Ration physical form continues to raise questions as corn silage processors, steam flaked corn, TMR mixing times, and fine ground corn choices are available. The challenge is to control fiber length to maintain rumen pH and health, optimize microbial growth, maintain dry matter intake, and dictate rate of passage through the digestive tract. Physical form of concentrate and forage must complement each other to achieve ideal nutrient intake, performance, and cow health. While forage particle size is critical in meeting rumen needs, this paper will focus on grain particle size. Review guidelines for the Penn State Forage Particle box to insure adequate function fiber levels are achieved.

Impact of Grain Particle Size

High producing cows receive grain as a major source of energy and starch in their diets. Cereal grains contain 45 to 80 percent starch. Optimizing starch utilization is critical to efficient milk production for several reasons.

1. Starch can be fermented in the rumen to volatile fatty acids (VFA) as a major source of energy for lactating cows.
2. If starch ferments too fast or excessively in the rumen, acidosis can lead to metabolic disorders and health problems.
3. Intestinal starch digestion results in a direct source of glucose needed for milk lactose production and source of alternative energy needs.
4. The amount of organic dry matter fermented in the rumen (especially starch) drives microbial protein synthesis.
5. Fecal losses of starch should be less than five percent of the total consumed (can exceed 15 percent due to improper processing or digestion.

Processing of grain will depending the source of starch, impact on feed intake, forage type, and level of non-fiber carbohydrate in the ration. Wheat, barley, and oats have faster rates of fermentation in the rumen while corn and sorghum are slower.

Goals of Grain Processing

The dairy manager and nutritionists are attempting to get the best of all worlds by maximizing total starch digestion in the animal, optimizing starch fermentation in the rumen to maximize VFA yield while avoid acidosis, and optimizing starch availability in the small intestine. If excess starch reaches the large intestine and is available, it can be fermented resulting in a less efficient energy resource. Fecal pH can drop below 6.0 which can used as diagnostic tool in the field. If corn is not process (fed as whole seed), 15 to 30 percent can appear in manure as whole grain. Because the starch and protein matrix is a tight complex, processing disrupts the matrix and exposes the nutrients to rumen fermentation and lower gut digestion. Reducing the particle size or increasing starch solubility (such as in gelatinized or high moisture grain) increases the rate and level rumen fermentation and digestion. A fine line exists between maximum rumen yield and an unhealthy rumen environment. The nutritionist must adjust ration levels of grain based on the extent of processing, feed system, forage resource, and rate of passage (dry matter intake).

Research in Grain Processing

Method and degree of grain processing affects the site and extent of digestion of starch. Optimal starch utilization is important to improving efficiency of production. Under processing grains increases feed costs and limits milk production, conversely, over processing reduces dry matter intake (DMI) and milk production. There is a limited amount of scientific research available to draw particle size recommendations from and few methods available to measure particle size and distribution accurately.

Studies have been conducted by cooperatives illustrating the importance of grinding particle size (Table 1). Farmland Coop compared cracked corn (2500 micron), ground corn (1100 micron), and a mix of the two corn forms (50 percent of each). Milk yield, milk components, and body weight change was optimal for the ground corn.

Table 1. Effect of corn processing on milk performance, dry matter intake, and weight change (Hutjens, 1999).

A second study by the CRF research farm compared a complete pelleted concentrate (complete), protein supplement plus fine ground corn (ground), protein supplement plus cracked corn (cracked), protein supplement plus steam flaked corn (steam), and blend of fine ground (45%) and steam flaked (55%) plus protein (Table 2). All protein supplements were pelleted. Rations were fed as a TMR with 46 percent forage (half haylage and half corn silage on a dry matter basis).

Table 2. Effects of corn processing on milk yield, dry matter intake, milk components, and MUN (Luhman and LaCount, 1998).

Dry matter intake was greatest for the complete grain mix. Milk production was significantly lower for cracked compared to other treatments. Milk fat test was statistically highest for ground and lowest for steam flaked while other rations were intermediate. Milk urea nitrogen levels were higher for cows fed cracked corn and lower for cows fed complete and steam fed cows. From an economic basis (including milk production, milk components, and cost of processing), complete, ground, and steam flake were $0.84, $0.83, and $0.81 improvement over cracked processed corn. The particle size of the ground and cracked corn are summarized in Table 3.

Table 3. Grain particle size of cracked and fine ground corn in CRF study (Luhman and LaCount, 1998).

Researchers at the Pennsylvania State University have used in situ incubations and sieves to address issues concerning degradability and particle size distributions of corn and soybeans. They demonstrated that a reduction in particle size of dry shelled corn increased ruminal degradability of dry matter (DM), crude protein (CP) and total nonstructural carbohydrate TNC Table 4. Heat treatment of the shelled corn by steam flaking increased ruminal DM and TNC degradability but lowered CP degradability.

