One is the Coldest Number: Determining How Group Size and Body Weight Affects Thermal Preference in Weaned Pigs (3 to 15 kg)

Authors: ROBBINS, LINDSEY - Purdue University; GREEN-MILLER, ANGELA - University Of Illinois; Johnson, Jay; GASKILL, BRIANNA - Purdue University

Submitted to: Animals
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/12/2021
Publication Date: 5/18/2021
Citation: Robbins, L., Green-Miller, A.R., Johnson, J.S., Gaskill, B.N. 2021. One is the Coldest Number: Determining How Group Size and Body Weight Affects Thermal Preference in Weaned Pigs (3 to 15 kg). Animals. 11(5). https://doi.org/10.3390/ani11051447.
DOI: https://doi.org/10.3390/ani11051447

Interpretive Summary: Knowledge of the thermal requirements of pigs is based upon data from >35 years ago and is not differentiated based upon group sizes. Because genetic selection has increased growth rates and metabolic heat production in modern pigs, it is likely that their thermal requirements have changed. Understanding the thermal requirements of pigs with current genetics is key to providing them with a comfortable environment and understanding what constitutes thermal stress. Therefore, the study objective was to determine the thermal preference of young pigs in various group sizes and weight categories. We hypothesized that group size would alter thermal preference and that this would vary by body weight. Specifically, surface area to mass ratio would be reduced in bigger groups and with heavier pigs, thus reducing overall heat loss and resulting in cooler temperature preferences. Overall, this study demonstrated that being housed in a group (either 2 or 4 pigs) and heavier body weight resulted in a cooler thermal preference in young pigs (8 to 15 kg). However, no interaction between group size and weight category appeared to affect thermal preference during times of inactivity. These data provide guidance on the thermal needs of 8 to 15 kg pigs when housed in various group sizes. Fortunately, body weight does not seem to play a role in thermal choice when pigs are inactive (which accounts for approximately 70% of their day) and housed in groups, thus attempting to create multiple microclimates for smaller pigs is not necessary if pigs are housed in groups.

Technical Abstract: Housing pigs within their thermal comfort zone maximizes productivity and performance. However, fundamental information on behavioral thermoregulatory responses of individual and group housed pigs is meager. As a gregarious species, pigs prefer to be near one another huddling. As pigs huddle together, they decrease their heat loss to the environment by decreasing surface area and increasing mass. Additionally, pigs gain weight rapidly as they age and as an individual grows, their ability to withstand lower temperatures increases. We hypothesized that group size would alter pig thermal preference and that thermal preference would change based upon body weight. Thirty-six groups of pigs (n = 2 pigs/group) were tested in a factorial design based on group size (1, 2, or 4) and weight category (small: 5.20 ± 1.15 kg; medium: 8.79 ± 1.30 kg; and large: 13.95 ± 1.26 kg) in both sexes. Treatment groups were placed inside a thermal gradient (4.6 m x 0.9 m x 0.9 m; L x W x H) that ranged in temperature from 18 to 30°C. Pigs habituated to the gradient for 24h. The following 24h testing period was continuously video recorded and each pigs’ location during inactivity (~70% daily budget) within the thermal apparatus was recorded every 10-min via instantaneous scan sampling. Data were analyzed using a GLM and Log10+0.001 transformed for normality. Tukey tests and Bonferroni corrected custom tests were used for post-hoc comparisons. Peak temperature preference was determined by the maximum amount of time spent at a specific temperature. Both group size (P = 0.001) and weight category (P < 0.001) influenced the thermal location pigs were observed in. Individual pigs preferred 30.31°C which differed from a group of 2 (20.0°C: P = 0.003) and 4 pigs (20.0°C: P < 0.001). The peak temperature preference of the small pigs (30.2°C) differed from the large pigs (20.0°C: P < 0.001) but did not differ from the medium sized pigs (28.4°C: P > 0.05). Overall, heavier pigs and pigs in larger groups preferred cooler temperatures.