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The dairy cow has a great ability to achieve high levels of feed intake relative to body size while maintaining a rumen fermentation environment within certain physiological limits. These limits facilitate a favorable symbiotic relationship between the ruminant host and rumen microorganisms. The ruminant host should provide the microorganisms an environment of limited oxygen, relatively neutral pH, constant temperature, periodic influx of water and organic matter, constant removal or absorption of end products and indigestible matter, and a retention time greater than microbial generation time. The feeding systems necessary in modern dairy cattle production have made it increasingly difficult to provide a rumen environment that stays within all of these constraints. The enormous energy requirements of high-producing dairy cattle require dairy farmers to feed dairy cattle ever greater amounts of dry matter containing increasing levels of concentrate feeds. One of the problems associated with this feeding system is an increased susceptibility to rumen acidosis due to the high starch and energy contents of the ration.
Ruminal acidosis is a condition where rumen pH falls below an optimal physiological range, and there are two distinct types of acidosis. The first, more severe, condition is acute ruminal acidosis, which is generally defined as ruminal pH below 5.0. The second, less severe and more common, condition is subacute ruminal acidosis (SARA), which is generally defined as ruminal pH in the range of 5.0 to 5.5. Acute acidosis is thought to be mainly caused by an increase in ruminal lactate, while SARA is thought to be mainly caused by an accumulation of volatile fatty acids. There are many negative side effects associated with SARA, including decreased dry matter intake (DMI), decreased milk production and milk fat content, increased risk of foot problems, and decreased feed efficiency. Thus, SARA has considerable economic impact on the dairy industry.
There are three major causes of SARA in dairy herds:
Dairy cattle can consume excessive amounts of fermentable carbohydrates either through high levels of concentrate in the ration or moderate levels of concentrates at high DMI. Adaptation of the rumen microorganisms is important in issues related to acidosis. The ruminant should be adapted slowly to ration changes, especially when going from high-forage to low-forage diets, to allow the rumen microorganism population to adapt and rumen papillae to lengthen. Lengthening of the rumen papillae is important in the increased absorption of volatile fatty acids.
The NRC based its fiber recommendations on NDF as it is the measure that best separates structural from nonstructural carbohydrates and is comprised of most of the compounds considered fiber. These recommendations are based on cows fed: a TMR, alfalfa or corn silage as the predominant forage, forage with adequate particle size, and dry ground corn as the predominant starch source. Forages are the major supplier of NDF in rations, and their slower fermentation (compared to grain) and physical characteristics are essential for maintaining ruminal health. The decreased digestibility of forage helps to maintain an optimal ruminal environment by diluting the effects of large amounts of VFA produced by starch fermentation. Saliva is an extremely important component to buffering the rumen, as lactating dairy cows can produce large volumes (>100 liters; 26 gallons) per day. Buffers secreted in saliva are quite strong and functional buffers at higher pH, but when ruminal pH drops to a range of low values (approximately 5.5), volatile fatty acids become the primary buffering system in the rumen.
Particle size, dry matter intake (DMI), and NDF and DM content of diets impact the dairy cow's rate of eating and the time spent eating. Chewing rate generally decreases and thus saliva secreted per unit of DMI increases when ration particle size, dry matter, and NDF are increased. Dairy nutritionists are constantly attempting to manage the chemical and physical characteristics of the forage and total diet to control saliva production and the rumen environment.
Nutritionists routinely measure NDF in forage and feeds along with the other primary analyses. In addition, diet and forage particle size can be measured, and the addition of dry matter, NDF, and starch content. NDF and starch digestibility is being investigated and may show some promise in mitigating acidotic problems by way of ration balancing, in the future. These tools, particle size, NDF and starch levels, allow us to balance for more effective use of fermentable carbohydrates in the rumen with more constant VFA production. Physically effective NDF (peNDF) is still important, but it is only a part of the equation. The peNDF value can be defined as the amount of a diet that is greater than 4 mm times the chemical NDF value of the diet. The peNDF is easy to measure, yet it is only concerned with the effect of a feed on chewing and the ruminal mat, which are mostly influenced by particle size, NDF, and dry matter content. Although fragility and specific gravity of feed particles probably have a small influence on peNDF as well, we currently do not have a simplified or easy measure of these parameters. The NRC (2001) did not publish a requirement for peNDF because of a stated lack of a standard, validated method to measure effective fiber of feeds or to establish requirements for effective fiber. A weakness of using peNDF is that NDF fractions are not chemically identical for all forages. The NDF composition (the ratio of hemicellulose to cellulose to lignin) of forage varies greatly and is affected by species, maturity, and storage method. This is probably part of the reason for the many contradictions in the literature about effects of peNDF on intake, milk production, milk fat content, and chewing behavior. It is also part of the reason for the problems associated with troubleshooting SARA issues on dairy farms. The interactions between forage particle size, fiber digestibility, and starch digestibility are large, and much more research is needed to elucidate these effects.
The final area affecting SARA in a dairy herd is the cow herself. While the majority of U.S. dairy cows receive a TMR, not all TMR diets provide cows a uniform and consistent diet throughout the day. Dairy cows are excellent at selectively consuming, or sorting, their rations when fed a TMR. Cows generally sort against long particles and for finer particles in their ration but have also been shown to sort for longer particles in rations under some situations. This behavior can create problems because cows are not only modifying the particle size of their consumed diet, they are also altering their NDF and rapidly fermentable carbohydrate intakes. Until recently, on farms and in research, the presence of sorting was usually determined by comparing the particle size distribution of TMR at feeding to its particle distribution at the end of the day. While these numbers have some (often limited) information, sorting throughout the day, primarily in the first 4 to 8 hours after feeding, can be much more important. Feeding rations of greater particle size and dry matter percentage have been shown to increase sorting. While sorting may not affect the overall daily intakes, the shift in diet starch and fiber during the day may impact ruminal pH levels for a part of the day. While actual intake of these components after 24 hours may be similar, feed efficiency, milk production, or milk components may be compromised. Therefore, when measuring sorting activity in lactating dairy cattle, it is most important to consider composition of the refusals at the end of a 4- and 8-hour period (which comprise over half of daily intake). Making modifications in chop length of the forage, processing of the starch component, or modifying ingredients to alter dry matter content of the TMR are just some of the changes that can be made to reduce the risk for sorting. Modifying feeding times, mixing procedures, and number of feedings during the day can also be part of the solution.
Professor of Dairy Science
Pennsylvania State University