Corn Starch Digestibility Revisited: Fermentation
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By Bill Mahanna, PhD, Dipl. ACAN, Pioneer Global Nutritional Sciences Manager
This is a continuation of the Feb. 9 Bottom Line column that covered intrinsic kernel characteristics that influence rumen and intestinal digestion. The focus this month will be on the modifying effect of processing methods like fermentation and flaking, issues surrounding research conclusions that can be drawn from the current body of published literature and considerations to keep in mind when selecting a corn hybrid.
With corn grain fed in the form of dryrolled grain, starch digestibility generally is lower for larger particles from more vitreous kernels. However, vitreousness has little, if any, effect on the digestibility of starch from corn that is moist, well fermented/processed (silage or grain) or adequately steam flaked (Owens and Soderlund, 2007; Firkins, 2006).
Starch digestibility from high-moisture corn appears to be closely related to the moisture content of ensiled grain. Higher moisture content (greater than 27%) extends the fermentation process and increases the total acid concentration. This results in an increase in both the ruminal and total-tract digestion of starch.
Several studies show further that a longer duration (weeks or months) of fermentation prior to feeding will increase starch digestibility (Benton et al., 2005; Newbold et al., 2006; Hallada et al., 2008).
Owens (2007) has proposed that the length of fermentation exerts its influence primarily by solubilizing ethanol-soluble zein proteins combined with softening or hydrolysis of other kernel proteins that may interfere with starch granule degradation.
Nebraska research (Benton et al., 2005) has shown that ruminal protein degradation closely parallels in situ dry matter digestibility. However, Newbold (2006), working with corn silage, showed that crude protein degradation increased with time of storage but was not highly correlated (P > 0.1) with starch digestibility.
For decades, feedlot nutritionists have relied on soluble crude protein (or nitrogen) content of high-moisture corn as a proxy for the rate of starch digestion by feedlot cattle. In several unpublished field trials in which soluble nitrogen was tracked over time in the Great Plains, typical nitrogen solubility levels began at ensiling (September) in the high 30th percentile, increased to the mid-40s by December and could reach the mid-60s by February. Feedlot nutritionists usually consider values between 50% and 60% water-soluble nitrogen (expressed as a fraction of total nitrogen) as ideal for feedlot rations.
High-moisture corn with a protein solubility of less than 40% is generally from lower-moisture corn (with less extensive fermentation) that exhibits lower starch digestibility more similar to that of dry-rolled corn. Feeding high-moisture corn with a nitrogen solubility that exceeds 60% often results in more acidosis and off-feed incidents (Soderlund, 2009).
As time in storage increases, feed conversion improves, but so do the incidences of digestive disorders.
Using high-moisture corn and corn silage analyses from individual months during 2002-06 posted on the Dairy One Laboratory web site (2007), Owens (2009) graphed the mean solubility of protein within corn grain and corn silage samples to examine whether protein solubility changed with time after harvest.
With the high-moisture corn, mean protein solubility of assayed samples increased progressively with time after harvest. Generally, samples of high-moisture corn with a higher moisture content (greater than 27%) have a protein solubility, reflecting more extensive fermentation and an increased concentration of silage acids. Lessextensive fermentation may explain why drier corn (less than 25% moisture) has lower rumen degradation.
With corn silage, protein solubility reaches a plateau some five or six months after harvest. This may be due to the higher moisture content of corn silage than high-moisture corn and the fact that corn protein is typically less than 60% of the total protein in corn silage compared to 100% of the protein in high-moisture corn grain.
The effects of the fermentation environment and proteolytic activity on freeing up starch granules for improved digestion can help explain conclusions from a recent literature review of starch digestibility trials by Owens and Zinn (2005) where ruminal disappearance for high-moisture, steam-rolled (or flaked) and dry-rolled corn averaged 76%, 54% and 49%, respectively. Summarized within processing method, the post-ruminal disappearance of starch leaving the rumen for high-moisture, steam-rolled and dry-rolled corns averaged 84%, 82% and 80%, supporting the concept that flaking quantitatively shifts the site of digestion from the rumen to the intestines, whereas the fermentation process shifted the site of digestion back toward the rumen.
Limited data on starch disappearance in the large intestine indicated that compared to high-moisture or steam flaked corns, much more of the dry-rolled corn starch was digested in the large intestine, possibly because proteolytic activity in either the small or large intestine exposed more starch granules for fermentation by bacteria in the large intestine. Total-tract starch digestibility from high-moisture, steam-rolled and dryrolled corns in this review averaged 96%, 94% and 91%, respectively.
