Starch Digestibility of Corn – Silage and Grain



Nonstructural carbohydrates (NSC) have been discussed in numerous proceedings from the Tri-State Dairy Nutrition Conference. Major sources include starch, sugars, and neutral detergent solubles, depending on the type of laboratory measurement made, which also has been widely discussed. Clearly, the amount of NSC fed depends on the amount of effective NDF in the diet and that which was actually consumed and not sorted. Effective NDF can be broken down into the ability of NDF to stimulate chewing to stimulate salivary buffer secretion and to dilute NSC to prevent excessive rate of volatile fatty acid (VFA) production. Moreover, the amount of NSC that should be fed depends on the amount of rumen-degraded protein and other factors affecting the efficiency of usage of energy available from fermentation. Although we know much more in general about these topics, there remains a major limitation in improving our feeding strategies on dairy farms: how do we reliably predict the amount of rumen-degraded starch (RDS) reliably and use that information to make rations work more consistently with diverse forages and bunk management capabilities? My major emphases will be to discuss recent research: 1) relating starch digestibility in corn silages differing in processing methods and nutritional qualities and 2) describing how processing of corn grain affects site of digestion. My goal is that this information will help nutrition advisers diagnose differences among farms to help improve the amount or efficiency of milk production under their particular circumstances.

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Starch Digestibility and Kernel Processing of Corn Silage

In the past 10 years, there has been considerable research on kernel processing (KP). Building from the foundational work of comparing KP versus unprocessed silages (Johnson et al.,1999), more current research has been documenting interactions in the efficacy of KP among chop length, maturity, and corn cultivars. Weiss and Wyatt (2000) found that KP increased apparent total tract starch digestibility from kernels while still allowing greater length of chop to increase the effectiveness of the NDF portion. The benefit of KP was more pronounced with a conventional hybrid than with a high-oil hybrid, which already maintained higher starch digestibility. If high-oil hybrids increased in frequency of planting, further work would be needed because increasing amylose/amylopectin ratio of corn could decrease starch digestibility in part because of increased chemical interactions with lipid (Svihus et al., 2005).

Based on a series of experiments by Wisconsin and Washington State researchers, it is well documented that KP is more beneficial with advancing maturity of the corn. In fact, the dry matter (DM) percentage remains a key diagnostic indicator, even with KP silages (Johnson et al., 2002a). Moreover, increasing DM was highly correlated with vitreousness of the corn kernel in the silage. Vitreousness is a term describing the type of endosperm in the corn kernel. Researchers manually fractionate the components of the starch kernel and measure the proportion of starch in the endosperm that is floury versus vitreous. The latter is associated with decreased susceptibility to amylase either originating from microbes or from the animal, in part because of increasing interaction with corn protein. However, when these two hybrids were selected to be different in vitreousness, there was not a major difference in ruminal starch digestibility, perhaps because they stated that vitreousness changed from before to after ensiling. In a companion study (Johnson et al., 2002b), the hybrid that was supposed to have kernels that were more vitreous (based on planting strategy) actually had greater apparent ruminal starch digestibilities. When the two different hybrids were harvested at two-thirds milkline, both had very comparable vitreousness values (48% of starch). The KP increased their measure of RDS in both hybrids but more in one than another. Still, differences were relatively minor in total tract starch digestibility. Moreover, data in their paper documented moderate relationships when total tract starch digestibility was regressed against the percentage of post-ensiling vitreousness values, but the low scale for digestibility (a range of only a couple of percentage units) compared against the scale for vitreousness (a range of approximately 70 percentage units) and the dispersion (variability) about the regression document little applicability of vitreousness as a predictor of starch digestibility for silages. Further work (Johnson et al., 2003) demonstrated that total tract starch digestibility was primarily a function of the amount of intact kernels remaining in silage, which obviously depends on whether or not the silage was processed.

