Litter surface airflow: untapped opportunity?

Improved airflow velocities and patterns over poultry house floors can lead to reductions of bacteria in the processing plant.

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Numerous farm and laboratory observations suggest that processors, producers and ventilation engineers have a promising opportunity: the correction of still, or sub-optimal, airflow two to three inches above such hard-to-ventilate floor areas as broiler house corners and other vulnerable litter locations. Beyond leading to important reductions in the numbers and risks of bacteria such as salmonella and E. coli in broiler and processing environments, such corrections may also reduce the bottom-line costs of broiler production.

Observation One: Farm Contamination Linked To Processing

The importance of farm contamination and the apparent significant multiplication of bacteria directly in broiler litter has previously been reported (1, 2, 3, 4, 5, 6). Studies by Kingston (1) and Mallinson (4) clearly indicate a close association between drag swabs of the litter surface and the level and types of salmonella found both in and on carcasses at processing. Consequently, much attention was given to learning which litter management factors most contribute to this problem and which serve to reduce them.


Observation Two: Litter Surface Dampness, Airflow Connected To Bacterial Loads

Study of the dynamics of bacterial contamination of litter surfaces reveals the importance of interrelated environmental conditions litter dampness (water activity/availability and total moisture content) and the velocity and pattern of airflow over litter surfaces. Numerous reports (6, 7, 8, 9, 10, 11, 12, 13, 14, 15) detail how these critical conditions are interrelated. Table 1 and Figure 1 illustrate some of the connections between water activity/availability, moisture content, bacterial populations, and airflow.

Previously unpublished data (Mallinson and Myint, 2005), Table 2, suggest high correlations between elevated E. coli counts in litter surfaces and correspondingly elevated counts for salmonella. Low E. coli counts were similarly correlated with low salmonella counts or the absence of detectable salmonella. These E. coli/salmonella associations lend extra significance to the airflow/E. coli relationship presented in Figure 1.

Farm contamination reaching processing plants likely has its most potent source in "hot spots" of salmonella and E. coli multiplication directly in the litter (15, 16). Interestingly, these hot spots appear to be those with still or sub-optimal litter surface airflow. Hot spots are important because they can significantly amplify low-level contamination that can inevitably come from new chicks, feed, rodents or previous flocks. Corners of houses were found to be common bacterial multiplication hot spots in and on the litter surface (12, 15). Poor airflow along walls and in eddies of airflow below fan vents were also found to be additional airflow-deficient hot spots.

In short, proper velocities and patterns of airflow reduce litter surface dampness from levels that promote exponential multiplication of salmonella and E. coli to levels that are essentially lethal to such bacteria. This change from dramatic multiplication to profound suppression can be of major importance to producers in terms of bird health. To the processor, it represents a substantial food safety/risk reduction consideration. At an ambient air relative humidity of less than 80 percent, the beneficial airflow velocities two to three inches above the litter surface were found to be at least 1½ to 2 mph (130 to 175 fpm). At an air humidity of greater than 80 percent, the beneficial airflow velocities were found to be to at least 3½ to 4 mph (300 to 350 fpm).


Observation Three: Farm Revenue

Salmonella-negative farms and those with better ventilation both situations likely interrelated have been associated with improved flock revenue. In a 1991 study of 43 broiler farms, Bender (17) reported that birds from drag-swab, salmonella-negative farms have a cost advantage over birds reared in a contaminated environment. The difference was 0.37 cent per pound for broilers and 0.70 cent per pound for roasters. Later studies of additional farms revealed the same economic advantage. This improvement is suspected to have been due to the presence of farm conditions such as litter surface airflow that not only reduce salmonella populations but also reduce exposure levels to other performance-robbing bacteria, such as E. coli, staphylococcus, clostridia and listeria. Several prominent studies have linked improved economic performance to better ventilation practices (18, 19, 20).


Other Observations

Regulatory Issues.  Inspection agencies are calling for more salmonella-reduction action at the pre-harvest level. Therefore, processors and producers may want to direct extra attention to the issue of litter surface airflow.

Additional Studies.  Measurement of the impact of litter surface airflow on bacterial populations and on bottom-line production costs should continue. It now appears to be the appropriate time to identify ventilation strategies and devices that improve airflow velocities and patterns in house corners and other vulnerable locations. Durable, accurate wind speed meters that can be conveniently positioned only inches above the litter surface and used with consistent results in the hands of different operators are recommended. Measurement of wind speed two to three inches above litter surfaces is different from measuring wind speeds two to three feet above house floors.

Analytical Choices.  Laboratory methods chosen are critical to the investigation of bacterial loads in the food animal production environment. The use of salmonella detection media, such as brilliant green-novobiocin, XLT4 and MM agar, is encouraged to reduce the problem of false salmonella-negative test results encountered with several other traditional agars (21, 22, 23). Delayed secondary enrichment techniques can similarly prove helpful in reducing false-negative test results (24).

Studies of the dynamics of bacterial contamination benefit from the use of techniques that provide counts or estimates of the number of salmonella and E. coli at different points in the environment (5, 11, 15, 25). Such techniques establish the levels of contamination at various sampling locations rather than only their presence or absence. Bacterial quantification is central to the dependable identification of areas of high risk and in the development of well-founded risk reduction strategies. In the future, automated enzyme detection systems and electronic biosensors may make the enumeration of broad-spectrum E. coli, total coliform and total viable bacterial counts more attainable.

Farm Studies.  People designing trials of various flock treatments for salmonella, E. coli and other pathogens, and evaluations of management practices for the reduction of condemnations, antibiotic usage and farm odor, should also include measurement of house floor airflow characteristics in their plans. As discussed here, floor airflow speeds and patterns may present an important confounding variable. Also, if optimal, proper airflow may represent a powerful complementing remediation practice.

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