Since the 1926 commercialization of hybrid corn Zea mays, steady advances in grain yield per acre have occurred. DuPont Pioneer periodically conducts "decade (grain) studies" using saved seed representative of the corn genetics of every decade from the 1930‘s to today. In DuPont Pioneer era studies conducted since 1972, corn yields showed no signs of plateauing and it is corn grain that contributes over 60% of the energy in corn silage. In these "decade" studies, genetic gains averaged about 1.5 bu/acre per year since 1963 (the "single-cross" era) in normal growing conditions, and 1.0 bu/acre per year under drought conditions. Genetic gains accounted for about 70-75% of total yield gains. Today‘s hybrids have improved stress tolerance, a higher grain-to-stover ratio, less silk delay and barrenness, better stalks and roots, smaller tassels, more upright leaves, better staygreen, and deeper roots than older hybrids. Corn yield gains show no signs of slowing. Growers can expect future gains to continue if corn research is supported at historic or higher levels (Butzen and Smith, 2014).
A corn silage version of DuPont Pioneer decade (grain) studies has been conducted at the University of Wisconsin (Coors et al., 2001; Lauer et al., 2001). This UW corn silage "era research" shows that as corn genetics have advanced, dry matter yield of both stover and whole plant have increased. Grain production has been the greatest driver of yields resulting in whole plant yields increasing faster than stover yield. Over time, cell walls (neutral detergent fiber, NDF) have comprised less and less of the whole plant, because of the dilution effect of higher grain yields. Stover, per se, has not changed significantly in percentage of NDF or in in vitro digestibility. In fact, unpublished work by DuPont Pioneer (Owens, 2011) indicates that a summary of published literature and DuPont Pioneer plot data shows that in newer genetics possessing improved late-season plant health, NDFD declined minimally over the maturity range of 30-40% dry matter, while starch increased at the rate of almost 1% unit per day (Owens, 2010).
Much of what has contributed to corn yield improvements has been improved stress tolerance allowing plants to respond better to higher planting populations (Wikner, 1996; Paszkiewicz and Butzen, 2001). Hybrid corn in the 1930‘s was typically planted at densities of 4-5,000 plants per acre; whereas today, hybrids can routinely withstand the population stress of over 35,000 plants per acre. Improved late-season plant health and kernel weight (grams per kernel) have also increased steadily since the 1950‘s. When these same modern genetics are exposed to moisture-stress, there is less improvement in yield, kernel weight, and staygreen. This fact, along with depleting agricultural water supplies, is driving seed companies to actively research mechanisms and genes controlling drought tolerance.
Corn Moisture Requirements
Estimates are that about 15% of the U.S. corn acres are irrigated. This means that 80-85% of the acres are at the mercy of Mother Nature. Corn has relatively high water use efficiency (dry matter produced per quantity of water used) compared to alfalfa, but because it produces more total dry matter, it can require more total moisture. A high-yielding corn crop requires between 20-24 inches of water and upwards of 28-30 inches in the more arid West. One inch of water per acre is about 27,000 gallons. A corn crop requiring 24 inches of moisture would require about 648,000 gallons of water. If that crop yielded a national average of 175 bu, each bushed would require about 3700 gallons of water. At some point during the growing season, 85% of all corn acres will experience some level of water deficit (Warner, 2011). Knowledge of the relationships between plants and their environment is vital to successful irrigation management (Kranz et al., 2008). Soil characteristics important to irrigation management include water holding capacity, water intake rate, and restrictive soil layers that might limit root penetration and/or water movement. Plant factors include crop development characteristics, rooting depth, and daily and total seasonal crop water use. Atmospheric factors are solar radiation, air temperature, relative humidity, and wind. Total available seasonal water supply is also important (Shanahan and Groeteke, 2011).
Irrigated corn grain yields are about 30% higher than non-irrigated yields attributing to irrigated corn accounting for nearly 20% of total U.S. corn production while occupying only 15% of acres (USDA, 2007). Much of the irrigated corn is cultivated in the semi-arid Great Plains region (Musick and Dusek, 1980) of the U.S., with corn occupying more irrigated acres in this area than any other crop (Norwood, 2000). However, recent concerns have been raised regarding declining surface and groundwater supplies (Clark et al., 2002) and increased pumping costs (Norwood and Dumler, 2002) in this region. For this reason, improving management practices under declining water supplies is critical for sustaining irrigation water resources (Shanahan and Groeteke, 2011).