Ferreira and Mertens (2005) selected 32 samples of widely diverse chemical and physical corn silages being used in the field. The percentage of starch in the particle fraction from a sieve of 4.75-mm (0.187 of an inch) pore size ranged from 8.7 to 100%. They developed an index based on the starch remaining in this sieve (roughly one-fourth or larger of a kernel) to represent slowly degraded starch, which correlated with in vitro disappearance of starch and non-fiber carbohydrate (the by-difference calculation). Although they stated that further in vivo testing is needed, this index should be considered for application. Because the Penn State shaker tends to separate higher (0.31 of an inch for the middle screen) and lower (0.05 of an inch) than 0.187 of an inch, it is unclear if this index is adaptable to that current Penn State system. In addition, Stone (2004) elaborated on the necessity of calibration of the Penn State shaker system; to be consistent, calibration probably also would be needed for the 4.75-mm index or any other index as it is adapted from the originating lab to any commercial lab (see later comments).

Other sources of variation might not be explained by individual experiments. Cooke and Bernard (2005) documented reasons for varying particle size (theoretical length of cut, TLC) and magnitude of KP (different roller clearance) in the field as farmers try to speed up the time for harvest (Table 1). Decreasing roller clearance from 8 to 2 mm increased total tract starch digestibility, regardless of TLC. However, the combination of the greater clearance and TLC seemed to negate the benefit on NDF digestibility. These latter values are lower than expected and might be a result of the digestibility marker used (indigestible ADF) or from climate (Georgia). However, the interaction of TLC and roller clearance seemed to be largely explained by lower energy-corrected milk and milk efficiency in that last treatment. Clearly, either 1 inch of TLC is too coarse, unless the extra power and time are used to fully process the kernels. Moreover, it would be expected that results would be further exaggerated when cows are in free stalls and sorting behavior would be worsened with increased chop length. This trial certainly documents why there could be differences among trials due to actual efficacy of chopping and rolling.

Table 1. Interaction of chop length and kernel processing on digestibility and lactation performance.1
TLC (inch): ¾ ¾ ¾ 1 1
           Item            Roller clearance (mm): None 2 8 2 8 Contrast
Starch digestibility, % 79.4 83.1 75.8 87.7 75.3 R
NDF digestibility, % 20.1 29.7 30.6 35.4 23.2 P, I
DMI, lb/day 48.2 48.0 48.8 48.6 48.2
ECM, lb/day 81.0 81.0 82.9 83.8 75.7 I
ECM/DMI 1.70 1.72 1.71 1.74 1.56 I
1From Cooke and Bernard (2005). TLC = theoretical length of cut.
2R = main effect of roller clearance, P = processing effect (first treatment versus average of last four), and I = interaction of TLC and R for the last four treatments. DMI = dry matter intake, ECM = energy-corrected milk, and NDF = neutral detergent fiber.

Starch Digestibility and Corn Silage Hybrid

Differences in total tract starch digestibility might result from differing types of corn silage hybrids. When a higher fiber hybrid was compared to a conventional hybrid, starch digestibility increased, although part of the response could have been due to varying corn grain concentration in the diet (Weiss and Wyatt, 2002). In a study comparing unprocessed or kernel-processed brown midrib (BMR) silage to KP conventional silage (Ebling and Kung, 2004), milk production was only increased by BMR compared with conventional KP silage if the BMR was also KP (Table 2). Total tract digestibility of starch was decreased when BMR silage was not processed, and this result seemed to be related to the number of intact kernels excreted in the feces. In fact, this trend also was seen in the Washington State and Wisconsin studies. Interestingly, if an in situ assay for starch digestibility of manually processed samples was extended too long, this difference was not significant even though the data more closely approximated the in vivo data. Moreover, fecal pH decreased with decreasing total tract starch digestibility, indicating continuing hindgut fermentation. Personally, I don’t have much faith in fecal pH as a diagnostic for rumen acidosis because shifting digestion from the rumen to the intestines should decrease pH. The in situ data highlight the need for standardization among protocols like this for improved predictability over a wide range of silages and conditions.

