By Bill Mahanna, PhD, Dipl. ACAN - Global Nutritional Sciences Manager
In recent discussions with various laboratory managers, it became clear that requests for lab results using methods directed toward starch digestion are on the increase.
These lab methods include corn silage kernel processing scores, grain particle size, fecal starch, seven-hour in vitro starch digestibility, Fermentrics and University of Wisconsin Feed Grain V2.0.
This column will attempt to prioritize the important drivers and methods that can be used to categorize and differentiate variation in starch digestibility.
When discussing starch digestion, it is important to clarify if the lab method is addressing the rumen, the intestines or the total tract.
Most literature reviews indicate that total tract starch digestion in lactating cattle exceeds 90% (Owens, 2005). Unfortunately, constructing relationships to processing methods is often difficult due to the frequent lack of quantitative particle size data.
If starch is fermented in the rumen, microbes use the energy to synthesize protein for the animal to digest downstream. When starch escapes the rumen and is digested in the small intestine, ruminal fermentation energy loss from methane and the heat of metabolism is avoided (Owens, 2005).
Starch absorbed as glucose from the intestines can have more than 20% greater caloric value than starch fermented to volatile fatty acids within the rumen. From an energy standpoint, total tract digestion of starch - not ruminal starch digestion - should be the primary concern (Owens, personal communication, 2010).
Ruminal starch digestion is typically lower for cows than for feedlot steers (Figure 1), presumably because dairy cows have a higher feed intake and consume more dietary forage - neutral detergent fiber (NDF) - which increases ruminal outflow rates (Owens and Soderlund, 2007). As a fraction of total starch digestion, the proportion of starch digestion that occurs post-ruminally from processed (flaking, high moisture) corn grain is greater for lactating cows than for feedlot cattle.
Grain processing methods that increase ruminal degradation of starch generally increase the digestibility of residual starch that enters the intestines, although there will be a reduced supply due to more extensive ruminal digestion of starch. Several reviews have suggested that intestinal starch digestibility decreases as starch flow to the intestines increases. However, when calculated within a processing method, post-ruminal digestion does not decline as the passage of starch to the small intestine (abomasal supply) increases (Owens, 2005).
Neither dry-rolled nor whole corn is particularly well digested post-ruminally. Poor processing of corn kernels, which hydrate and escape the rumen in the liquid phase, may predispose cows to jejunal hemorrhagic syndrome if the reduced starch surface area limits pancreatic amylase activity, thereby allowing excessive amounts of starch to flow to the lower intestinal tract.
This increases the substrate to support growth of toxin-producing organisms such as clostridia or aspergillus. Assuming adequate particle size, it may be beneficial, in some feeding situations, to shift the site of starch digestion downstream to the intestines if the rumen microbial protein yield is adequate and acidosis is a concern.
In Europe, harder flint grain is actually preferred in dairy diets to reduce the incidence of acidosis in lactating cows being supplemented with wheat and barley-based concentrates.
Some researchers believe that ruminal escape starch is directed toward body fat rather than milk production in that glucose increases insulin, which stimulates glucose incorporation into fatty acids. Most of the studies referenced have been with glucose infused into the small intestines of mature steers or lambs with limited production requirements for energy. There is some debate as to whether glucose would be used for production needs in animals that have a high energy demand such as early-lactation dairy cows (Owens, personal communication, 2010).
The primary determinants of starch digestibility, in order of importance, are: (1) particle size, (2) kernel moisture and length of ensiling time for fermented corn and (3) a distant third, the vitreousness (prolamin content) of dry grain (Feedstuffs, Feb. 9 and March 9, 2009).
The use of corn silage kernel processing scoring continues to increase among labs offering this important measurement. This makes sense given the variability in corn silage processing, which is influenced by equipment differences and varying kernel maturity over extended harvest windows. Modification in roller mill designs (e.g., shredlage) may reduce the need for this method in the future (Feedstuffs, Aug. 13, 2012).
I often question producers and nutritionists about the particle size of their grain or, in particular, high-moisture corn and am often disappointed with the lack of quantitative data.
However, it is encouraging that Dairyland Labs (Taysom, personal communication, 2013) reported that submissions by dairy nutritionists for grain particle size analyses are on the increase. The Table demonstrates the variation in grain particle sizes fed to lactating dairy cows.
