Simulation model helps sort through energy cost variables — nursery vs. wean-to-finish housing.
As energy costs escalate, many pork producers are asking whether the economics of wean-to-finish barns vs. traditional nurseries and finishers has shifted.
To help answer the question, Iowa State University (ISU) Agricultural Engineer Jay Harmon built a simulation using a model developed with the cooperation of Murphy-Brown's western division located in Ames, IA.
“There are many variables that can be shifted around to affect the outcome, so some assumptions have to be made,” explains Harmon. “Actual energy usage may vary from the model results, but using a model helps to examine the magnitude of the impact of making changes.”
For the nursery, Harmon used a 1,000-head building with a preheated hallway. The nursery room was 50 × 72 ft. The wean-to-finish unit used in the simulation was a 40 × 200 ft., curtain-sided structure, also holding 1,000 head.
“I calculated what energy would be used during one week of operation based on the average January and April temperatures in central Iowa, which are 18.5°F and 49.8°F, respectively,” he explains. The model also assumed that both structures were ventilated at the proper rate.
Harmon ran the model using two different pig placement weights — 15 lb. and 35 lb. Here is what the simulation shows:
15-lb. pig placement: Both facilities were set at 85°F and 2 cfm/pig (cubic feet/minute/pig) for ventilation rate.
“In actuality, 2 cfm/pig is pretty difficult to achieve in a wean-to-finish unit, because most producers try to do this with two, 24-in. fans, which cannot effectively be run slow enough to accomplish that low ventilation rate. Consequently, wean-to-finish facilities are often over-ventilated. In a nursery, smaller fans are used, and therefore, over-ventilation is less likely,” Harmon explains.
In this scenario, the nursery would use about 200 gal. of propane for one week in January, while the wean-to-finish unit would use about 440 gal.
In April, with the same settings, the nursery would use about 70 gal. of propane for one week and the finisher would use about 200 gal., he says.
Although the actual numbers might vary, Harmon is confident that the wean-to-finish scenario will use about twice as much propane as the nursery.
“Heat loss through the building shell is the biggest difference,” he adds. “The nursery heaters will cycle as long as the outdoor temperature is lower than 63°F, and the wean-to-finish heaters will cycle as long as the outdoor temperature is lower than 73°F.”
35-lb. pig placement: Both the nursery and the wean-to-finish unit were set at 72°F and 2.5 cfm/pig ventilation rate.
In January, the nursery required minimal supplemental heat because the heaters only cycle when the outdoor temperature is below 16°F. The finisher would use about 190 gal. of propane per week because the heaters run whenever the outdoor temperature is below 38°F. Minimal supplemental heat would be required in either facility in April, he says.
The Bottom Line
If both buildings are managed similarly, the wean-to-finish facility would tend to use twice as much propane as the nursery for the first week after 15-lb. pigs are placed.
Finishing buildings are more likely to be over-ventilated because many use two, 24-in. fans on variable control, so actual savings could be greater. A better option is to use one, 24-in. fan so it will run fast enough to function properly. Nurseries generally use smaller fans and are more easily turned down to the proper rate. Over-ventilation is a huge source of energy loss; however, the cost of under-ventilating can be just as devastating due to the impact it may have on health and performance of the pigs.
Many wean-to-finish buildings use zone heating to conserve energy. If the temperature for 15-lb. pigs can be turned down 10°F — from 85 to 75°F — propane use would be reduced by about 25% (100 gal. for the first week, in this case). However, some of this savings would be offset by the cost of the brooder operation, Harmon reminds.