Table 2. Starch digestibility and milk production by dairy cattle fed processed conventional or brown midrib (BMR) corn silage.1
Control BMR
Processed Processed Unprocessed
In situ starch disappearance, %
           3 h
           12 h
           30 h



Total tract starch digestibility, % 99.0a 98.7a 88.5b
Number corn kernels per lb feces 7.3b 5.5b 36.4a
Fecal pH 6.92a 6.97a 6.81b
Dry matter intake, lb/day 51.5b 57.0 53.9ab
Milk, lb/day 91.1b 97.5a 93.5ab
abMeans in the same row with dissimilar superscripts differ statistically (P < 0.05).
1From Ebling and Kung (2004).

Kinetics of digestion and passage of BMR hybrids (and probably other corn hybrids) need to be taken into consideration. Oba and Allen (2000a,b) factorialized conventional versus BMR silage with dietary NDF level. The BMR treatments decreased the apparent rumen digestibility of starch, perhaps because the BMR hybrid was drier or had less corn grain in the diet. Interestingly, increasing the amount of either silage (i.e., increased NDF) decreased both the digestion and passage rates of starch from the rumen. Clearly, the interaction of silage with rumen turnover is a factor that needs further research (and will be developed more in the next section).

Starch Digestibility of Processed Corn Grain

Considerable work has been done to characterize grain structure, including chemical and physical aspects of starch composition and interrelationship with other components of the kernel (Svihus et al., 2005). Firkins et al. (2001) have quantified differences in corn processing methods for ruminal and total tract digestibility, microbial N flow to the duodenum, and milk production characteristics (Tables 3 and 4). In general, more aggressive processing of corn grain will increase ruminal digestibility of starch, but this tends to be at the expense of ruminal NDF digestibility. There was considerable compensation of digestion in the intestines such that the overall benefit on total tract organic matter digestibility was relatively minor: about 3 to 5 % units might be expected. Previously, I determined that this improvement might provide enough energy to support about 5 lb/day of milk (Firkins, 1997a). However, these studies were nearly all with fixed levels of corn in the diets within studies, and most studies provided low or no corn silage. Therefore, negative rumen effects from highly processed corn might have been overestimated, and any benefits of using less aggressively processed corn grain in heavy corn silage diets was not ascertained. Although we showed that increasing DM intake decreased the percentage of corn grain starch that was degraded in the rumen, increasing intake still increased the amount of starch being broken down and fermented in the rumen on a lb/day basis. Consequently, it is becoming more and more important to balance diets to account for RDS and perhaps diluting highly available starch with slower degrading by-products, especially when cows are likely to sort against forage and slug feed in freestalls.

Table 3. Ruminal and total tract digestibilities of nutrients and duodenal flow of microbial N by lactating dairy cows fed different corn sources.1
Rumen, % Total tract, %
Corn Source Starch,
  NDF    OM, 
Microbial N,
  NDF   OM,
Dry, cracked or rolled 44.6 48.1 52.3 276 85.0 52.0 66.6
Dry, ground 52.3 44.9 48.6 257 90.7 49.0 67.8
Dry, ground finely 91.4 51.2 69.8
Steam-rolled 88.8 49.8 67.2
Steam-flaked 56.9 41.9 52.8 296 94.2 48.2 68.6
High-moisture, rolled 86.8 47.1 60.1 236 94.2 50.0 71.9
High-moisture, ground 98.8 50.4 73.9
1Adjusted for effects of experiment and other significant variables (Firkins et al., 2001). All data are on an apparent basis (not accounting for endogenous or microbial contributions) except organic matter (OM) digestibility in the rumen. Note that 56.9% for steam-flaked corn is probably too low (should be around 65%), as explained in the paper.
Table 4. Lactation performance for Holstein cows fed different corn sources.1
Corn Source DMI, lb/day Milk, lb/day Protein, % Fat, %
Dry, cracked or rolled 49.5 68.0 3.09 3.59
Dry, ground 50.8 69.3 3.18 3.53
Dry, ground finely 48.2 71.3 3.02 3.49
Steam-rolled 48.6 70.2 3.10 3.49
Steam-flaked 50.2 71.5 3.10 3.36
High-moisture, rolled 49.9 71.5 3.17 3.54
High-moisture, ground 50.8 74.6 3.17 3.37
1Data are adjusted for effects of experiment and other significant variables (Firkins et al., 2001). To interpret these data for milk, for example, the actual data were scaled to an average dry matter intake (DMI).

