Purpose: We conducted a field experiment to determine the distance coliforms can be transferred by air during manure application, and the resulting rate of contamination on vegetables.
Methods: Romaine lettuce and tomato were planted in a field at the OARDC. Main plots (8 m2) were arranged in a randomized complete block design. Liquid bovine manure was spread in a 5-m-wide band perpendicular to expected wind direction, next to the first row of plots. Agar plates located 24 cm above the ground and at the point nearest the manure spreading, 15, 30, and 122 m downwind, and 15 m upwind were left open for 5-15 min after spreading. Lettuce leaves and tomato fruits were collected before and immediately after manure spreading, then on post application days 1, 3, 5, and 7. Air temperature, wind speed and direction were recorded on each sampling day. Dilutions were prepared from homogenized vegetable samples and plated; all plates were incubated at 37°C for 36 hours, and colony counts (CFU/ml) were determined.
Results: The number of airborne bacteria on open agar plates was highest at the point nearest the manure spreading. Counts decreased significantly at 15 m and 30 m downwind (P < 0.05), but after 30 m, the number of bacteria on open plates stayed constant (P = 0.6). Number of bacteria on vegetable samples did not vary with distance (P = 0.3). Wind speed and direction were not correlated with number of bacteria (P = 0.4).
Significance: This study’s findings can be used as evidence to support farm food safety policy and to develop future strategies to prevent and control microbial hazards.