The size of grow-finish groups of pigs had limited effect on stress responses at loading and unloading and transport losses in market weight pigs in a study at Iowa State University.

At loading, pigs raised in small groups of 36 pigs had reduced open-mouth breathing and skin discoloration compared to pigs kept in large groups of 324 pigs/pen.

But following a one-hour journey to the packing plant, pigs raised in large groups throughout grow-finish had reduced skin discoloration when compared to pigs raised in small groups.

Although pigs raised in large group sizes showed higher stress responses than pigs raised in small groups at loading and lower stress responses at unloading, there were no differences in transport losses between group sizes at marketing.

For the study, pigs were transported to three commercial wean-to-finish facilities at 18 days of age and marketed at about 200 days of age. The pigs were from a standard commercial, terminal genetic line and selected to be free of the HAL-1843 stress gene.

Small groups (SG) were housed in single-sex pens of 36 pigs. Large groups (LG) were housed in single-sex pens of 324 pigs.

Pigs were moved from their home pen to the loading ramp by the same four-person loading crew, and all handling methods met the production system’s standard operating procedures. Groups of four to six market hogs were removed from their pen, moved down the building’s center aisle and loaded onto the transport trailer using sort boards and electric prods, when necessary.

Aluminum straight-deck trailers (Wilson Trailers, Sioux City, IA) were used for both trials. The National Pork Board’s Transport Quality Assurance program guidelines were followed for trailer setup and ventilation. Fresh wood shavings were used as bedding.

Hogs were analyzed at loading and unloading for open-mouth breathing, skin discoloration and muscle tremors and at the farm for non-ambulatory condition.

SG pigs had less open-mouth breathing and skin discoloration at loading than LG pigs (Table 1). At unloading, skin discoloration was higher for SG pigs, but no differences were observed between group sizes for open-mouth breathing or muscle tremors.

LG pigs had a higher incidence of open-mouth breathing and skin discoloration at loading, while at unloading a higher incidence of skin discoloration was observed in SG pigs.

No differences between SG and LG pigs were noted for fatigued, non-ambulatory and total losses at the harvest facility.

Two pigs were injured from the LG pigs and none from the SG pigs, and no dead on arrivals were found in either group size.

Total transport losses for both treatment groups in this study were very low compared to estimated national averages (0.69%; Ritter et al. 2009).

Further research is needed to determine whether larger, grow-finish group size is a useful strategy on grow-finish sites experiencing high numbers of transport losses or numbers higher than the estimated national average.

Researchers: L.M. Gesing, A.K. Johnson and K.L. Stalder, Iowa State University; H. Hill, S. Abrams, A. Whiley and M. Faga, Iowa Select Farms; R. Bailey, JBS Swift and Co. and M.J Ritter, Elanco Animal Health. For more information, contact Gesing by phone (563) 568-9651 or e-mail