Site of Starch Digestion

We must remember that simply shifting starch digestion out of the rumen has several ramifications that might negate its effect. As explained previously, theoretically it is more efficient to digest starch and absorb glucose from the small intestine than to produce VFA, of which only propionate and a minor amount of branched chain VFA can be net precursors for glucose synthesis. However, with the fast passage rates of high-producing dairy cattle, most processing methods that decrease RDS also decrease total tract digestibility of starch by 5 to 10% units (Firkins et al., 2001). Also, it is becoming clearer that increasing protein supply to the small intestine increases intestinal digestibility of starch (Abramson et al., 2005). Therefore, shifting too much starch to the small intestine without coupling RDS to energy available for microbial protein synthesis could theoretically further decrease the benefit of shifting starch digestibility to the small intestine. Further, calculations estimating efficiency of energy availability from digestibility of starch in the rumen versus the small intestine might be exacerbated by assuming a constant methane output (Harmon and McLeod, 2001). For example, even if molar proportion of propionate were increased from 20 to 25% of total VFA, this might seem only moderately significant. However, increasing RDS should increase total VFA production. Also, if the corn grain were one-third of the diet, this 5% unit increase was actually diluted to the extent that non-grain components of the diet would be fermented to propionate (and not changed by corn processing). With propionate fermentation, there is no stoichiometric possibility for methane being produced. Consequently, I think that increasing RDS probably has a much less negative effect on energy or nutrients available to support milk production than currently projected.

Optimizing Rumen-Degraded Starch

The amount of RDS consumed must be maintained to an adequate level to prevent ruminal acidosis and to prevent a major decrease in the efficiency of microbial protein synthesis. The former situation is very obvious to all readers of this paper and won’t be addressed (although it is highly important). In contrast with other corn processing methods (grinding, flaking, and rolling) in Tables 3 and 4, there have been several more recent studies reinforcing our summary that feeding high-moisture shelled corn (HMC) versus dry ground corn has either decreased the amount of microbial protein flowing to the duodenum or decreased its efficiency (see papers discussed later). Given that protein from soybean meal or other more expensive rumen-undegradable protein (RUP) sources is more costly than energy from grain and also that microbial protein has an excellent profile of amino acids (NRC, 2001), depressed efficiency of microbial protein synthesis really translates to depressed efficiency of conversion of dietary protein into milk protein (Firkins and Reynolds, 2005). Conversely, increased substitution of beet pulp for HMC linearly decreased the amount of microbial protein flowing to the duodenum (Voelker and Allen, 2003c). Insufficient RDS clearly would therefore limit metabolizable protein for the cow, even though total organic matter digestibility was increased (Voelker and Allen, 2003b). Had those researchers prepared diets lower in crude protein (18.0%), it is tempting to speculate that milk protein yield might have been decreased. Taken in total, both excessive and insufficient RDS should decrease the amount of microbial protein flowing to the duodenum.

Considerations for High-Moisture Corn in Silage and Grain

Balancing diets for an optimum RDS depends on predictions for processed grains (Tables 3 and 4) but also on predicting the RDS from corn silage, reducing forage particle sorting, adequate feeding frequency, etc. Unfortunately, there are not very firm estimates for RDS in corn silage. Consequently, many of you are using or wondering about using in vitro estimates for forages or grains on your clients’ farms. Although Weiss and Wyatt (2002) reported no net benefit for in vitro NDF digestibility compared with summative prediction equations, Oba and Allen (1999) developed a prediction equation for the benefits of increasing in vitro NDF digestibility on DM intake and milk production. I am aware of only one study with published equations predicting improvement of starch digestibility and corresponding feed intake and milk production based on in vitro analyses. Ferreira and Mertens (2005) have several predictions that might be of use for some laboratories. However, I caution that these have not been scrutinized with diverse in vivo data; even so, when adapting these published equations to commercial labs, considerable care must be taken to standardize grinding procedure, amount of feed incubated, incubation time, and other variables. Taylor and Allen (2005a) suggested that in vitro starch digestibility should be used only as a general ranking of corn grains. With any in vitro procedure, there are considerable differences among run (even within labs — and this would be more severe among labs) (Firkins, 1997b). In particular, users should be aware that a standard forage should be run with each in vitro batch to make sure that the results are standardized to an average or common calibration value. Moreover, some Michigan State publications have explained that rate of starch digestibility might not be constant with varying concentrations of starch (i.e., second order kinetics). Therefore, I also conclude that these types of procedures should be used mainly to rank corn silages and corn grains.

