kernel milk as a trigger for checking corn whole plant moisture and predicting silage harvest.
Silo Structure	Recommended moisture content	Kernel milk "trigger"
	%	%
Horizontal bunker silos	70 to 65	80
Bag silos	70 to 60	80
Upright concrete stave	65 to 60	65
Upright oxygen limiting	60 to 50	40
Silage moisture decreases at an average rate of 0.5% per day. Days between early dent and 50% kernel milk ~12 d; between 50% kernel milk and 0% milk (black layer) ~13 d. "trigger": kernel milk stage to begin checking silage moisture (90% of HLY)

Figure 1. Relationship between whole plant moisture and kernel milk. Each data point is one hybrid in an environment.