– Francis L. Fluharty, Ph.D., Department of Animal Sciences, The Ohio State University (excerpted from Energy Use in Digestion, and Increasing the Digestibility of Forages with Processing Technologies)
Feed costs are rising, and corn price projections are currently maintaining between $4.95 and $5.35 per bushel on corn futures through December 2010. This equals $.088 to $.095 per pound, or $176 to $190 per ton. Dried distillers grains and corn gluten feed are currently in this same price range, and the prices of other alternative feeds are keeping pace on an energy and protein basis, so there are no cheap supplemental feeds for cow-calf producers, stocker cattle operations, or feedlots. Therefore, forage-based operations must utilize cost effective management tools that maximize forage digestibility.
In 1999 I participated in a research project which reported on the effects of limit-feeding a high-concentrate diet which was formulated to provide daily energy and protein intakes equal to lambs grazing alfalfa, resulting in the same ADG between the two groups. The lambs fed the high-concentrate diet had lower intestinal organ weights, and retained more than twice as much nitrogen (a measure of protein status) on a daily basis compared with lambs that grazed alfalfa, resulting in carcasses with a larger loin-eye area and more lean yield, demonstrating the large energy and protein requirements of the intestinal organs. Therefore, dietary manipulation can be used to alter nutrient utilization and muscle and fat deposition through altering tissue requirements. These reductions in maintenance energy and protein requirements are the basis for the enhanced feed efficiencies achieved with most limit-fed, grain-based feeding systems, and are partially responsible for the greater feed efficiency and average daily gain which occur when growing cattle are taken from forage-based grazing systems to grain-based feedlot diets. These same principles offer potential with harvesting or post-harvest processing technologies that reduce the particle size of forages, thereby increasing their digestibility and increasing microbial protein.
In contrast to cattle being fed grain-based diets, the size of the rumen limits the amount of energy that can be consumed with forage-based diets, and digestible energy intake decreases with increasing forage maturity. From a practical standpoint with unprocessed forages, the large particle size of mature forage reduces the energy available to the animal. For digestion to occur, the microorganisms in the rumen must first be associated with the forage, and then attach to the forage. Digestion normally occurs from the inside of the forage to the outer layers. Limitations to the speed at which this occurs include the physical and chemical properties of the forage, the moisture level of the forage, time for penetration of the waxes and cuticle layer, and the extent of lignification. Undigested feed is broken down through the process of rumination and re-chewing until it is either digested, or small enough to pass from the reticulo-omasal orifice. Most particles leaving the rumen are smaller than 1mm, although particles as large as 5 cm may leave the rumen. It is, therefore, not hard to understand how reducing the large particle size of many mature forages to 1mm to 5 cm can increase digestibility.
With mature forages, and crop residues such as wheat straw, or corn stover that are harvested for feed, lignin content can be a limiting factor. Wheat straw is approximately 14% lignin, and corn stover is approximately 7% lignin. However, in many parts of the world, straw is fed as a roughage source to ruminant animals. While long-stem straw has a very low digestibility, varying between 35% and 55%, Dr. M.G. Jackson, Professor of Animal Nutrition at the G.B. Pant University of Agriculture and Technology, Pantnagar, India, reported ‘the grinding of straw increases consumption leading to higher digestible energy intakes of the order of 30%. In terms of net-energy intake the increase is somewhat more than this because the net-energy value of straw is increased by grinding.’
Dr. Steven Loerch, at The Ohio State University, investigated the potential of using processing technologies to improve the utilization of prairie hay. Dr. Loerch reported that “One effective option producers rarely consider is hay chopping. Chopping hay allows the cows to eat 25-30% more energy. Costs of chopping hay (equipment, labor, etc.) should be compared to costs of purchasing supplemental energy. For some producers, this may be a cost effective option. I came to realize the potential of hay chopping from an observation at the OARDC Beef Center in Wooster. Steers fed a chopped hay based diet gained 2.5 lbs/day while those fed round baled hay (same hay source) in a rack gained less than 1.5 lbs/day.” This can be explained on the basis of more surface area being available for degradation, allowing for a more rapid rate of digestion; a faster rate of passage of indigestible components from the rumen allowing for an increase in feed intake, and the possibility that these factors allowed for an increase in propionate production due to a faster rate of digestion, and an increased rate of passage of indigestible components.
Finally, harvesting techniques have been found to result in improvements in forage digestibility. Williams et al. (1995) harvested wheat forage with a mower conditioner, at an early head stage of maturity, and allowed it to wilt for either 0, 6 or 20 hours prior to being cut with a forage chopper and ensiled. They reported that wilting for 20 hours resulted in lower fermentation acids, a higher concentration of water-soluble carbohydrates, and improved fiber digestibility compared with either direct-cut or wilting for 6 hours. Hintz et al. (1999) reported that maceration, an intensive forage conditioning process that shreds forage thus reducing rigidity and increases field drying rates by as much as 300% by disrupting the waxy cuticle layer of the plant and breaking open the stem, resulted in an increase in surface area available for microbial attachment in the rumen, a decreased lag time associated with NDF digestion, an increase in NDF digestion, while having a decrease in the acetate:propionate ratio, which would be positive for growing and finishing animals.
If forage is evaluated on a price per pound, rather than a price per ton, the necessity to maximize digestibility becomes apparent. If corn is $5.04 per bushel, it is $.09 per pound. Likewise, if dried corn gluten feed or distillers dried grains are $180 per ton, they are $.09 per pound. If hay is $180 per ton, it is $.09 per pound. Normally, the digestibility of corn, corn gluten feed, and distillers dried grains are all much higher than even the highest quality hay. Therefore, in order for forages to be economically competitive, they must be managed, harvested, and potentially processed to their optimum digestibility.
In summary, high prices for all feeds necessitate that beef producers adopt practices and forage harvesting and processing technologies which reduce the animal’s energy and protein requirements by increasing the digestibility of forages through providing more sites for bacterial attachment.