Oba and Allen (2003b) reported an interaction in the rate of starch digestibility in the rumen when two levels of corn grain (either dry ground or high moisture) were fed; this would mean that a rate being inputted into a model such as Cornell Penn Minor (CPM) might need to be manually changed depending on the feeding level. Also, users evaluating kinetics of grain degradation in situ should be aware that fine-grinding also increases the instantaneously soluble fraction (Rémond et al., 2004). Clearly, the ramifications of trying to transfer information like this into practice would be a real challenge with today’s resources. Therefore, despite the increasing sophistication of ration evaluation software, there are still some limits regarding their inputs and the continuing need to retain the services of a good, discerning nutrition consultant.

Rumen Digestion Characteristics of High-Moisture Corn

As stated previously, much more data have been collected for HMC since data in Tables 3 and 4 were generated. I caution that the data for HMC might exceed your general expectations because HMC sources on farms are probably of lower quality than those from these research trials. When HMC or dry ground corn was fed at 21 or 32% of the diet (Oba and Allen, 2003a,c), the HMC decreased meal size. This corn source was only 63% DM, indicating a very highly available source. Interestingly, they noted an interaction in the average amount of starch consumed per meal. At the lower inclusion level, cows consumed nearly 1 lb of starch per meal. However, at the 32% level, those fed dry ground corn consumed 1.6 lb/meal and for HMC 1.3 lb/meal (a statistical interaction). These data indicate that the amount of RDS impacts meal size. However, the mean ruminal pH still was greater than 6.1, which might seem less indicative of rumen issues. If we stop there, we could conclude that the cow regulates her own RDS (Allen’s group has a series of experiments documenting why short-term intake is regulated by the amount of propionate being produced in the rumen to be metabolized in the liver). For my perspective, though, even an average pH of 6.12 was related to over nine hours in which the pH was below 6.0, and his group has shown that increasing volatility of pH is correlated with reduced efficiency of microbial protein synthesis. This kind of roller-coaster up-and-down swings of pH and energy availability for ruminal microbes caused the efficiency of microbial protein synthesis to be about 20% higher for dry ground corn. In addition, they noted that microbial efficiency was positively related to ruminal starch passage rate, which was faster for dry ground corn than HMC.

Voelker and Allen (2003a,b,c) replaced HMC with beet pulp, which decreased the digestion rate and increased the passage rate of starch from the rumen. This effect might have been a result from decreasing the percentage of starch from HMC grain compared with that from corn silage. This treatment structure shifted starch digestibility from the rumen to the intestines, resulting in no net decrease in total tract digestibility. Efficiency of microbial protein synthesis was not affected, but again, it was negatively correlated to digestion rate of starch and positively correlated with starch passage rate. Increasing passage rate should increase passage of adherent bacteria and increase efficiency of microbial protein synthesis by decreasing the fraction of energy spent on cell maintenance functions. For such a dramatic response in starch digestibility, though, the digestibility of starch from their corn silage must have been very low in the rumen yet high in the total tract. This underscores the need for more definitive information on starch digestion kinetics for corn silage.

When HMC and dry cracked corn were processed to the same particle size, the retention time in the rumen was greater for cracked corn (Krause et al., 2002). They described the effects of particle size and hydration on digestion and passage kinetics. Even though the retention time in the rumen and the total tract was longer for the dry cracked corn, total tract starch digestibility was still lower. This trial is important because it is one of the few to standardize against the effects of particle size. Yet passage rate did not decrease significantly when semiflint corn grain was processed to have increasing particle size (Rémond et al., 2004). In our review (Firkins et al., 2001), HMC seemed to stimulate chewing time, indicating that the larger corn kernels are residing in the rumen longer and being remasticated. Therefore, the greater potential from low pH resulting from increased ruminal availability of coarser rolled HMC typically fed would be compensated at least in part by increased salivary buffering. Consequently, the risk of acidosis might not be as great as thought, particularly if feed intake or meal pattern is affected. In contrast, nutrition advisers should strongly consider that the amount of microbial protein contributing to metabolizable protein will likely be lower in diets with high amounts of HMC, and protein supplementation might need to be modified for high producers.

