Forage particle length affects many aspects of rumen function as well as dry matter intake (DMI). Regardless of current milk prices, input costs or other variables, milk production remains a dairy producer’s highest priority, and DMI is fundamental to achieving high milk production.
Maximum DMI can only be reached by maintaining optimal rumen fermentation through proper diet, mixing and feeding. To understand how forage particle size impacts rumen health and DMI, it’s helpful to understand how the rumen works.The rumen acts as a fermentation vat. The rumen microbial population is responsible for fermentation and is sensitive to changes in rumen pH. Proper pH is needed to optimize the number of microbes and increase digestibility. Rumination or “cud chewing” produces saliva, which helps maintain rumen pH.
Consumption of too-finely processed forages decreases rumination. Diets with too many small particles result in feed spending less time in the rumen and less microbial digestion. This can reduce digestibility and cause digestive upsets.
Particle size also directly affects the rumen mat, which is formed by small particles adhering to longer-stem forages that float in the rumen. This attachment allows the smaller particles to remain suspended in the rumen to be properly fermented. Without adequate fiber, particles may sink into the less-desirable part of the rumen where they cannot be digested as easily.
Ideal particle size depends on the forage, but as a general rule, corn silage should be chopped at ⅜- to ¾-inch. Forages with small particle sizes have been shown to lower butterfat tests. Besides depressing fat tests, finely chopped forages can cause metabolic disorders such as ruminal acidosis and displaced abomasums.
Diets that predispose cows to acidosis also increase the risk of feet and leg problems such as laminitis. Cows suffering from laminitis will spend less time eating, leading to drops in DMI and milk production.
The length of chop also affects fermentation in the storage structure. Drier corn silage may need to be cut shorter to facilitate packing.
It is important to watch particle size regardless of the forage you are feeding, but even more so with a highly digestible forage like BMR corn silage. Because of the lower lignin and higher digestibility of BMR, it is necessary to manage the rate of passage through the rumen by maintaining adequate effective fiber. Chopping at a longer particle size will help slow digestion and prevent upsets.
Follow these chopping recommendations to ensure adequate effective fiber in BMR corn silage (see Table 1).
Another factor in corn silage particle size is mechanical processing. Silage processing can improve ensiling characteristics as well as improve starch digestion by exposing the corn kernel to rumen bacteria. Although processing offers the potential for better nutrient utilization, it also can reduce particle size.
Because BMR hybrids have moderately textured kernels, less aggressive processing is recommended to maintain adequate particle size in BMR corn silage.
So what is the correct forage particle size? There is no one answer that fits all dairy operations. The bottom line is to find what works on your dairy to optimize rumen health and milk output. Meet with your farm advisory team to discuss chopping plans before harvest. PD