At the recent dairy science meetings in San Antonio, Texas, there were at least six research papers that dealt with ration particle size. These studies typically evaluated physically effective fiber using laboratory instruments like a Ro-Tap or field tools such as the Penn State Separator or the recently introduced Miner Institute Z-Box.
This research and the associated tools are critical in helping us learn to how to manage chop length and feed delivery such that we are properly stimulating cud chewing and reducing costly bouts of subclinical acidosis. As we move into an era of high-cost grain and corn silage, nutritionists are increasingly looking for similar research and tools to help evaluate particle size and digestibility of starch components in the ration.
Laboratories commonly use grain sieves to determine the mean particle size in microns of dry or high-moisture corn. Dairy nutritionists are starting to advise clients as to their preference (typically 600-800 microns) and caution against a high standard deviation indicative of either too many large pieces or too many fines.
Recently, laboratory methods (e.g., corn silage processing scores [CSPS]) have been commercialized that allow nutritionists to better understand the particle size distribution of the starchy components in corn silage.
This is important information because the extent of kernel damage affects the degree of pericarp disruption, along with the cell wall surrounding individual starch granules and the protein matrix surrounding multiple starch granules. Facilitating the physical association of amylase with amylose and amylopectin for enzymatic hydrolysis within each starch granule is critical to the ruminal and post-ruminal starch degradation process (Hall, 2000). Several researchers have also theorized that particle size reduction is more important for the digestibility of starch than for fiber.
However, nutritionists are not the only ones who need to understand the effects of particle size. For example, laboratories have a choice of starch digestibility (STRD) procedures ranging from in situ or in vitro procedures where samples are finely ground (1 mm) to methods that use unground, as-fed samples (e.g., DSA method; Shaver, 2002).
Laboratory managers are often asked about the relationship of their STRD values, measured on ground samples, to the actual on-farm silage that will be presented to rumen microflora. Methods using unground samples are often criticized for not factoring in the particle size reduction that occurs naturally when cows masticate silage.
Most laboratories prefer using ground samples for corn silage because of the potential error associated with subsampling this heterogeneous crop consisting of fiber, cob and grain. The question nagging lab managers is how fine the sample should be ground.
Insight into the answer comes from a collaborative study conducted by Washington State University and the University of Idaho (Johnson et al., 2002) that addressed the relationship between 48-hour, in situ analysis of dried samples ground to 2, 3, 4 or 8 mm and in vivo dry matter disappearance (DMD) and lactation performance of cows fed two corn silages of distinctly different harvest maturities.
The study concluded that dried samples processed through an 8 mm screen in a Wiley Mill maintained a high correlation with both in vivo DMD, dry matter intake and lactation results. Conversely, there was a very poor relationship to in vivo results with analyzed samples that were ground to less than 8 mm (Table 1).
Recent unpublished data from Sapienza Analytica LLC (Sapienza, 2007) suggests that utilizing a 1 mm grind (Table 2) also significantly alters A-fraction estimates. This is important because most models credit the A-fraction as rapidly and completely digested.
Others argue that escape from either in situ bags or filter paper is more related to particle size than it is to rate of digestion. The effect on A-fraction estimates and the agreement to in vivo results in the Washington/Idaho study has led several labs to abandon 1 mm grinding of corn silage in favor of processing samples through a 6 mm Wiley mill screen. Laboratories prefer the 6 mm screen because it is more commonly available and approximately 75% of the particles will range from 6 to 8 mm when oven-dried material is ground through a 6 mm screen.
Further justification of the use of a 6 mm grind comes from another recent study conducted at Sapienza Analytica. It looked at the mean particle length (MPL) of rumen contents in fistulated Jersey steers fed a ration consisting of corn silage, dry hay, ground corn grain and soybean meal. Rumen contents were sampled from just beneath the floating rumen mat six hours post-feeding.
Interestingly, MPL of the (non-mat) rumen contents was very similar to MPL of corn silage dried and processed through a 6 mm screen and four times longer (1.1 versus 0.3 mm) than MPL of samples ground to 1 mm.
Additional research is needed in this area, but the early implications are that samples ground to 6 mm represent an MPL very similar to digesta in the rumen that has been exposed to normal chewing and rumination.
Mertens (2002) published an equation for estimating corn silage total tract (ruminal and intestinal) starch digestibility (ttSTRD) derived from published experiments in which STRD was measured.
The following Mertens equation is based on silage dry matter and percentage of starch retained on a Ro-Tap > 4.75 mm screen: corn silage ttSTRD (% of starch) = 117 - 0.586 x (corn silage dry matter %) - 0.00144 x (corn silage dry matter %) x (% starch greater than 4.75, with a maximum of 98%).
When this equation was used on 32 diverse corn silages, it generated STRD values ranging from 85 to 98%.
Dairyland Laboratories Inc. has worked in conjunction with Sapienza Analytica to further refine the Mertens equation by including additional results from 106 corn silage samples analyzed for dry matter, 12-hour ruminal STRD, ttSTRD and CSPS (Taysom, 2007). Adjustment factors for ruminal STRD and ttSTRD were calculated using a principal component regression analysis based on the relationships among (1) measured ruminal STRD, (2) measured ttSTRD, (3) calculated ttSTRD (Mertens logic with expanded database) and (4) the relationship of calculated ttSTRD to measured ttSTRD.
This gives Dairyland Labs the ability to report 12-hour ruminal STRD and ttSTRD estimates adjusted for the kernel particle size distribution of individual silages. Table 3 shows that when applying these adjustment factors, well-processed silage (CSPS > 90%) has essentially no adjustment, while poorly processed silage (CSPS < 33%) receives an 11-point reduction in the value of the ruminal starch digestion coefficient.
Hall, M.B., J.P. Jennings, B.A. Lewis and J.B. Robertson. 2000. Evaluation of starch analysis methods for feed samples. J. Sci. Food Agric. 81:17-21.
Johnson, L.M., J.H Harrison, D. Davidson, J.L. Robutti, M. Swift, W.C. Mahanna and K. Shinners. 2002. Corn Silage Management 1: Effects of hybrid, maturity and mechanical processing on chemical and physical characteristics. J. Dairy Sci. 85:833-853.
Mertens, D.R. 2002. Fiber: Measuring, modeling and feeding. Proceeding of the 64th Cornell Nutrition Conference, Wyndham Hotel. Syracuse, N.Y.
Sapienza, D.A. 2007. Personal communication. www.sapienzanalytica.com.
Shaver, R.D., and P.C. Hoffman. 2006. Corn silage starch digestibility - What's new. Proceeding of the NRAES-181 Silage for Dairy Farms, Penn Harris Hotel, Harrisburg, Pa.
Taysom, Dave. 2007. Personal communication. www.dairylandlabs.com