Corn production uses water through evapotranspiration (ET). In this process, water is removed directly from the soil surface to the atmosphere by evaporation and through the plant by transpiration. Plant transpiration is evaporation of water from leaf and other plant surfaces. For corn, evaporation often accounts for 20-30% and transpiration accounting for the remaining 70-80% of total ET over the course of a growing season. Transpiration involves a continuous flow of water from the soil profile, into the plant roots, through plant stems and leaves, and into the atmosphere. This serves to cool the crop canopy and prevent leaf tissues from reaching lethal temperatures. Additionally, water from transpiration provides positive pressure inside cells that gives plants much of their structure and ability to stand. Finally, the transpiration stream carries water-soluble nutrients like nitrate and potassium from the soil into the plant, providing essential nourishment for plant growth (Shanahan and Groeteke, 2011).
Both evaporation and transpiration are driven by a tremendous drying force the atmosphere exerts on soil or plant surfaces. Hence the magnitude of daily ET will vary with atmospheric conditions. For example, high solar radiation and air temperatures, low humidity, clear skies and high wind increase ET, while cloudy, cool and calm days reduce ET. Seasonal water use is also affected by growth stage, length of growing season, soil fertility, water availability and the interaction of these factors. Although the amount of daily water use by the crop will vary from season to season and location to location, it will generally follow the pattern shown in Figure 2.
Figure 2. Long-term daily average (black line) and individual year (green line) corn water use by growth stage in Nebraska (adapted from Kranz et al., 2008)
When water supplies do not fully compensate for crop ET, grain yields are reduced compared to fully irrigated corn. To maximize yields and returns under limited water supplies, growers must understand how corn responds to water, and how changes in irrigation and agronomic practices can influence water needs depending on growth stage, irrigation timing, crop residue, hybrid genetics and plant populations. The impact of water stress on corn grain yield varies with crop growth stage (Figure 3). During the vegetative growth of the corn plant, it is relatively drought tolerant and can survive on upward of 60% soil water depletion in the root zones without a significant impact on grain yield. However, silage yields will be reduced due to shorter plants when moisture-stressed during vegetative growth stages. The corn plant needs the most moisture from about silking through the blister stage (Figure 2). After blister stage, the plant is again fairly immune to water deficiency and irrigation can be terminated when the kernel milk line is at about 50% (Figure 2). Growers may be able to delay the first irrigation as late as tasseling in years of lower evaporative demand provided soil water reserves are ample at planting and irrigation systems have the capacity to rapidly correct soil water deficits (Shanahan and Groeteke, 2011).
Figure 3. Yield susceptibility to water stress for corn (adapted from Sudar et al., 1981)
In recent years, the seed industry has been actively engaged in utilizing advanced genetic tools to mine and advance native drought resistance in pursuit of more drought-tolerant hybrids. Several of these products are now on the market and demonstrate upwards of a 5% average grain yield advantage over leading commercial hybrids when water was limited during flowering or grain fill to less than 66% of optimum crop moisture (Warner, 2011). Transgenic approaches to drought tolerance are also being actively pursued by several seed companies but regulatory hurdles must be met before they will reach the marketplace. In general, the tremendous research dollars spent on corn breeding and research compared to any other crop, along with the introduction of biotechnology traits, has been the key driver in the continuous improvement in agronomics and yield of corn.
Weather patterns, if trending toward either warmer/colder or wetter/dryer have potential to impact corn yields over time. Increased weather variability within single seasons could also affect yield trend. Using the DuPont Pioneer propriety software, EnClass®, Pioneer breeders were able to evaluate historic weather patterns and model their expected impacts on yield from 1950-2011. This analysis of weather records determined that the effects of weather on yield was minimal, contributing an upward bias of only 0.02 bu/acre per year during the period studied (Butzen and Smith, 2014).
Crop Management Advances
In addition to genetic and technology trait gains, corn yields have benefitted from improvements in cropping practices. Those most beneficial and widely adopted by growers include: 1) earlier planting, which reduces moisture stress during pollination and ear fill, and lengthens the growing season; 2) use of seed treatments that contain a fungicide and insecticide, and may also include a nematicide, growth promoter, or other active ingredient, 3) increasing use of foliar fungicides to limit leaf diseases, 4) use of improved planters to achieve more consistent depth and coverage of seed, more equal plant-to-plant spacing to reduce competitive effects among plants, more timely planting of a higher percentage of corn acres at higher ground speeds, 5) improvements in irrigation practices and number of acres of irrigated production, 6) improved fertility practices, including higher rates of nitrogen fertilizer, 7) variable-rate technologies that allows growers to plant specific hybrids and place fertilizer where most beneficial, 8) narrower row spacing and 9) increase in systematic tiling (Butzen and Smith, 2014).
