Materials and methods
The mean pH values for Control and PMR cows for each measurement in a 24-h acetylcholine receptor over the duration of the 14-day measurement period are shown in Fig. 1. The typical daily pattern involved a drop in ruminal pH following the morning feed, followed by a small increase in pH before the afternoon feed. A second drop after the afternoon feed was followed by a slow return to maximum values overnight. Daily ruminal pH values are summarised in Table 1. There were no significant differences between means for Control and PMR groups, and no effect of feeding rate for any of the outcomes.
The mean (±SD) corrected serum Hp concentration was 0.062 ± 0.27 mg/mL for Control cows (n = 62) and 0.053 ± 0.20 mg/mL for PMR cows (n = 64). There was no effect of feeding method (P = 0.98) or feeding rate (P = 0.74). None of the 15 rumen-fistulated cows had serum concentrations of Hp >0.009 mg/mL (lowest detectable concentration) whereas 17/110 non-fistulated cows did.
The percentage of time that the pH is < 6.0 and 5.6 have both been reported to be relevant for determining sub-optimal rumen function and the presence of SARA, respectively (Mould et al., 1983; Nocek, 1997; Olson, 1997). The indwelling pH boluses allowed normal behaviour of the cows and could record many more values of pH than would have been possible using manual sampling so parameters of time below pH 5.6 and 6 could be precisely evaluated as well as the ‘area’ (time × pH) under those values. This gave a measurement of not only how low the pH went, but for how long it was at this low value. Gozho et al. (2005) defined SARA as a ruminal pH < 5.6 for > 3 h/day. This is 12.5% of the day and, by this definition, even though there was no significant difference between PMR and Control cows in the time spent below the defined ruminal pH thresholds, 12/15 rumen-fistulated cows in the study had SARA. However, there were no health problems recorded in these cows.
Working with the same cows, Auldist et al. (2014) also found no difference between PMR and Control cows for milk yield or composition, body condition score, liveweight or ruminal volatile fatty acid and ammonia concentrations. All of these parameters have been well-described as potential indicators of SARA, or at least of sub-optimal rumen function. (Kolver and de Veth, 2002; Kleen et al., 2003; Nordlund et al., 2004). Additionally, Auldist et al. (2014) measured ruminal pH manually at 2-h intervals for a 24-h period during the same experiment and found no difference in ruminal pH parameters between PMR and Control cows, apart from PMR cows recording a higher maximum pH. Those authors did report that with increasing level of supplement, the area under pH 6 increased. This also highlights the problem of having a defined threshold for SARA when the dynamic nature of the rumen and its pH (and also the way the cow is able to adapt to fluctuations in the pH over time) means that an absolute threshold of optimal ruminal pH or a level of pH defining SARA may be less clear than is often assumed.
The results from this experiment indicate that lower ruminal pH values or more time spent below pH 5.6 were not associated with increased concentrations of Hp. In the time period from 8 days before to 7 days after blood collection, 9/126 blood sampled cows were sick (eight with mastitis, one lame). Four of the eight cows with mastitis during that period were found to have Hp concentrations greater than the upper limit of the normal reference range (<0.1 mg/mL; Ceciliani et al., 2012), whereas only seven of the remaining 118 cows with no signs of clinical disease had serum Hp concentrations >0.1 mg/mL. This supports the evidence in the literature that for conditions resulting in high levels of acute inflammation, such as mastitis (Eckersall et al. 2001), serum Hp concentrations may be a useful marker.
Materials and methods