Short communication: Sodium salicylate negatively affects rumen fermentation in vitro and in situ.

Administration of sodium salicylate (SS) to cows in early lactation has a positive effect on whole-lactation milk production but a negative effect on metabolism in some cases. The objective of this trial was to determine whether SS directly affects rumen fermentation. Experiment 1 was designed to investigate the effects of direct inclusion of SS in a 24-h batch culture, and experiment 2 was designed to test the fermentative ability of rumen fluid from heifers who had received SS. In experiment 1, we combined strained and pooled rumen fluid from 3 heifers in a 2:1 ratio with McDougall's buffer, and added 150 mL of the inoculum to each flask (n = 5/treatment) with 2.5 g of fermentation substrate similar to a lactating cow ration, ground to 1 mm. We then added premixed treatments (1-mL volume) to achieve the desired final amount of SS (CON1 = 0 mg, LOW = 125 mg, MED = 250 mg, HI = 375 mg). In experiment 2, 6 heifers (n = 3/treatment) were drenched daily for 3 d, either with 62.5 g of SS dissolved in water (SAL) or an equal volume of water (CON2). Rumen fluid was collected from each heifer and was not pooled. After the fluid was mixed 2:1 with McDougall's buffer, 150 mL of inoculum was added to the fermentation flasks (n = 4/heifer) with 2.5 g of fermentation substrate. This experiment was performed the day before SS treatment began and repeated 1, 13, and 35 d after the end of the treatment period. We also performed an in situ experiment at each of these time points. In the first experiment, inclusion of SS resulted in a decrease in dry matter disappearance (DMD) over 24 h, as well as an increase in final pH. We detected no difference between treatments for gas production asymptotic volume, rate, or lag. In the second experiment, we detected a significant treatment × day interaction for DMD: we observed no difference between groups during a 24-h batch culture on the day following treatment, but SAL resulted in lower DMD on d 13 and d 35. We detected no treatment effect on the final pH of the batch culture or on any gas-production parameters. We observed a tendency for SAL to decrease the DMD rate in situ on the day after treatment. These results indicate that SS administration has a negative effect on rumen microorganisms.