Subacute ruminal acidosis (SARA) was induced by replacing 25% of the total mixed ration intake [dry matter (DM) basis] with pellets consisting of 50% wheat and 50% barley. This reduced dietary forage content (DM basis) from 39.7 to 29.8% and increased the dietary concentrate content from 60.3 to 70.2%. Induction of SARA reduced the 24- and 48-h in situ neutral detergent fiber (NDF) degradabilities of grass hay numerically from 31.5% to 24.6% (P = 0.29) and from 51.3% to 36.9% (P < 0.05), respectively. The 24- and 48-h in situ NDF degradabilities of legume hay were reduced from 35.3 to 26.3% (P < 0.05) and from 49.0 to 35.8% (P < 0.05), respectively. The 24- and 48-h in situ NDF degradabilities of corn silage were reduced from 44.0 to 37.2% (P < 0.05) and from 56.1 to 44.8% (P < 0.05), respectively. This study suggests that induction of SARA by excess feeding of wheat/barley pellets reduces the rumen digestion of NDF from grass hay, legume hay, and corn silage.

The effects of monensin, administered either as a controlled release capsule (CRC) or a premix, on attenuating grain-induced subacute ruminal acidosis (SARA) and on ruminal fermentation characteristics in Holstein cows receiving a total mixed ration were investigated in two experiments. In both experiments, six multiparous, rumen-fistulated Holstein cows were used in a two-treatment, two-period crossover design with 6-wk periods. In Experiment 1, treatments were either a monensin CRC or a placebo CRC. In Experiment 2, treatments were either a monensin premix or a placebo premix. In both experiments, at the beginning of wk 4 SARA was induced in experimental cows for a 10-d period with a grain challenge model, and ruminal pH was measured continuously using indwelling pH probes. The administration of monensin either as a CRC or a premix had no effect on ruminal pH characteristics. Neither monensin CRC nor premix had an effect on ruminal volatile fatty acid concentrations, but reduced the acetate:propionate ratio. Monensin premix-treated cows were observed to have increased milk yield, largely as a result of a higher dry matter intake in monensin-treated cows compared to control cows. Milk fat content and yield were lower in monensin-treated cows compared to placebo-treated cows during SARA. In conclusion, there is no evidence that monensin was efficacious in raising ruminal pH during SARA under the conditions employed in this study.

. 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

. W. 2001. Effect of post-ruminal infusion of glutamine on plasma amino acids, milk yield and composition in lactating dairy cows. Can. J. Anim. Sci. 81: 229-235. The objectives of this study were to examine the effect of supplying post-ruminal L-glutamine in mid-lactation Holstein dairy cows on plasma amino acid profile, dry matter intake, milk yield and milk composition. The experiment was designed as a 4 × 4 Latin square with four 2-wk periods. Cows were continuously infused post-ruminally with graded levels of L-glutamine (L-Gln) for 5 consecutive days during the second week of each period. During the last 24 h of the infusion, blood plasma was collected every 6 h for the determination of plasma amino acid concentrations. Plasma Gln concentration increased (P < 0.05) with increasing L-Gln daily infusions. Infusion of 0, 100, 200, and 300 g d -1 resulted in blood plasma Gln concentrations of 289.5, 299.2, 356.4 and 386.2 µmol L -1 , respectively. The level of Gln infusion also resulted in a decrease in the blood plasma concentration of phenylalanine, but the concentrations of all other amino acids were not affected. The administration of Gln did not affect dry matter intake, milk yield and milk composition.

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