APPROXIMATELY 80% of the cooling produced in a modern tunnel-ventilated poultry house is the result of wind speed moving over birds, according to recent research by Dr. Brian Fairchild and Dr. Mike Czarick at the University of Georgia.
In a study funded by the USPOULTRY Foundation, Fairchild and Czarick found important interactions among airspeed, static pressure and air velocity distribution in tunnel-ventilated broiler houses.
Understanding these relationships is very important in modern broiler houses, where airspeeds can be in excess of 600 ft./min.
According to the researchers, airspeeds across the cross-section of a house may vary by 30% or more, resulting in significant differences in bird cooling at different areas of the house (e.g., side wall versus center).
In new poultry house construction, airspeeds of 600-900 ft./min. are being targeted, but little work has been done to document factors affecting air velocity in tunnel-ventilated poultry houses, Fairchild and Czarick explained. The objective of their study was to determine which factors affect cross-sectional air velocity distribution in tunnel-ventilated broiler houses.
In the past, air velocity profiles in poultry housing were measured at one level, the researchers said, noting that their study used a grid of 15 anemometers (ceiling to floor and wall to wall) located 50 ft. in front of the tunnel fans to measure the cross-sectional air velocity.
Pressure sensors were installed at the pads, inlet, quarter house and three-quarter house to measure static pressure. Air velocity and static pressure were monitored with several fan capacities varying between 100% and 50% of all tunnel fans operating.
These measurements were taken in a total of 27 poultry houses that were less than two years of age belonging to four companies. Included were 24 broiler houses, one commercial layer house, one pullet house and one breeder house, Fairchild and Czarick reported.
High static pressure makes fans work harder but move less air, they explained. Therefore, static pressure was kept as low as possible in the past so fans could continue to operate at optimum efficiency.
In this study, the data show that static pressure increases as house air velocity increases, Fairchild and Czarick said. In the newer houses, the cross-sectional air velocity did not vary by more than 10% across the width of the house. This was due to the smoothness of the side-wall construction and having few obstructions down the length of the house, the researchers noted. In houses with smooth side walls, the average airspeed in the house can be estimated by measuring air movement at the side feed lines.
As more obstructions are introduced and side wall smoothness decreases, the researchers said additional measurements across the width of the house are required to estimate average house air velocity.
Static pressure increased as air moved down the length of the house. While static pressure is usually measured at the center of the house, it is important to remember that the total pressure the fans are working against is slightly higher when measured at the rear of house near the tunnel fans, they noted.
In older houses (airspeeds of 400-500 ft./min.), the largest factor affecting static pressure was the tunnel inlet, Fairchild and Czarick reported. If the inlet was too small, then pressure would be high. In those cases, the producer would increase the size of the tunnel inlet to maintain a low static pressure.
In modern high-airspeed houses, the tunnel inlet is not the main factor involved in increasing static pressure. Instead, the transitional pressure — where the air makes the turn into the house — is the main contributor to total static pressure, the researchers said.
They explained that in houses with airspeeds of 600 ft./min. or more, increasing the tunnel inlet size will not result in lower airspeeds. In fact, oversizing the tunnel inlet (usually done in length) can actually make airspeed distribution along the length of the tunnel inlet worse.
According to Fairchild and Czarick, this set of data indicated that tunnel openings in the house can be smaller than the evaporative cooling pad opening, which improves the air distribution along the length of the inlet without significant effects on the static pressure against which the fans are working.
They concluded that poultry companies targeting higher airspeeds should design their fan requirements at higher static pressures than the traditional design pressure of 0.1 in. of water column. House static pressure should be measured near the fans at least once a year.
Being mindful of these influences on air velocity can maximize the air movement in poultry houses and minimize the losses during hot weather.
The U.S. Poultry & Egg Assn. and the USPOULTRY Foundation also announced the completion of a research project they funded at Auburn University that has found a potential marker for selecting against broilers' susceptibility to deep pectoral myopathy (DPM), commonly called "green muscle."
Drs. Roger J. Lien, Sarge F. Bilgili and Joe B. Hess of Auburn explained that DPM is a condition in which the breast tender of broilers is found, during processing, to be discolored. This meat must be trimmed and condemned.
According to the researchers, it is estimated that 0.5% of breast tenders are condemned because of DPM, creating a loss of $50 million per year.
They were able to define some of the factors involved in causing this condition, including broiler strain, broiler gender and the temperature at which the birds were raised. Of particular importance, the researchers found that the level of a certain serum enzyme was correlated with the development of green muscle and suggested that this enzyme might be used as a marker for genetic selection of broiler strains that are less susceptible to the condition.
DPM is becoming more common in broilers and is caused by wing flapping at least a few days before slaughter, since discolored lesions take 24-48 hours to develop, or several weeks earlier, since tissue damage is often permanent, Lien et al. added. Broiler DPM will likely continue to increase since breast yield selection and heavier processing weights are both contributing factors that are increasing.
Creatine kinase (CK) is a muscle enzyme normally found in elevated amounts in plasma after muscle damage, the researchers said, explaining that its levels are elevated following DPM induction in turkeys and broiler breeders. CK may be useful as a non-invasive marker for detection in broilers, Lien et al. said.
In a previous project, Lien et al. developed a standardized technique to induce DPM called "encouraged wing flapping" (EWF).
The project objectives were to:
1. Determine if temperature, time of day or strain influenced susceptibility to DPM induction by increases in light intensity and human disturbance or EWF.
2. Determine the effects of age, sex, strain and bodyweight on DPM lesion development.
3. Determine if baseline or changes in CK levels with age- or EWF-induced CK level changes in very young broilers can indicate subsequent DPM susceptibility.
4. Determine the time course of CK level elevations following DPM induction.
Light intensity increases and disturbances similar to those occurring when farmers check broilers did not increase DPM incidence, Lien et al. reported. The prevalence of DPM was greater at normal temperatures — which resulted in greater growth than at high temperatures — and was greater in strains selected for breast yield.
Induction of DPM three to five days before slaughter resulted in a 10-20% decrease in subsequent growth. Broilers gradually developed DPM susceptibility from four to seven weeks, with males developing susceptibility a week earlier than females and bodyweight differences not being a primary factor, the researchers said.
Plasma CK levels in the weeks before induction of DPM were not a good predictor of susceptibility because they increased much more dramatically from one to four days after EWF in birds in which DPM was induced. Therefore, plasma CK levels after EWF could be used as a non-terminal marker for DPM susceptibility in genetic selection programs, Lien et al. said.
They concluded that plasma CK level changes in response to EWF at ages too early to induce DPM did seem to be related to subsequent susceptibility, which also suggests a potential use of CK levels to screen birds for DPM susceptibility without actually inducing lesions.