Seasonal Initial Concentrations and In-Field Decay Rates of <i>Escherichia coli</i> and Bovine <i>Bacteroidetes</i> in Beef Cattle Manure

Eight naturally deposited beef cow manure patties were sampled during summer (July 19 to August 9, 2010), fall (October 26 to November 19, 2010), winter (January 14 to February 18, 2011), and spring (May 5-27, 2011) to determine whether hypothesized seasonal differences existed in the initial concentrations and decay rates of Escherichia coli (E. coli) and bovine Bacteroidetes (BoBac). E. coli concentrations were estimated as culturable colony forming units (CFU) and with a quantitative polymerase chain reaction (qPCR) assay targeting the 23S ribosomal gene. BoBac was quantified with a qPCR assay targeting a 16S ribosomal gene sequence associated with cattle manure.

Initial concentrations for culturable E. coli varied several orders of magnitude during each season, but were significantly lower when the animals grazed fresh forage (3.6 and 4.3 log₁₀CFU/g-dry-manure in fall and spring, respectively) versus receiving hay and grain because of dormant pastures (6.4 log₁₀CFU/g-dry-manure in winter). Average initial E. coli 23S gene abundance was also highly variable but lower in the spring and fall (7.1 and 8.5 log₁₀copies/g-dry-manure) than in the winter (9.4 log₁₀copies/g-dry-manure). Average initial BoBac 16S gene abundance was much less variable but again lower during grazing (9.9 log₁₀copies/g-dry-manure in both spring and fall) versus during supplemental feeding (11.0 and 11.2 log₁₀copies/g-dry-manure in summer and winter, respectively).

Linear regressions of aggregated log transformed concentration data were used to calculate seasonal decay rate coefficients. The decay rate for culturable E. coli was highest in the winter (-0.094 log₁₀CFU/g-dry-manure/day) and significantly lower in the fall and spring (-0.028 and +0.018 log₁₀CFU/g-dry-manure/day, respectively). The same was true for E. coli 23S gene abundance (-0.086, -0.026, and +0.023 log₁₀copies/g-dry-manure/day in winter, fall, and spring, respectively). The decay rates were far higher for BoBac 16S gene abundance which had an opposite seasonal trend, being much higher in the summer (-0.33 log₁₀copies/g-dry-manure/day) than in the winter (-0.10 log₁₀copies/g-dry-manure/day).

The fact that initial bacterial concentrations and decay rates vary seasonally should be considered when modeling the fate and transport of the regulatory fecal pollution indicator E. coli and the fecal pollution source tracking BoBac gene sequence.