Ruminant and ruminal responses to feeding the gram-positive ionophore, monensin, have been researched extensively over the past 20 years. A proportion of many such in vivo experiments have used a change-over design. In doing so, the researcher either paid no attention to or was reasonably confident that any possible carry-over effects would have dissipated. Evidence does exist which leads to an estimation of duration to maximum treatment effects but such comparable evidence surrounding the duration of carry-over to monensin treatment is not available.
An in vivo trial was proposed at the Centre for Dairy Research (CEDAR), to investigate the ruminal effects of feeding monensin to lactating dairy cows with 4-week periods of which 3 weeks was for change-over and adaptation. Therefore, before conducting such an expensive experiment, in terms of both time and money, an in vitro study using the rumen simulation technique (Rusitec) was carried out to determine if 3 weeks was considered adequate to eliminate carry-over effects when measuring volatile fatty acid (VFA) molar proportions.