Table 4. Particle size distribution and effective ruminal degradability of dry shelled corn processed differently (Lykos and Varga, 1995).

¹CC = cracked corn, CCC = chick cracked corn, FGC = fine ground corn, and SFC36 = steam flaked corn with a density of 0.36 kg/L.

²Means in the same column with different superscripts differ (P<0.05).

Grinding soybeans, either raw or roasted, compared to cracking increased ruminal degradability of DM, CP and TNC (Table 5). Heat treatment by roasting improved CP and TNC ruminal degradability but had little effect on DM degradability.

Table 5. Particle size distribution and effective ruminal degradability of soybeans processed differently (Lykos and Varga, 1995).

¹SB = soybean, RSB-C = roasted SB cracked, RSB-G = roasted SB ground, SB-C = raw SB cracked, and SB-G = raw SB ground.

²Means in the same column with different superscripts differ (P<0.05).

A study by Aldrich (Akey Inc.) compared corn samples (n=21) of various particle size and distribution for in situ digestibility . Dry matter disappearance (DMD) is correlated with starch disappearance/digestibility. Particle size was negatively correlated with DMD at 16 h of ruminal incubation. A particle size of 400 m resulted in a DMD of >90%. One sample >3300 m had a DMD of 14%. Researchers suggests that the goal should be >70% DMD at 16 h which corresponds to approximately 80% starch digestibility. An average particle size of 500 m resulted in about 80% DMD.

The relationship between ruminal starch degradation and the physical characteristics of corn grain was examined by French researchers. Two corn types, dent and flint, varied in the proportion of coarse particles, measure on sieves ranging in size from 90 to 500 mm, when ground at the same speed and length of time. Particles greater than 250 mm were considered course, while particles less than 125 mm were considered fine. The dent corn samples on average compared to flint corn samples had a smaller proportion of coarse particles (61.9 vs. 69.6%), a larger proportion of fine particles (15.6 vs. 9.0%) and a lower vitreousness (51.4 vs. 71.8%). Ruminal starch degradability averaged 61.9% and 46.2% in dent and flint corn types respectively. The researchers concluded that ruminal starch degradability could be predicted by vitreousness (R2=0.89) or by a combination of apparent density and 1,000-grain weight (R2=0.91).

Processing corn as fermented grain remains an attractive alternative in the Midwest as growing conditions result in immature grain, ear dropping is reduced, land preparation for the next spring can be completed earlier, and energy value per unit of dry matter is higher compared to dry corn. Fermenting corn increases the solubility of starch and protein in the rumen. Coworkers at the University of Maryland and USDA studied the performance, ruminal fermenation and site of starch digestion in early lactation cow (n=34) fed corn grain harvested and processed differently (Table 6). Diets were 42.4% corn, 45.3% alfalfa silage, 10.6% SoypassÔ and 1.7% minerals and vitamins. Corn was fed as dry ground corn, dry rolled corn, high moisture ground corn and high moisture rolled corn, with a mean particle size of 618, 1725, 489, 1789 mm, respectively. Both grinding and moisture increased starch digestibility in the rumen. Grinding increased DMI and tended to increase yields of milk, protein, lactose, and solids non-fat. Moisture content of the corn did not affect intake or milk production. ). Digestible, metabolizable, and heat energy as a percent of gross energy intake were higher for high moisture corn compared to dry corn. The net energy values for the four types of corn (substitution calculation base using NRC for dry ground corn) were 2.00, 1.96, 2.16, and 2.42 Mcal/kg for DR, DG, HR, and HG, respectively.

Table 6. Comparison of rolled and ground dry and high moisture corn (Knowlton, 1998).

Canadian researchers summarized studies with processing barley grain. It is suggested that the processing index (PI) is a practical means for measuring degree of processing in steam processed barley describe the extent to which starch will be available to degradation by ruminal microbes. By definition, the PI is a "measure of weight of a given volume of barley after rolling as a percent of the weight of the whole barley." A course rolled grain has a higher PI than fine rolled grain. In a study where cows were fed 31% barley silage, 8% alfalfa hay and 42.5% steam rolled barley, decreasing the PI from 81 to 72.5% resulted in a 14% increase in milk production because of the increased DMI and dry matter digestibility (Table 7). Decreasing the PI from 72.5 to 64% resulted in an additional 9.6% increase in milk production. However, further processing of the barley from 64 to 55.5% decreased DMI and milk production by 6.2%. The researchers suggested that the optimum PI for steam rolled barley is 64 to 68% in order to maximize DMI, digestibility and milk production. It was noted that PI would not be applicable to ground grains or dry rolled grains because the spaces between the larger grain particles would be filled with fines and not be a reliable indicator of particle size distribution.

Table 7. Milk production, dry matter intake, and digestibility in lactating cows fed steam rolled barley at different densities (Corbett, 2000).