Two experiments were conducted to study the effects of a subacute ruminal acidosis (SARA) model on diet choice in dairy cows. In the first experiment, 25% of the ad libitum dry matter intake (DMI) of the total mixed ration (TMR) was replaced with wheat-barley pellets (WBP, 50% ground wheat, 50% ground barley). Rumen pH was measured continuously via in-dwelling probes in 4 mid to late lactation cows. This diet change reduced rumen pH by 0.14 +/- 0.02 pH units (mean +/- SE) and increased time below pH 6.0, from 319 +/- 36 min(-1) to 641 +/- 36 min(-1). Hence, the nutritional model successfully induced SARA. The second experiment determined if inducing SARA increases the feed preference for long alfalfa hay compared with alfalfa pellets. The 2 wk of inducing SARA were separated by 1 control wk. Four cows on either SARA and control diets were given a choice of 2 feeds, 2 times per d, for 30 min. The preference ratios (PR = Amount of Hay consumed/Amount of Hay + Pellets consumed) for alfalfa hay during two SARA weeks was greater (0.85 +/- 0.03) compared with the control week (0.60 +/- 0.03). In SARA weeks, average rumen pH was 0.23 +/- 0.03 units lower, and time below pH 6.0 and 5.6 was higher compared to control. These results suggest that when given a choice of feeds, dairy cows alter their diet selection to attempt to attenuate SARA.

. W. 2001. Effect of subacute ruminal acidosis on in situ digestion of mixed hay in lactating dairy cows. Can. J. Anim. Sci. 81: 421-423. Induction of subacute ruminal acidosis (SARA) by replacing 25% of the total mixed ration (TMR) intake [dry matter (DM) basis] with pellets consisting of 50% wheat and 50% barley, reduced the in situ DM and neutral detergent fibre (NDF) digestion of mixed hay. In situ 48 h NDF disappearance was reduced from 39.5 to 30.9% (P < 0.05).Key words: Sub-acute ruminal acidosis, fibre digestibilty, dairy cows Plaizier, J. C., Keunen, J. E., Walton, J-P., Duffield, T. F. et McBride, B. W. 2001. Incidence de l'acidose subaiguë du rumen sur la digestion in situ d'un mélange de foin chez la vache laitière en lactation. Can. J. Anim. Sci. 81: 421-423. Lorsqu'on induit une acidose subaiguë dans le rumen en remplaçant 25 % de la ration mixte totale (équivalent en matière sèche) de l'animal par des agglomérés composés à 50 % de blé et à 50 % d'orge, on observe une diminution de la digestion in situ de la matière sèche et de la cellulose au détergent neutre (CDN) présents dans le mélange de foin. La quantité de CDN éliminée in situ au bout de 48 heures était passée de 39,5 % à 30,9 % (P < 0,05). Mots clés:Acidose subaiguë du rumen, digestibilité de la cellulose, vache laitière It is expected that subacute ruminal acidosis (SARA) affects the productivity of dairy cows by reducing the fibre digestion, because low rumen pH negatively affects cellulolytic bacteria. This is likely due to reduced growth yield and increased maintenance energy requirements of these bacteria (Shi and Weimer 1992). Plaizier et al. (1999) continuously monitored rumen pH throughout the transition period in dairy cows. Diets consisted of corn silage, high moisture corn, mixed haylage, mixed hay, soybean meal and commercial supplements and were fed ad-libitum. Average dry matter intake (DMI) was 11.2 kg d -1 during the week before calving and 16.0 kg d -1 during the week after calving. The forage to concentrate ration was switched from 70:30 to 55:45 at calving. One week prior to calving average daily pH was 6.83, average daily time with rumen pH below 6 was 25.5 min d -1 , and average daily time with rumen pH below pH 5.6 was 5.6 min d -1 . During the first week after calving average daily pH was 6.51, and average daily time with rumen pH below 6 and average daily time with rumen pH below pH 5.6 were 312 min d -1 and 59.6 min d -1 , respectively. These results show that rumen pH dropped considerably after calving (Plaizier et al. 1999). This drop in rumen pH is expected due to the higher concentrate content of the post-calving diet, resulting in a higher rate of volatile fatty acid (VFA) production. This temporally increases the VFA concentration in the rumen, until the absorptive capacity of the rumen mucosa for VFA has been increased.Many studies of the effect of pH on fiber digestion have been conducted in vitro. Calsamiglia et al. (1999) found, using dual-flow continuous culture fermenters, that the NDF

The effect of inducing subacute ruminal acidosis (SARA) on the free-choice intake of sodium bicarbonate (SB) was investigated in four midlactation Holstein cows in a switchover experiment with four 1-wk periods. The SARA was induced by replacing 25% of the ad libitum intake of total mixed ration (TMR) with pellets containing 50% ground wheat and 50% ground barley and restricting access to TMR from 0700 to 1700 h. Control consisted of feeding TMR ad libitum. Powdered SB was provided for ad libitum consumption. Rumen pH was measured continuously using indwelling pH probes. Induction of SARA reduced (P < 0.05) the average daily rumen pH from 6.08 to 5.87, increased (P < 0.05) the average duration of rumen pH below 6 from 547 min x d(-1) to 916 min x d(-1), and increased (P < 0.05) the average duration of rumen pH below 5.6 from 132 min x d(-1) to 397 min x d(-1) (P < 0.05) but did not significantly affect SB intake. Average intake of SB was 26.8 g x d(-1) during SARA and 34.5 g x d(-1) during control. These low SB intakes must not have substantially affected rumen pH. Sodium bicarbonate intake differed significantly (P < 0.05) between cows. These data indicate that cows did not select SB in order to attenuate SARA.