Considering the ruminal acidosis concerns on many dairies and the potential energetic advantages from increasing intestinal starch supply, some researchers have proposed a goal of decreasing — not increasing — the rate of starch fermentation and ruminal digestion. Except, perhaps, for diets containing significant amounts of dryrolled corn grain, increasing the extentof ruminal starch digestion will likely not improve the productive efficiency of ruminants (Owens and Zinn, 2005; Owens, 2009).
Measurements of the rate and extent of starch fermentation in the rumen and digestion in the intestines are based largely on in vitro (test tube) and in situ (Dacron bag) methods using finely ground grain. Compared with in vivo results, such methods appear to underestimate the extent of slowly degraded starch and overestimate the proportion of rapidly fermented starch (Svihus et al., 2005) probably due to the very fine particle size (1 mm grind) of test samples versus grain particles entering or recovered from the rumen.
Starch digestion in the small intestine is ignored with most in vitro methods, as is compensatory starch digestion in the large intestine that would contribute to total-tract starch disappearance, especially with dry-rolled corn. Estimating the extent of ruminal digestion from the digestion rate requires some estimate of ruminal residence time, a factor that varies with diet, ruminal segregation and level of feed and neutral detergent fiber intake (Owens, 2009).
Furthermore, the role aerobically sensitive rumen protozoal and fungal populations play in starch digestion will be unaccounted for unless in vitro methods are extremely conscientious about maintaining temperature, anaerobic conditions and the activity of rumen fl uid as it is transferred from donor animal to the in vitro vessel (Sapienza, 2009).
Results from meta-analyses of feeding and digestion trials with cereal grains often prove difficult to interpret given the wide diversity among trials in grind size, particle size distribution, kernel moisture, presence of stress cracks in heat-dried kernels and kernel density. Such factors usually are not reported in research publications.
In one trial, Szasz et al. (2006) found that when high-moisture corn was rolled wet, particle size was smaller for the hybrid that was more vitreous when mature — opposite of what was expected from rolling dry grains that differ in vitreousness. In their study, more than 96% of the variation in total-tract organic matter digestibility could be explained by geometric mean diameter of the high-moisture corn particles.
In the absence of detailed chemical and physical analyses of grain samples both before processing (starch content, kernel size, absolute density) and after processing (geometric mean particle size and distribution, exposed surface area), it is difficult to draw valid conclusions from the published literature.
Care is needed to ensure that sample handling and ranges being tested in research studies are realistic. For example, extrapolating results from studies on kernel maturity (Correa et al., 2002; Ngonyamo-Majee et al., 2008) to the feeding of fermented high-moisture corn or corn silage is open to question when kernels are assayed as unfermented grain and not exposed to the modifying effects the fermentation process can have on both the pericarp and the endosperm protein:starch matrix.
Other studies have investigated starch digestibility using extremes in vitreousness ranging from 3% to 66% (Taylor and Allen, 2005a,b,c) or from 25% to 66% (Allen et al., 2008). Although such wide extremes in vitreousness (and, presumably, prolamin content) may aid in the understanding of how one specific mechanism can limit starch digestion, caution should be exercised when applying these findings (or production expectations) to field situations where rations are built around commercial hybrids with a much narrower range in vitreousness (typically 55-65%).
One in vivo study by Corona et al. (2006) used four hybrids within the typical range in vitreousness (55%, 61%, 63% and 65%). Increasing vitreousness of dry-rolled corn failed to significantly affect ruminal disappearance of starch but, surprisingly, tended to reduce postruminal digestion.
IF the concentration of total zeins (prolamins) in corn grain alters starch accessibility in vivo, one would expect that the site and extent of starch digestion would be proportional to the concentration of zeins or the zein:protein ratio.
In an unpublished steer digestion trial, grains from isogenic hybrids that differed in zein content were fed. Reducing the concentration of zeins failed to increase starch disappearance in the rumen, small intestine, large intestine or total tract when these grains were fed as either dry-rolled or steam-flaked grain. However, with dry-rolled grain, ruminal degradation of dietary protein tended to be greater for the hybrid with reduced zeins, indicating that protein digestibility had been increased (Owens, 2009).
Failure of a reduction in zeins to improve starch digestion by steers fed isogenic corn hybrids in this trial makes one question the practical importance of total prolamin concentration relative to other factors (particle size, pericarp shielding and degree of disulfide linkage) that can alter accessibility of starch for digestion and the energy availability of dry-rolled corn grain.
