Timing for corn silage harvest is tricky. The question is how much starch to target?

“First, there is a lot of pressure on producing more starch per acre, and if you delay harvest, it's something you can achieve, because as the plant keeps maturing, starch content keeps increasing over and over,” said Luiz Ferraretto, assistant professor at the Department of Animal Sciences, University of Florida. “If you delay harvest, you may end up with more total starch per area which is really useful if you consider that the more starch you have in corn silage, the less grain you need to add to the diet.”

However, a delayed harvest and higher starch levels can also bring problems, like decreased quality.

“Time of harvest should be a balance between yield and quality, and unfortunately, as you delay harvest quality normally decreases for two reasons,” he said. “The first is, at the same time starch is accumulating, prolamin proteins are also accumulating in the kernels. They surround starch and inhibit digestion.”

Neither bacteria in the rumen nor the enzymes in the small intestines can reach the starch when prolamin proteins are surrounding it. So producers may end up with a theoretical increase in the level of starch in corn silage, but this starch is not accessible and therefore not beneficial for the dairy cows, he said.

The second issue caused by delayed harvest is also related to digestibility.

“As I said, the kernels are accumulating starch and prolamin proteins, and at the same time, the stover portion of the plant is still accumulating lignin,” he said. “If you increase the level of lignin, the probability that you will decrease NDF [neutral detergent fiber] digestibility is very high. So you may end up with lower starch and lower NDF digestibility rather than having more starch overall.”

An additional concern is that dry matter content is accumulating every day. If there is a decision to harvest later to achieve more starch, or if harvest is delayed due to weather or a broken harvester, the result could be very dry material, which can impact fermentation.

“When silage is too dry, uniformity of particles at harvest is much lower, so sometimes you end up with very long particles in the silo that can compromise packing density. Therefore the silo has more oxygen inside, and it takes longer to start the fermentation process,” Ferraretto noted.

Also, when the silage material is too dry, fermentation is poor because bacteria in the silo needs water to move across the silo as well as to replicate.

“The dry material creates more oxygen and poor conditions for bacterial growth. Fermentation is much worse when you delay harvest,” he said.  “You lose the benefits of the fermentation process – to get a better preservation and to help increase digestibility.”

Ferraretto says to target timing to maximize quality, because high forage quality helps to formulate the diet.

“Normally my recommendation is to start with half of kernel milk line or about 33% to 35% dry matter so you don't end up with very dry material,” he concluded. “To have more starch is great, but the problem is you don't have more digestible starch. From a nutritional perspective, the focus should be on starch digestibility rather than content.”