Vitreousness of Corn Grain

In addition to increasing the surface area of starch granules, grinding also helps to break up protein complexes that inhibit starch degradation. In particular, corn kernels that have a greater contribution of vitreous endosperm have more zein protein, which is more resistant to proteolytic attack than other proteins that are greater in floury endosperm. Taylor and Allen (2005a) selected two corn grains widely different in percentage of vitreousness (Table 5). The site of digestion was shifted from the rumen to the intestines for the vitreous corn. Because the postruminal digestibility was about 8% lower when expressed relative to that entering the intestine, the total tract digestibility was about 5% lower. In a companion study (Taylor and Allen, 2005b), the vitreous corn tended (P < 0.12) to increase efficiency of microbial protein synthesis. Their correlation analysis showed that this efficiency was inversely correlated with starch digestion rate and positively related to ruminal pH and starch passage rate, which would be expected based on other reports from Allen’s group. Even though these grains were selected to be diversely different, the differences were within the ranges of values seen for unselected corn grains comprising much of the literature (Firkins et al., 2001). As I interpret these data and compile the MSU results, I conclude that increased vitreousness is not a dramatic problem, particularly in diets with moderate to high levels of corn silage and with RDS approaching the maximum that can be used efficiently in the rumen. Slowing of ruminal availability of starch should reduce energy spilling and perhaps even provide a vehicle for adherent bacteria to pass from the rumen. A main factor to consider is total tract digestibility of starch and total organic matter, which were 4.6 and 3.3% units different, respectively. In this study, the mean particle sizes of the two corn grains were 1.4 and 1.6 mm.

Table 5. Differences in digestibility between floury and vitreous corn grains in dairy cattle.1
Floury Vitreous
Vitreousness, % 3.0 67.2
Starch disappearance in vitro, %/hour 7.7 1.8
True starch digestibility in the rumen, % of intake 62.1 46.3
Apparent postruminal digestibility
          % of intake
          % of duodenal flow


Apparent total tract digestibility, % of intake 96.3 91.7
1All means were P < 0.05 except disappearance rate (statistics not done) (Taylor and Allen [2005a]).

In another experiment (Rémond et al., 2004), a semiflint corn grain was ground to differing mean particle sizes (0.7, 1.8, and 3.7 mm). Increasing mean particle size of this corn grain decreased the apparent digestibility of starch in the rumen from 58.6 to 49.8 to 35.5%, but there was no compensation in the intestines because total tract digestibility still decreased from 91.4 to 86.0 to 69.5%. In contrast, when dent corn was ground (3-mm screen) or coarsely rolled (0.6 or 3.5 mm), there was a lower difference in rumen (69.8 and 53.5%) and total tract starch digestibility (97.3 and 89.2%), but results from such a large range in particle size were clearly much less than in the previous experiment with semiflint corn. Combined with the previously discussed effects of vitreousness, I conclude that dry corn can be poorly digested in the total tract if it contains a lot of vitreous starch, unless it is ground to be less than about 1.5 mm mean particle size. And then, based on their data, grinding to less than about 1 mm seems to further improve starch digestibility.

