Any good forage crop should have high dry matter yield, high protein content, high energy content (high digestibility), high intake potential (low fiber), and optimum dry matter content at harvest for acceptable fermentation and storage. With the exception of high protein level, corn silage exhibits these characteristics better than other forages.

A number of management decisions are required to produce high yielding and high quality corn silage. Beginning with the seed planted in the ground, hybrid selection has been shown to account for significant differences in animal performance and economic return. Other agronomic practices such as planting date, plant density, row spacing, weed control, etc. all contribute to successful production of high yielding, high quality corn silage. All will be for naught though unless corn is harvested and ensiled at the proper whole plant moisture to ensure fermentation for long-term storage.

The production difficulty farmers often encounter is timing harvest so that the proper moisture for ensiling is obtained for the storage structure (Table 1). If corn silage is too wet then yield is often reduced, silo seepage occurs and the silage is sour tasting resulting in lower intake by livestock. If corn silage is too dry then yield is often reduced, heat damage and mold more easily develops in the silo because fermentation is inadequate, and the silage has lower protein, digestibility and the vitamins A and E.

Table 1. Recommended silage moisture contents for various silo structures and suggested kernel milk stage "triggers" to begin checking silage moisture.
Silo structure	Recommended moisture content for ensiling	Kernel milk stage "trigger"
	%	%
Horizontal bunker silos	70 to 65	80
Bag silos	70 to 60	80
Upright concrete stave	65 to 60	60
Upright oxygen limiting	60 to 50	40
Silage moisture decreases at an average rate of 0.5% per day during September

Historically, up until the early 80's corn silage had always been harvested at Historically, up until the early 80's corn silage had always been harvested at "black layer". Wiersma et al. (1993) reported that whole plant moisture was predictable using kernel milkline. They suggested that silage harvest should begin around 50% kernel milk and be completed by 25% kernel milk. However, many farmers reported silo seepage to occur when using this recommendation. Many modern corn hybrids released since the research of Wiesrma et al. (1993), have the "stay-green" trait where the kernels are mature (black layered), the ear turns brown and grain drydown occurs even though the stover remains green. This trait maintains the integrity of the plant longer into the fall improving combine-ability. Unfortunately the relationship between whole plant silage moisture and kernel milk may no longer hold and may be the reason farmers are observing increased silo seepage when using kernel milk.

Evidence to suggest that the relationship between whole plant silage moisture and kernel milk may no longer hold is shown in Fig. 1. Each data point is the mean of three replicates of a hybrid grown at a location (environment). The regression line drawn through the data points fits nicely with the observations of Wiersma et al. (1993), but the variability for any particular kernel milk stage is high. For example, at the kernel milk stage of 50%, whole plant moisture varies from 50 to 74% whole plant moisture. In most cases, corn hybrids would be either too wet or too dry at 50% kernel milk.

Figure 1.

The variability within the relationship between whole plant moisture and kernel milk stage is usually observed regardless of production environment year (Fig. 2). For the 1992 and 1997 production years, on average, the regression line through the data points is typical of the relationship between whole plant moisture and kernel milk stage described by other workers. But, wide whole plant moisture variability exists for each kernel milk stage. Again, corn silage harvested at the 50% kernel milk stage could be either too wet or too dry for a concrete stave silo.

Figure 2.

There is some thought that the relationship between whole plant moisture and kernel milk stage may not vary as much for a particular hybrid. In our database, there are some hybrids where the relationship is good, while the majority of the hybrids show a poor relationship between whole plant moisture and kernel milk (Fig. 3 shows a few examples). A total of 56 corn hybrids were evaluated where there had been at least five environments of testing. Of these, only 10 hybrids had R² values greater than 0.75. An example is Cargill 4327 which shows a good relationship between whole plant moisture and kernel milk stage. Aproximately, 82% of the hybrids exhibited a poor relationship between whole plant moisture and kernel milk stage.

Figure 3.

In conclusion, there appears to be no good, easily identifiable plant indicator for accurately predicting whole plant moisture for ensiling corn. The only way to truly measure the moisture of corn in a field is to measure by weighing it directly. It is impracticle to characterize all corn hybrids as to the influence of the "stay-green" trait because of the variability caused by environment and management.

But can we still use something like kernel milk stage as a plant indicator for predicting whole plant moisture? I think we can if we think about it as a "trigger" to go out and begin measuring the field for whole plant moisture. Generally the amount of time it takes to go from dent to black layer is about 25 days (12 days from dent to 50% kernel milk stage, and 13 days from 50% kernel milk stage to black layer). Once we know where a field is at for whole plant moisture, we can use an avaerage dry-down rate for the month of Septemeber of 0.5% drying per day. Hot dry weather may speed up silage dryown, while cool, wet weather will slow it down.

The kernel milk "triggers" to begin moisture sampling fields are described in Table 1 for various storage structures. Approximately 95% of the time the field will be too wet for the storage structure when using the suggested triggers. But, you will know where you are at with that field and by using the drydown rate of 0.5% per day, you can predict when to either test the field again or begin chopping the field for ensiling.

Literature Cited

Wiersma, D.W., P.R. Carter, K.A. Albrecht, and J.G. Coors. 1993. Kernel milkline stage and corn forage yield , quality and dry matter content. J. Prod. Agric.6:94-99.