K. M. Krause — School of Veterinary Medicine, UW-Madison
D. V. Dhuyvetter — Ridley Block Operations
G. R. Oetzel — School of Veterinary Medicine, UW-Madison
INTRODUCTION
Subacute ruminal acidosis (SARA) is defined as periods of moderately depressed ruminal pH (between 5.2 and 5.6) that are between acute and chronic in duration (Cooper and Klopfenstein, 1996). The most consistent and immediate clinical sign of SARA is depressed feed intake, evidently because excess organic acids disrupt rumen function (Cooper et al., 1995, 1996) and cause malaise (Provenza et al., 1994). Periods of low ruminal pH impair the function of the fibrolytic microflora (Shi and Weimer, 1992) and decrease ruminal fiber digestion in vivo (Krajcarski-Hunt et al., 2002). These factors combine to reduce milk yield and hence, profitability for dairy producers.
Subacute ruminal acidosis may be caused by feeding excess NFC, a rapid increase in dietary content of NFC, or insufficient ruminal buffering (NRC, 2001). Dietary buffers, particularly sodium bicarbonate (SB), have been added to dairy cattle diets in an attempt to meet this shortage in ruminal buffers and decrease the incidence of SARA. Buffers can either be force-fed to cattle (i.e., added directly to the cow’s mixed ration or grain mix) or offered free choice. Force feeding of buffers may result in greater expense than necessary, because not all cows require the same extent of dietary buffering. Free-choice feeding of buffers could be economically efficient, because in theory it allows cows to consume buffers only as needed. This requires that the cow has the “nutritional wisdom” to consume buffers in proportion to her need for dietary buffering. However, Cottee et al. (2004) reported that cows, when subjected to SARA, showed no difference in preference of an SBsupplemented water source to unsupplemented water. significant decrease in mean ruminal pH of 0.28 pH units and significant increases in time and area below ruminal pH 5.6. Similarly, Keunen et al. (2003) found no preference for free-choice SB in cows induced with SARA. These studies indicate that cows do not attempt to correct an imbalance in rumen environment by increasing their intake of SB. Because undiluted SB may be unpalatable, cows with low ruminal pH may not consume enough free-choice SB to elicit a positive reinforcement. When SB was included in a pelleted, high-energy density feed, Cooper et al. (1996) found that feed intake of sheep increased. In addition, Phy and Provenza (1998) found that after feeding rolled barley, lambs preferred pellets with SB (2% as-fed basis) to pellets with NaCl. These findings suggest that incorporating SB into a highly palatable free-choice supplement might increase intake of SB during a bout of SARA.
Sodium bicarbonate can be incorporated into a lowmoisture molasses block. Mixing SB with molasses has the advantage of masking the flavor of bicarbonate and encouraging free-choice consumption. Furthermore, intake of a low-moisture block is only possible by licking the surface area, which will stimulate saliva production and thereby the production of endogenous buffers. The objective of this experiment was to evaluate the effect of a free-choice, low-moisture buffer block on ruminal pH and milk production in cows challenged with a bout of SARA.
