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• Response of weaning at an early age on rumen pH, rumen fermentation, dietary intake, growth rate in the post-weaning phase, and rumination behavior employing the Hr-Tag rumination-monitoring system (Hr-Tag; SCR Engineers Ltd, Netanya, Israel) in Simmental calves. • Weaning Simmental calves at 45 d of age compared with 60 d, fed whole milk in the pre-weaning period, did not impair rumen fermentation after weaning, and overall the rate of VFAs clearance have a major impact on ruminal pH. • Daily rumination time measured by Hr-Tags positively correlated to pH and negatively correlated to concentration of total VFAs, acetate, and propionate. A positive linear relationship was also found between daily rumination time and the molar proportion of acetate:propionate ratio. • The use of Hr-Tags could give quick information on rumen health and thus to identify in a timely fashion those calves at a greater risk (i.e. marked decrease of rumen pH) during this critical phase, from liquid to only solid feeds. The present experiment aimed to investigate (1) the effect of weaning age on rumen fermentation, dietary intake, growth, and rumination behavior at 20 d after weaning in Simmental calves, and (2) the relationship between rumination time measured by the Hr-Tag rumination-monitoring system (Hr-Tag ; SCR Engineers Ltd, Netanya, Israel) and rumen fermentation variables. Ten Simmental calves were randomly assigned at birth to be weaned at 45 (early weaning, EW) or 60 d of age (normal weaning, NW). EW calves were offered 6 L of bulk whole milk/d from d 2 to 38 and 3 L/d from d 39 to 45, while NW were offered 6 L of bulk whole milk/d from d 2 to 53 and 3 L/d from d 54 to 60. Individual calf starter intake was recorded daily and average daily gain (ADG) was calculated taken into account the BW at 6 d after weaning and final BW at 20 d after weaning. On d 20 after weaning, samples of rumen fluid were obtained via an esophageal tube. These samples were used to determine pH and volatile fatty acids (VFAs) concentration. Throughout the entire experiment, the Hr-Tag system continuously recorded the rumination time (RT) by Hr-Tag loggers positioned on the left side of the neck. No treatment differences were detected for total VFAs concentration and molar proportion of acetate, propionate, and butyrate (P > 0.05). Overall in the period -25 to 20 d relative to weaning time, daily RT did not differ between NW and EW calves and it increased around weaning in both groups. Hr-Tags revealed no differences in daily RT pattern (2h-RT) during the 3 d before rumen collection, while during the day (24 h) 2h-RT significantly varied overall in both EW and NW groups. Daily RT measured by Hr-Tags positively correlated to pH (r = 0.98; P < 0.001), and negatively correlated to concentration of total VFAs (r = −0.90; P < 0.001), acetate (r = −0.85; P < 0.001), and propionate (r = −0.90; P < 0.001). On the contrary, a positive linear relationship was found between daily RT and the molar proportion of acetate:propionate ratio (R2 = 0.76; P = 0.002). Results herein provide evidence that early weaning (45 d) in Simmental calves fed whole milk in the pre-weaning period did not impair rumen fermentations 20 d after weaning and BW gain. Surprisingly, the use of Hr-Tags could prove interesting for the prediction of rumen fermentation characteristics and patterns in calves after weaning. The collected data could be used to obtain quick information on rumen functionality allowing the identification in a timely fashion of those calves at a greater risk (i.e. marked decrease of rumen pH) during this critical transition phase, from liquid to only solid feeds, when rumen of calves is not completely developed and efficient. The provision of these results from Hr-Tags in calves may give some input for further research involving a greater number of calves to better establish correlations with phenotypic variables. Moreover, this device could be adopted in studies focused on the effect of different pre- and post-weaning diets or weaning management. [ABSTRACT FROM AUTHOR]