The dairy industry still lags behind the feedlot industry with regard to sensitivity to grain particle size distribution or establishing clear goals such as having less than 5% whole kernels and 10% fines.
Roller mills have traditionally processed grain faster, providing a narrower particle size distribution and more consistent product compared to a hammermill or tub grinder (Figure 2). Rolling corn would be preferred for obtaining high starch digestibility since it results in fewer coarse particles and fewer fine particles. Rolling also will reduce the prevalence of fine particles that can sift or separate in the mixer or the bunk, which can increase feed sorting and potentially result in acidosis, especially with dry rations and poor bunk management (Owens, personal communication, 2013).
Figure 1. Effect of corn processing (dry, fermented, flaked) on the site of starch digestion in dairy cows and beef steers.
Figure 2. Effect of hammermill versus roller mill on grain particle distribution.
Figure 3. Impact of processing dry corn on total tract starch digestion.
When a sample arrives in the laboratory, the analytical path is different for fermented or unfermented grain. The concentration of ammonia (NH3-N) is used as the benchmark in that the presence of ammonia defines the intensity and length of fermentation. Samples devoid of ammonia are categorized as unfermented.
Starch digestibility in fermented samples is based on particle size and ammonia content. Feed Grain V2.0 predicts about a 20% unit difference in ruminal starch digestibility and a 6% unit increase in total tract digestibility when comparing high-moisture corn that is finely ground (1,000 microns) and long fermented (7% ammonia) to corn that is coarsely ground (2,500 microns) and recently ensiled (1% ammonia).
Starch digestibility in unfermented, dry corn grain is based on particle size and prolamin content. The prolamin concentration is an indicator of the level of kernel hardness (texture, vitreousness or protein:starch matrix). 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 (black layer and earlier).
The inclusion of vitreousness or kernel texture for dry corn grain only 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.Consequently, a finer grind should solve problems involved with vitreousness and high prolamin content attributed to hard corn kernels (Owens, personal communication, 2013).
Research from France (Ramos, 2009) with relatively high-vitreous (flinty) corn showed that grinding removes most of the negative influence of vitreousness in dry corn (Figure 3).
The Feed Grain V2.0 hierarchy of analytical inputs brings into question the validity of seed companies promoting soft-texture, low-vitreous or low-prolamin corn used for corn silage or high-moisture corn. These companies often show kernel texture data on fully mature dry corn; however, they lack data on hybrid vitreousness at corn silage (three-quarters milk line) or high-moisture corn (black layer) maturities.
Unfortunately, many producers and nutritionists are being shown university starch digestibility studies comparing genetic extremes in vitreousness such as 3-66% (Taylor and Allen, 2005a, b and c) or 25-66% (Allen et al., 2008). These comparisons certainly 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.
It would make 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 or high-moisture ensiled grain hybrids.
Since all undigested starch is found in feces, the concentration of fecal starch is a convenient and reliable index of starch digestion and a proxy to assess if processing was adequate (Owens and Basalan, 2012).
Dairyland Labs reported that requests for fecal starch analyses have doubled in the last three years, and about 28% of samples exceeded the 5% starch hurdle at which an intervention such as reprocessing the grain to reduce its particle size may need to be implemented.
The primary determinants of starch digestibility are particle size, kernel moisture and length of ensiling time for fermented corn and the vitreousness (prolamin) of dry grain.
Monitoring kernel processing in silage and particle size in high-moisture or dry corn, along with analyzing a composite fecal sample from 8-10 cows, may be the most economical way to begin troubleshooting where starch digestibility issues may be originating. Other lab methods the offer an integrated approach to the most important factors affecting starch digestibility and help to quantify the increase in starch digestibility over time in fermented storage and the importance of a fine particle size in unfermented corn.
Exercise caution regarding claims of low-vitreous, soft-textured or low-prolamin corn, especially when the crop is intended for corn silage or high-moisture grain.
This article was originally published in February 2013 Feedstuffs issue, and is reproduced with their permission.
The foregoing is provided for informational purposes only. Please consult with your nutritionist or veterinarian for suggestions specific to your operation. Product performance is variable and subject to a variety of environmental, disease, and pest pressures. Individual results may vary.