Both agronomic and nutritional factors must be considered when selecting grain or silage hybrids. Nutritionists often are unaware or unfamiliar with agronomic considerations involved with grain or silage production because their responsibility is to optimize performance and profitability of animals fed harvested crops. Conversely, some seed companies do not fully recognize the importance of nutritional factors because they will be judged based on plant standability, disease/pest resistance and yield.
Solely focusing on either quality or quantity traits fails to recognize the complex interactions among hybrid genetics, harvest maturity, feed type (dry versus fermented), processing method, particle size and potential associative effects with other ration ingredients. For example, based on analyses across modern hybrids, grain yield and kernel density were correlated positively (P < 0.01; Owens, 2009). This suggests that direct selection for reduced kernel density would reduce grain yield.
Attributes desired for hybrids will also differ depending on the grain processing method they will undergo. For corn fed as dry, ground grain, the starch from less-vitreous dent hybrids likely will be more extensively digested in the rumen than starch from more-vitreous dent (or fl int) hybrids. As discussed earlier, this increase may or may not be of benefit depending upon other ration ingredients, the need for microbial protein production and the possible desire to shift starch digestion to the intestines to reduce the potential for ruminal acidosis.
In contrast, when selecting hybrids for corn silage, high-moisture corn or steam fl aking, more-vitreous dent hybrids appear to be preferable because the processes of ensiling (protein matrix solubilization and acid hydrolysis) or flaking (physical gelatinization) will minimize most adverse effects associated with vitreousness (Szasz et al., 2006; Owens, 2009).
Research across hybrids suggests that larger kernels may lead to improved totaltract organic matter digestibility when corn grain is fed in either the dry-rolled, flaked or high-moisture form.
In a Nebraska study (Jaeger et al., 2004), seven hybrids were dry rolled and fed to steers. Average daily gain did not differ among hybrids, but feed efficiency was superior for hybrids with a larger mean kernel weight. Some nutritionists have developed grain purchase and discount guidelines, such as a minimum of 35 g/100 kernels, as a proxy physical measurement index for ensuring larger kernel size and density (Soderlund, 2009).
The economic and ruminal health implications of starch digestibility are immense. Digestion of corn grain by dairy cattle appears to be limited primarily Reprint Feedstuffs, March 9, 2009 3 by large particle size and the pericarp shielding the endosperm. In addition, the starch fermentation rate can be reduced in mature, dry corn depending on the level of protein shielding (presence of prolamin proteins, commonly called zeins) that surround and link the starch granules in the portion of the endosperm containing more of the dense, vitreous starch.
Fine grinding will increase starch digestion by cattle, but more extensive processing methods such as ensiling or fl aking will greatly minimize or obliterate the effect of vitreousness on starch digestion. In addition to increasing totaltract digestion, extensive processing also can shift the site of starch digestion.
Concerns about a slight reduction in ruminal starch digestibility have driven researchers in Europe and South America (where fl inty hybrids are more common) - and, more recently, in North America - to examine the effects of vitreousness (and zein content) on starch digestibility of corn grain samples that differ in vitreousness (absolute density). It has also spurred a new lab test, mTZM, to better quantify the corn zeins (prolamins) that slow starch digestion.
Assuming that measuring prolamin content is repeatable in the lab, in vivo animal studies with real-world rations are needed to assess corn prolamin effects on milk production and to rank cereal chemistry parameters in relation to other factors known to affect starch digestibility such as kernel moisture/ maturity, kernel size, starch content, grain processing method and grain particle size/distribution.
Studies investigating the extremes in vitreousness (and, presumably, in prolamin content) have provided valuable insight into the biochemical and physical mechanisms that limit the rate of starch digestion. Caution should be exercised when attempting to apply these observations to field situations that use rations containing productive, commercial hybrids with a more moderate level of density and prolamin content. Furthermore, research results based on studies with dry, unfermented kernels typically do not apply when grain is processed (fermented as high-moisture corn and corn silage or steam fl aked) prior to feeding.
Published animal research on starch digestibility has been limited, making it difficult to draw general conclusions given the lack of reporting for important factors such as kernel size, mean particle size and distribution. Extrapolating from in vitro and in situ measurements often proves faulty. Nonetheless, the infl uence on digestibility from intrinsic differences in the chemistry of cereal grains is an area ripe for further investigation.
However, until there is a better understanding of the variability and magnitude of cereal characteristics needed to infl uence animal performance, there appears to be room on most dairies for a near-term focus on quantifying and managing the variability routinely experienced in corn grain and corn silage related to kernel moisture/maturity, starch content, particle size/distribution and the increase in starch digestibility associated with the duration of ensiled storage.