There are laboratory measurements that can help improve the characterization of corn grain sources. Vitreousness can be visually appraised, and the assay often reported is from dissecting out the endosperm and determining the starch contribution as a percentage of the total. I recommend that if this procedure is done, then its main purpose would be to help nutrition advisers know when to fine-grind corn. Gelatinization is being evaluated, but I have not seen conclusive published data for a wide range of conditions. However, gelatinization is clearly related to ruminal availability (Svihus et al., 2005), and extrusion of corn seems to increase gelatinization compared to dry grinding. The question remaining to be answered, though, is how well gelatinization is related to total tract digestibility. I would suspect that unless gelatinization potential is moderate to high, the only major result is a shift in site of digestion from the intestines to the rumen with minor impact on nutrition and milk production; if gelatinization potential is low, then it might be a valuable diagnostic, but its use as a predictor might best be used to convince users to grind corn finely. When assessing gelatinization, it is important to note that the sample needs to be taken from the grain actually consumed because starches can undergo retrogradation as processed grain dries and cools (Svihus et al., 2005), and such a process might leave it as bad or worse than before.

Some Considerations for Sugars and Rumen-Degraded Starch

Feeding smaller amounts of molasses and other products with sugars might compensate for slower degradation rate in some corn grains. In the studies by Broderick and Radloff (2004), an optimum amount of about 6 and 5% total sugar was proposed for dried and liquid molasses experiments. However, when I looked at their data (Table 6), I noted that increasing total sugar increased organic matter digestibility much more in the first than the second trial. Assuming 100% digestibility of the molasses product, I used a statistical procedure to estimate the total tract digestibility of organic matter of the HMC that was replaced by molasses products. This substitution procedure predicted the total tract OM digestibility of the HMC to be only about 65 and 89% in the trial with dried and wet molasses, respectively. Even if these values are not absolutely accurate, this exercise indicates that substitution of sugars for grain that is less digestible should provide more benefit than when the grain it replaces is already highly available in the total tract (and by correlation, in the rumen).

Table 6. Digestibility and lactation performance when high-moisture corn grains were replaced by increasing levels of dried or liquid molasses in two trials.1
Dried, % of DM Liquid, % of DM
0 4 8 12 Contrast 0 3 6 9 Contrast
Total sugar, % of DM 2.4 4.2 5.6 7.2 2.6 4.9 7.4 10.0
pH 5.63 5.80 5.66 5.82 NS 6.07 5.90 6.02 6.06 NS
OMD, % 58.8 60.1 61.1 63.1 L 64.7 63.1 66.4 65.1 C
NDFD, % 37.5 37.8 38.6 41.1 L 36.6 36.3 44.6 37.2 L
DMI, lb/day 55.7 56.5 57.9 57.2 L 55.9 61.8 57.4 59.0 Q,C
Milk, lb/day 83.6 82.5 85.6 80.7 C 95.9 100.1 96.8 93.3 Q
3.5% FCM, lb/day 90.6 92.8 93.9 88.7 Q 101.2 102.7 96.8 93.3 L
Protein, lb/day 2.62 2.53 2.64 2.44 C 2.90 3.15 3.01 2.84 Q
1From Broderick and Radloff (2004). NS = not significant, L = linear, C = cubic, and Q = quadratic responses (P<0.05). OMD = organic matter digestibility, NDFD = NDF digestibility, DMI = dry matter intake, and FCM = fat-corrected milk.


Taken together with other studies described, I conclude that there should be an optimum NSC availability in the rumen that is consistent with efficient rumen microbial metabolism. Clearly, the amount of RDS depends on the maturity, DM percentage, and processing of corn silage. Some silages might be lower in RDS than others, so we still need to develop or improve methodology to predict starch
availability in silages in a systematic laboratory analysis that will help nutrition advisers to better account for varying RDS. Until more work is done, the Wisconsin index (Ferreira and Mertens, 2005) for particle size should be considered to predict total tract starch digestibility. Vitreousness of corn grain in silage seems to be of relatively little value. In contrast, vitreousness or perhaps gelatinization of dry corn grain should be considered, particularly to help users know when to grind corn more finely. Using these considerations for coarse adjustment of rations, the amount of RDS can be fine-tuned with more slowly available by-products or increased moderately with small amounts of sugars according to individual herd or group needs. As ration balancing and feeding systems continue to improve in reliability and repeatability, nutrition advisers will still have to use their knowledge of nutrition to continue to keep pace with other feeding management practices.

Author Information

Jeffrey L. Firkins
Department of Animal Sciences
The Ohio State University


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