High-quality corn silage contributes greatly to supplying the energy and fiber needs of high-producing dairy cows, reducing purchased feed costs from grain and byproduct supplements and generating milk revenues for dairy producers.
The purpose of this article is to review selected recent developments and strategies that may influence the nutritive value of corn silage through increases in starch and neutral detergent fiber digestibility (NDFd).
Physical and chemical factors may affect starch digestibility in corn silage. First, the starch endosperm is protected by a seed coat which, if intact, is highly resistant to microbial attachment; breakage of the seed coat is required. However, even the exposed endosperm is not fully digested due to existence of a starch-protein matrix formed by the chemical bonds of zein proteins with starch granules.
Therefore, strategies to break down or decrease zein proteins in corn kernels are crucial for enhancement of starch digestibility in corn silage.
Factors that influence physical and chemical barriers which impede maximum starch digestibility in corn silage include kernel processing, maturity at harvest, extended storage and endosperm properties. NDFd in corn silage, however, is limited primarily by the cross-linking of lignin to other fibrous components.
Therefore, decreasing lignin-to-NDF ratio is crucial to optimize NDFd in corn silage. The main factors altering this ratio are hybrid type, maturity at harvest, cutting height and the crop growing conditions, but we cannot control the latter.
Maturity at harvest
To let corn plants stand longer in the field with the purpose of obtaining greater yields of starch is a common practice. In a recent University of Wisconsin meta-analysis of corn silage trials with lactating dairy cows, however, reduced starch digestibility was observed in diets containing corn silage harvested with more than 40 percent dry matter (DM).
This may be related to an increase in the proportion of vitreous endosperm in the kernel associated with greater maturity.
Alternatively, a reduction in the extent of fermentation for drier corn silage may lessen proteolysis of zein proteins during fermentation. Increased kernel vitreous endosperm proportion, along with increased DM content of corn silage, also increases kernel hardness which, in turn, may cause kernels in very dry corn silage to be less susceptible to breakage during kernel processing at harvest.
This would explain why processing increased starch digestibility for diets containing corn silage with 32 to 40 percent DM at feedout but not when corn silage was above 40 percent DM in the same meta-analysis.
In addition, as maturity progresses, lignin content in corn plants increases. Also, other challenges related to the harvesting of drier corn silage (greater than 40 percent DM), such as packing issues and poor aerobic stability, must be considered when targeting for more mature corn silage.
Recently, research at the University of Delaware highlighted that yeasts obtained from spoiled silage reduced the capacity of the rumen microbes for digesting NDF. Therefore, greater maturity beyond 40 percent DM at harvest may limit not only starch but also NDFd of corn silage. Combined, these results suggest proper maturity at harvest is required to maximize the nutritive value of corn silage.
Kernel processing
Researchers from the University of Wisconsin also reported greater starch digestibility when corn silage was processed using 1- to 3-mm roll gap settings compared with 4- to 8-mm processed and unprocessed corn silage. This is related to increased surface area for bacterial and enzymatic digestion of finer particles.
Although degree of kernel processing was effective across a wide range of DM contents (as previously discussed) and theoretical length of cut (TLOC; 9 to 29 mm or 0.37 to 1.13 inches) settings on choppers, it did not overcome the negative effects of very high DM content on total tract starch digestibility and was ineffective at very long TLOC.
Recently, a new method of harvesting corn silage, shredlage, targets harvesting of corn silage at a longer TLOC while still maintaining or improving the degree of kernel processing.
A recent research trial from the University of Wisconsin compared a BMR corn hybrid harvested either using a shredlage processor (26 mm/1.02 inches of TLOC settings) or a conventional processor (19 mm/0.75 inches of TLOC settings). Both treatments were harvested using the same harvester.
Greater kernel processing and corresponding starch digestibility was observed for corn shredlage compared with conventional-processed corn silage. Furthermore, feeding corn shredlage enhanced lactation performance by dairy cows and may be a potential tool for dairy producers and nutritionists desiring to feed higher-corn silage diets without compromising kernel breakage and energy availability for corn silage harvested at greater length of cut.
However, despite the greater proportion of coarse fiber particles for corn shredlage, milkfat content and rumination activity were not increased.
Cutting height
Varying the cutting height is another harvest practice option to increase the nutritive value of corn silage. Researchers from Penn State University summarized data from 11 studies and concluded that by increasing the cutting height from 18 to 48 cm (7 to 19 inches), more lignin and NDF was left in the field, and thereby NDFd and starch content were both increased about 5 percent; thus milk per ton estimates were increased.
However, increased cutting height reduced DM yield per acre about 7 percent, and thus milk-per-acre estimates were reduced about 2 percent. Therefore, farm priorities for maximum yield versus higher quality should be used to determine if this harvest practice is feasible for individual farms or specific fields within farms.
Storage time
It was recently established that extended time in storage increases ammonia-N, soluble CP and starch digestibility in corn silage of various hybrids, maturities and chop lengths. These data support the use of inventory planning so newly harvested crop would be fed only after four months or more in storage.
However, extended storage did not alter the negative effects of vitreousness and maturity at harvest on starch digestibility in a recent study from the University of Wisconsin.
Although prolonged storage of corn silage would be a valid management practice overall, it requires proper silo management during filling, packing and covering to ensure beneficial fermentation patterns. While allowing an extended ensiling period may be beneficial for increasing starch digestibility, research does not support the same fate for NDFd.
Overall, data from several sites across the U.S. demonstrate that extended storage does not change or slightly reduces NDFd in corn silage.
Hybrid selection
Hybrid selection remains a reliable strategy to increase NDFd in corn silage. BMR hybrids, for example, consistently show greater NDFd than other hybrids in hybrid performance trials. In addition, cows fed corn silage-based diets with BMR hybrids of the bm3 gene mutation hybrids had greater total tract NDFd, DM intake, milk and milk protein yields in another meta-analysis review from the University of Wisconsin.
However, feed efficiency (pounds of milk per pound of DM intake) was similar among hybrids, and starch digestibility was lower for BMR hybrids in this meta-analysis. Perhaps the incorporation of a floury endosperm in hybrids containing high NDFd would be beneficial.
Bringing it home
In summary, the research literature emphasizes the importance of hybrid selection, proper maturity at harvest, kernel processing and extended storage length to achieve maximum starch digestibility in corn silage. Furthermore, hybrid selection, proper maturity at harvest and increased cutting height are viable tools to enhance NDFd in corn silage.
However, yield and nutritive value of hybrids vary from year to year and in different regions. Therefore, evaluating the results from hybrid performance trials conducted near your farm and over several years is crucial prior to selecting a new hybrid for your herd. In addition, decisions about extended storage lengths and increased cutting heights require proper inventory planning. PD
Luiz Ferraretto is a post-doctoral research associate with The William H. Miner Agricultural Research Institute. Randy Shaver is a professor and extension dairy nutritionist with the University of Wisconsin – Madison Department of Dairy Science.
References omitted due to space but are available upon request. Click here to email an editor.
PHOTO: Harvesting corn silage. Photo by Lynn Jaynes.
