Variations in fecal pH and fecal particle size due to changes in dietary starch: Their potential as an on-farm tool for assessing the risk of ruminal acidosis in dairy cattle.

This study evaluated the variations in fecal pH and particle size due to changes in dietary starch, and the potential of these variations as a tool to detect the risk of subacute ruminal acidosis (SARA) in dairy cows. Nine ruminally cannulated, non-lactating, non-pregnant Holstein cows were used in two 6-week experimental periods. In each period, cows were first fed a forage diet for 1 wk., then transitioned over 1 wk. to a 65% concentrate ration, which they consumed for 4 wk. continuously. Measurements were conducted when cows consumed 17.3, 21.9 and 28.8% dietary starch. Fecal pH and particle size were measured at 0, 4, 8 and 12 h relative to feeding in days 7, 11, 21, 28, 35, and 42 of each period. Ruminal pH was measured every 15 min. Data were analyzed with SAS, the statistical model included concentrate level, time of sampling and period as fixed effects and cow was considered as random effect. Data showed an interaction between dietary starch level and time relative to feeding on fecal pH, with a shift in its pattern due to diet change. Specifically, during low starch feeding (17.3%), fecal pH was highest before feeding and decreased thereafter, reaching lowest value 12 h post-feeding (P < 0.05). With the 21.9% starch diet, fecal pH did not change significantly after feeding. However, during high starch feeding (28.8%), fecal pH decreased, being lowest before feeding and increased (P < 0.05) during the day reaching highest values at 8 and 12 h post-feeding. Fecal pH was lower (P < 0.01) during the days of high starch feeding; though, it was maintained relatively stable through this timeframe. Increasing dietary starch decreased the proportion of small fecal particles (0.5 to 1.18 mm), but increased the proportion of large (>1.18 mm) and soluble particles (<0.5 mm). There were significant correlations among ruminal pH, fecal pH and fecal particle size of feces collected before feeding. For example, fecal pH was correlated with minimum and daily mean ruminal pH; the proportion of fecal particles 0.5 to 1.18 mm correlated with minimum and daily mean ruminal pH (P < 0.01). Log-linear dependency analyses indicated a strong effect of starch intake on fecal pH so that for every kg increase in starch intake, there was a reduction in fecal pH by 0.38 units. In addition, regression analysis showed that the proportion of fecal particles between 0.5 and 1.18 mm showed strong dependency on the ratio dietary physically effective fiber to starch (P < 0.01). Ruminal pH also correlated with fecal particle size of frozen/thawed samples, but with lower strength than fresh samples. Overall, evaluating variations of fecal pH and particle size holds potential as a non-invasive on-farm tool for assessing rumen pH and SARA risk.