Some nutritionists question if breeding for improved agronomic traits, such as standability, has negatively impacted corn stover (cell wall) nutritional composition and digestibility. In conventional corn hybrids, there is no obvious association between either fiber or lignin concentration and stalk lodging. Distribution of structural material may be as important, or more important, than concentration of structural components, per se (Allen et al., 2003). The University of Wisconsin Departments of Agronomy and Dairy Science jointly led a 1991-95 UW Corn Silage Consortium that was jointly funded by all the major seed companies. A review of their findings (Coors, 1996) indicates there was genetic variation for nutritive value among adapted U.S. corn hybrids with both silage yield and grain yield potential and that forage quality and agronomic traits were not highly correlated.
The heritability of fiber digestibility in conventional corn silage hybrids is quite high; however, the genetic variation to apply selection pressure against is relatively narrow in high yielding corn genetics. The introduction of brown mid-rib (BMR) corn as a non-GMO, recessive gene trait to improve fiber digestibility in corn silage is testament to the fact that significant improvements in fiber digestibility could not be achieved by traditional selection methods. Corn hybrids with BMR mutants have less lignin and a lower proportion of iNDF than isogenic conventional corn silages. Research conducted at the Miner Institute (Grant and Cotanch, 2011) indicate that, presumably, the more fragile fiber in BMR is what drives higher intakes in early lactation cows who lack the ability to satisfy energy needs from typical dry matter intakes. Rations need to be balanced differently when using BMR corn silage, particularly in terms of starch supplementation, total NDF and physically-effective fiber levels. Despite the lower lignin in BMR resulting in higher fiber digestibility, BMR genetics are also at the mercy of Mother Nature just like conventional silage genetics. Excessively wet growing conditions prior to silking (vegetative stages) typically increases plant height and reduces fiber digestibility, while growing conditions after silking appeared to only exert an effect on grain yield (Mertens, 2002). Bolinger (2010) summarized data from Michigan State University silage plots harvested in a relatively wet growing season (2006) compared to the same hybrids harvested from the same plot in a relatively dry growing season (2007). Hybrids averaged 6.5 points higher in 24-hour NDFD in the drought year. It was interesting to note that, as expected, the highest NDFD in both seasons was a BMR hybrid, but the BMR fiber digestibility was also reduced in the wet growing season (Mahanna, 2010). It has been proposed that with irrigated crops, silage growers might stress the crop for water during pre-tasseling to increase NDFD (without reducing plant height too much) and applying the conserved water more liberally during kernel starch filling periods of plant growth. More research is definitely warranted as to when to irrigate the corn plant to manipulate both silage yield and nutritional value.
Corn Silage Harvest and Feeding Advances
High-chopping is a management option to potentially increase fiber digestibility and concentrate more starch in corn silage. In a review of high-chop research by Wu and Roth (2004) at Pennsylvania State University, they found an average increase of 6.7% in NDFD and a 5.9% increase in starch content when comparing 19-inch versus 7-inch chop heights. Leaving the less digestible, lower stalk internodes in the field resulted in an average dry matter loss per acre of 7.4%. There does appear to be a significant genetics-by-growing-season interaction suggesting that hybrids need to be analyzed for NDFD at various chop heights just prior to harvest because not all hybrids respond the same to specific growing environments. Several lactation studies with high-chop corn silage indicate higher milk production and but reduced milk fat content. This is likely the result of researchers not reducing the starch level in the high-chop treatments; unlike what field nutritionists would do when recognizing the increased starch level from high-chopped corn silage.
Changing Starch Digestibility
Several research studies have put credence to field experience that starch and protein degradability increase over time in corn silage. However, the effects of fermentation should not be viewed as an acceptable alternative to adequate pericarp damage from proper kernel processing at harvest. Using newly available starch digestibility laboratory methods (e.g. 7-hour starch digestibility or Fermentrics™) or tracking water-soluble nitrogen levels correlated to increasing starch digestion, can help nutritionists monitor these changes and make appropriate ration adjustments. Understanding these changes can help nutritionists better formulate cost effective rations as well as prevent potential sub-acute acidosis problems caused by longer-fermented silages (Mahanna, 2007).
More Mature Kernel Harvest
As late-season plant health of the corn plant continues to improve from both genetic advancements and management practices (e.g. foliar fungicides), it allows producers the ability to delay harvest and obtain more starch from advancing kernel maturities without sacrificing significant declines in NDFD. A recent study by Seglar et al., (2014) showed that as kernels matured from the half milk line to black layer, kernel weight increased an average of 24% and starch by 27%, suggesting that premature harvest of corn silage dramatically reduces starch content.
