High producing dairy cattle require large amounts of dietary energy in order to meet the demands of milk production. High energy diets, however, can put the cow at risk for subacute ruminal acidosis (SARA). High energy diets are often high concentrate diets, low in neutral detergent fiber (NDF) and high in starch. Feeding more grain and less forage increases the production of volatile fatty acids (VFA) in the rumen, as grains are generally more rumen digestible than forages and they are rapidly digested as well. The optimal rumen pH in a dairy cow is between 6.0 and 6.2 (Van Soest, 1994). It is well documented that when cattle consume a diet high in highly fermentable carbohydrates and low in structural fiber, rumen pH often falls below 6.0 (Owens et al., 1998) as the production of VFA exceeds the cows' ability to absorb them. Each cow has an inherent capacity to buffer and absorb acid, and that capacity determines how far her ruminal pH will fall after a meal containing large amounts of fermentable carbohydrates (Krause and Oetzel, 2006). While it is critical to meet the energy requirements of high-producing cows, acidosis should be avoided to ensure high milk production, efficient use of feed, and rumen and overall cow health.
Forages in general and long forage particles in particular promote chewing and saliva secretion, which buffers VFA production from feed digestion (Mertens, 1997). Therefore, forage particle size and the amount of fiber in the diet can have a significant impact on ruminal pH. Cows consuming longer forage particles spend more time eating (per pound of dry matter) and ruminating, which increases the flow of saliva and its associated buffers into the rumen. Diet fiber content is a primary factor affecting chewing activity and is believed to be an indicator of the diet's effect on rumen health and function (Yang et al., 2001). Compared to high forage diets, high concentrate diets are more rapidly fermentable due to their composition, contain less physically effective fiber, and may result in suboptimal rumen conditions of low overall pH. These conditions may limit digestibility of some feed components.
Unfortunately, cows generally sort against long particles due to their less palatable nature and sort for finer particles in the ration. This behavior can lead to metabolic problems such as SARA. Cows consuming the finer particles of the ration are reducing their particle size consumption and, in effect, their NDF intake (Leonardi and Armentano, 2003; Kononoff and Heinrichs, 2003). These sorted diets contain more fermentable carbohydrates and less effective fiber than the formulated ration. Diet sorting, however, can be beneficial during periods of acidosis. There is growing evidence that dairy cows will select feeds with high ruminal buffering capacity in an attempt to attenuate the effects of low ruminal pH to improve rumen health and dry matter intake (DMI). Keunen et al. (2002) demonstrated that lactating dairy cows induced with SARA increased their preference for long alfalfa hay over pelleted alfalfa. Cows sorted for longer particles in a TMR when challenged with a high starch diet (Beauchemin and Yang, 2005; Yang and Beauchemin, 2006). DeVries et al. (2008) also used a rumen challenge model to induce SARA in early and mid-lactation Holstein cows. Changes in eating behavior were measured by determining the particle size distribution of offered feed and refusals. After the rumen challenge, cows in both groups changed their sorting behavior. DeVries et al. (2008) determined that early lactation cows generally increased their sorting for medium particles and against short and fine particles and exhibited no change in sorting long particles when challenged. A Penn State study (Maulfair et al., 2013) showed that cows with long diets could sort for smaller particle size feed components and cows with short diets sorted for longer components of the diet. These studies suggest that early lactation cows altered their sorting behavior to consume a diet that may help attenuate their bout of SARA or inadequacies in their diets. In addition, supplementing the TMR with long hay maintained DMI during incidents of and recovery from periods of low rumen pH in a recent Penn State study that we conducted.
Diet sorting by dairy cattle may be beneficial during times of SARA or misbalanced diets in some instances. Providing longer forage particles and effective fiber may improve rumen pH and maintain DMI during incidents of SARA.
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