Corn silage continues to gain popularity as a forage for dairy cows in California. Well ensiled corn silage has a high nutrient density based upon a mixture of proteins, rumen fermentable fiber, and starch that can be fermented in the rumen or digested in the intestine. However there are a number of non-traditional silage hybrids available, that have different nutritional characteristics, and some have been suggested to be superior for silage. In addition, growers have options relative to selecting the time of harvest and whether to process at harvest.
There are four non-traditional hybrids that may be of interest to California corn growers. They differ in their nutritional characteristics and may be useful under some conditions.
High Oil Hybrids: High oil hybrids contain about 8 to 10% oil in the kernels versus about 4 to 5% in conventional varieties. Due to the high energy value of oil, versus other nutrients, this will tend to increase the energy value of the corn silage. However, the increase in oil comes at the expense of a decrease in starch and there is some indication that high oil hybrids have a higher stover to ear ratio than conventional hybrids, which will tend to decrease the energy value of the silage.
Controlled research studies with dairy cows have not shown consistent influences on intake of dry matter (DM) or production of milk in lactating dairy cows. Part of the reason may be due to dilution of the oil, which is in the kernels, in the whole plant and further dilution of the whole plant in the total ration of the cows. Thus diets based upon high oil hybrids will only contain about 0.5 to 1.0% more oil than conventional hybrids. This level is probably too low to be reliably detected in controlled research studies. Nevertheless, high oil varieties should theoretically be advantageous under conditions where the added fat and/or oil level of the diet is low, too low to negatively effect growth of rumen microorganisms, and levels of non-structural carbohydrate (NSC) are relatively high. Conversely, if NSC levels are already low, due perhaps to high fiber commodities in the ration, then high oil varieties may have a negative influence on animal performance by reducing the starch level of the diet thereby reducing rumen microbial growth.
Waxy Hybrids: Waxy hybrids contain 100% amylopectin in their starch, versus about 75% in conventional varieties which also contain about 25% of the less digestible amylose. It has been proposed that the presence of the more digestible amylopectin gives an advantage to the waxy varieties. However, controlled research with dairy cows has not shown consistent increases in DM intake or milk production. This may be partly due to dilution of the increased concentration of amylopectin in the whole plant, about 10% of corn silage DM, and further dilution in the diet fed to the cows (to only 3 to 4% of DM intake). In addition, corn silage amylose tends to be fermented in the rumen, or digested in the intestine, to a high degree. Indeed, if the starch in the waxy hybrids were in fact more easily fermented in the rumen, this could be negative if levels of NSC in the diet were already high enough to cause acidotic conditions in the rumen.
High Lysine Hybrids: High lysine hybrids contain a higher level of lysine than do conventional hybrids. This has been suggested to be important, as lysine has been frequently suggested as an amino acid that may limit milk production of lactating dairy cows. However, due to degradation of some of the lysine by rumen microorganisms, and dilution of the lysine from the corn silage in the diet actually fed to the cows, the quantitative impact of high lysine varieties on delivery of lysine to the intestine is small. In addition, a productive response would only be expected if the cows were limited by intestinal lysine supplies. There is little controlled research with lactating dairy cows evaluating high lysine hybrids.
Brown Midrib Hybrids:
In contrast to the previous hybrids, which attempt to influence the quality of corn silage by manipulating the nutritional profile of the kernels, brown midrib hybrids attempt to influence the nutritional value of corn silage by manipulating the nutritional profile of stover. Brown midrib hybrids can have as much as 50% less lignin in the stems and leaves, which is associated with increased rate and extent of rumen fermentation of fiber. Controlled research studies with dairy cows have shown consistent increases in DM intake of diets containing brown midrib hybrids and milk production and/or body status is generally improved. However, brown midrib varieties tend to grow more slowly in the field, are less sturdy in the field, and generally have a higher ratio of stover to ears.
There has been some interest in many parts of California in mechanically processing corn silage prior to ensiling in order to smash all of the kernels thereby preventing whole kernels from being fed to the cows and, potentially, appearing in manure. While there is very little controlled research in this area, there is a perception that kernel processing of corn silage prior to ensiling improves the quality of the fermentation process, results in higher digestibility of corn silage by the cows, and results in higher milk production. These benefits have all been attributed to the effect of kernel processing on smashing the corn kernels prior to ensiling.
Do the perceived benefits of kernel processing make biological sense and is it worth the extra charge of $1 to $2 per ton of corn silage? Clearly kernel processing smashes the kernels, and this may make more starch from the corn available to silo microorganisms thereby allowing more rapid production of lactic acid leading more quickly to stable silage. However, corn silage typically reaches a low pH very quickly without kernel processing, and there is no evidence that a more rapid pH decline would be beneficial, even if it did occur. There is no evidence, except in the most physiologically mature corn silages, that there is a significant passage of starch (i.e., kernels) from corn silage through the digestive tract of dairy cows. However starch from corn silage harvested at less than physiological maturity frequently escapes digestion by rumen microorganisms, due to delayed release in the rumen, caused because much of it is in semi-intact or intact corn kernels. Thus the primary effect of kernel processing will seldom be to increase the digestibility of corn silage overall, but will be to shift the site of digestion of the starch from the small intestine to the rumen. This may be good if dietary levels of rumen degradable NSC are low, due to a stimulatory effect on rumen microbial growth. Conversely, it may be bad if dietary levels of NSC are high, as it will stimulate growth of rumen microorganisms that produce propionic and lactic acid. High levels of propionic acid in the rumen are associated with reduced levels of fat in milk and high levels of lactic acid can cause lactic acidosis, which is associated with health problems such as increased frequency of 'off-feed', reduced overall DM intake, and laminitus. In addition, shifting the site of starch digestion from the intestine to the rumen, due to kernel processing, will reduce the amount of starch digested in the small intestine.
Intestinally digested starch is converted to glucose after absorption from the intestine, and it is glucose that supports production of milk lactose, which drives up the yield of fluid milk.
There is evidence to support kernel processing for corn silage harvested at physiological maturity. However under most situations, kernel processing of corn silage harvested at less mature stages will not result in improved silage fermentation, DM intake, or milk production and may, under situations where the diets already contain substantial quantities of NSC, result in reduced animal performance both in terms of short-term milk production and long-term health.
Maturity: Corn silage can be thought of as two crops in one. It is comprised of the ear, the more digestible portion which increases in proportion with maturity while its digestibility declines, and the stover, the less digestible portion which decreases in proportion with maturity while its digestibility also declines. The interaction of the increasing ratio of the more digestible ear, the decreasing ratio of the less digestible stover, and the decreasing digestibility of both may make it difficult to decide when best to harvest. However, within the normal harvest window typically employed for corn silage, the crop is only slowly changing in composition and nutritive value, as the period of the most rapid changes typically occurs well in advance, and well past, the period that the crop would normally be harvested. Indeed, data available from several sources suggests that the proportion of the whole corn crop that is digestible is constant during this period, even though a greater proportion of the digested DM may be coming from starch and less from fiber.
November 5, 1999