There has been recent interest in the endosperm type (e.g. floury versus vitreous) of kernels found in corn silage. Harder texture kernels typically have more vitreous endosperm accompanied by higher levels of zein proteins (prolamin) surrounding the starch granules. Despite some of the marketing claims by some seed companies about the improved ruminal starch digestibility of floury endosperm hybrids, a study (Seglar et al., 2014) of commercial corn hybrids grown in different years at two different locations and harvested at three maturities indicated that neither the kernel density, prolamin content nor prolamin:starch ratio of kernels reliably predicted seven-hour ruminal starch digestibility.
Advocates of floury genetics often show kernel texture data on fully mature dry corn, lacking data on hybrid vitreousness levels at corn silage maturities (half to three-quarter milk line). It is further misleading to promote university starch digestibility studies comparing genetic extremes (e.g. 3-66%) in vitreousness (Mahanna, 2013). These comparisons make sense for researchers investigating the mode of action of starch digestion. However, vitreous ranges this wide simply do not exist in commercially viable North American corn hybrids that typically exhibit a range in vitreousness of 50-70% in fully mature kernels and even less of a range in kernels at silage harvest maturity.
University of Wisconsin researchers (Hoffman et al., 2012) have developed an integrated analytical approach to starch digestibility called Feed Grain V2.0 that is available at select laboratories. This approach reinforces the relative importance of: 1) kernel particle size, 2) extent and length of fermentation and finally, 3) endosperm differences (vitreousness or hard kernels). In Feed Grain V2.0, starch digestibility in fermented samples are based on particle size and ammonia content (more ammonia, the longer the fermentation). Starch digestibility in unfermented, dry corn grain is based on particle size and prolamin content. The prolamin content is not considered in high-moisture corn, snaplage or corn silage starch digestibility calculations because of the small variation and minimal effect vitreousness (kernel texture) has on grain harvested at relatively early kernel maturities (pre-black layer).
The inclusion of vitreousness or kernel texture for dry corn grain is consistent with a review by Firkins (2006) indicating that vitreousness of corn grain in silages (fermented grains) was of relatively little value, whereas vitreousness of dry corn grain should be considered, particularly to help users know when to grind corn more finely. At the same particle size, starch digestion is similar for soft and hard corn. More vitreous (hard) corn simply yields larger, more slowly digested particles than softer corn, particularly if it is ground. Research from France (Ramos, 2009) with relatively high-vitreous (flinty) corn compared to North American hybrids showed that grinding removes most of the negative influence of vitreousness in dry corn. The body of research to dates suggests it makes more sense for producers and nutritionists to focus attention on corn yield, agronomic strengths/weaknesses, particle size and fermentation quality rather than the minor effects of kernel texture, especially in silage hybrids (Mahanna, 2013).
Kernel Processing Advances
Laboratory methods now exist (e.g., corn silage processing scores [CSPS]) which allow nutritionists a better understanding of the particle size distribution of kernels in corn silage. Mining laboratory data on how well kernels were processed in submitted corn silage samples indicates upwards of 40% are significantly under-processed. At the same time, producers often desire even longer corn silage fiber particle size in high corn silage diets in an attempt to improve effective fiber and avoid the necessity of adding long fiber such as hay or straw to help establish a rumen mat to stimulate rumination to help buffer the rumen environment. The commercial release of Shredlage® processors in 2010 allowed for excellent kernel damage even when chopping at upwards of 26-30mm (compared to standard 19mm). The design of the teeth on the Shredlage rolls rip and tear rather than smashing kernels apart like conventional rolls. Shredlage rolls also have more grooves on one roll than the other which adds even more differential without changing the speed of the rolls more than the 30% differential set at the factory (Olson, 2013). Two lactation studies by Shaver and co-workers have proven the merits of this alternative approach to processing corn silage (Mahanna, 2014, 2012). It is very encouraging that chopper manufacturers like John Deere and Claas are now also offering unique roller mill designs or modification kits to speed up roll differentials, to finally give dairy producers both effective fiber and kernel damage needed in high corn silage diets.
Choppers with NIRS
It is entirely possible in the near future to "dial-in" desired NDFD or starch content of corn silage with choppers outfitted with on-board Near Infrared Reflectance Spectroscopy (NIRS). As silage is exiting the chopper spout, it could be analyzed for important constituents and the cutter head tied into this information forcing the head up or down to modify either NDFD or starch content of the silage being harvested.