Pigs (Sus scrofa), domesticated perhaps as early as 10,000 bc, were common to early agrarian societies throughout the Middle East and Europe.1,2 Because they are largely self-sufficient, pigs were allowed to range freely on pastures and in woodlands; a fact particularly well-documented in art and literature from the Middle Ages.3 Pigs were not native to North America and were introduced by Christopher Columbus and other early explorers and settlers.4 They quickly adapted to the New World and by 1847 the US pig population reached approximately 35 million (as opposed to the human population estimated to be 20 million).5 Beginning in the 19th century, producers began to provide small shelters (2.4 m × 2.4 m or smaller) called “cots” or “colony houses” in pastures to protect animals from adverse weather conditions.6 Constructed by the farmers themselves, or available in local lumberyards, cots were cheap, lightweight, and easily moved as animals were rotated between pastures.6
Experiments in the early 1900’s led Danish producers to conclude that indoor housing (“intensive” production) provided more efficient use of land, protected animals from weather, eliminated fighting, and improved feed conversion.7 In 1919, Spencer8 commented on a 2-story barn he observed near Aarhus, Denmark, in which the pigs were fed downstairs and slept in an upstairs area they reached by walking up a ramp.
In the northern regions of the United States, the adoption of indoor housing was driven by the fact that newborn piglets could only survive if farrowed in the summer months. Because farrowing was seasonal, this periodically resulted in an excess supply of market-weight “summer pigs” and, consequently, low prices. To break this seasonal cycle, producers began to implement indoor production as a way to improve newborn piglet survivability in the colder months and create the opportunity to market pigs throughout the year.
For most of the 20th century, pig barns in the northern regions of the United States were similar in design, ranging in size from 2.4 m × 4.2 m to 7.3 m × 14.6 m, with pen sizes typically 1.8 m × 2.4 m or 2.4 m × 2.4 m, and breeding barns up to 8.5 m × 24.4 m.6,9 Although extensive (outdoor) production remained in wide use, O. Burr Ross’ writing in 1960 was prescient:
While the practicality of confinement systems of swine production has been demonstrated over and over again by research institutions, universal acceptance by swine producers has been slow. I believe most of the hogs of tomorrow will be raised under some sort of confinement program.6
These words were prophetic and confinement production became the norm in the United States and elsewhere. Concurrent with the transition from outdoor to indoor production, the swine industry changed from predominantly small, individual farrow-to-finish herds to large, specialized production operations.
The progression toward larger farms necessitated changes in housing and management systems and led to the emergence of 2- and 3-site production systems.10 The first multi-site farm in the United States was constructed in 1988, but by 1995, 60% of production systems with more than 10,000 pigs in inventory were designed as multi-site systems.10 This evolution process continues with the new generation of confinement facilities. We will learn much about the current US industry, ie, type, size, number, and location of US swine farms, as the data collected through the 2022 US Department of Agriculture (USDA) Census of Agriculture are released. The objective of this research was to supplement the census with a snapshot of current barn designs, in terms of the number of pigs per pen and per barn, in the US swine industry.
Materials and methods
Survey design
The objective of the survey was to collect information on the mean number of pigs per pen and total barn capacities on production sites in the United States. The Iowa State University Office of Research Ethics determined that Institutional Review Board approval was not required because the survey collected no information about people. Participation in the survey was voluntary and efforts were made to include individual pig producers, integrated production companies, and swine veterinarians, ie, no restrictions were placed on participants in terms of scale of production. Two questionnaires, one for swine producers and one for swine veterinarians, were published online using Microsoft 365 Forms and made available from February 2, 2022 to April 14, 2022. Using email listservs and conference announcements, the questionnaire for producers was distributed under the auspices of the National Pork Board and the questionnaire for veterinarians was distributed through the American Association of Swine Veterinarians. Participation was also solicited in a widely distributed electronic article published by the National Hog Farmer on March 1, 2022, with a link provided to the online survey. Finally, the Iowa State University Veterinary Diagnostic Laboratory solicited participation from all clients who had ever submitted one or more swine-related cases.
Data analysis
Survey responses from veterinarians and producers were analyzed separately. For exact numeric responses, eg, the number of “Farm sites represented”, percentiles were calculated using Microsoft Excel. Means and percentiles for the mean, maximum, and minimum respondent-summarized values were also calculated using Microsoft Excel for survey questions such as the “Average number of sows per pen”, “Maximum number of grower-finisher pigs per barn”, or “Minimum number of pens in a grower-finisher barn”. The mean number of growing pigs per pen was calculated by dividing the “average number of grower-finisher pigs per barn” by the “average number of pens in a grower-finisher barn” for each individual response. The means (95% CI) and percentiles for these estimates were calculated using R Studio11 (version 42023.12.1+402).
Results
A total of 147 swine producers provided responses to the survey (Table 1). Among 134 respondents providing location information, 77 were from the US North Central region (Illinois, Indiana, Iowa, Minnesota, Nebraska, North Dakota, South Dakota, and Wisconsin), but all areas of the United States were represented. The survey represented a wide range in the number of production sites managed by a respondent, eg, the median value was 2 production sites, but respondents at the 90th percentile reported 60 farm sites. The majority of producers (n = 99; 67.3%) had breeding herd(s) on their farm(s). Among this group, 65 (65.7%) reported using pen gestation. The mean number of sows per pen was 30, but respondents at the 10th percentile reported 4 sows/pen versus 65 sows/pen at the 90th percentile. Likewise, most producers (n = 130; 88.4%) raised grower-finisher pigs, with means of 26 pens/barn and 1333 pigs/barn, respectively. The mean number of pigs per pen was calculated as 75 (95%CI, 57-93). The individual calculated values ranged from 11 (10th percentile) to 146 (90th percentile).
| Survey questions | Responses | Percentiles | Mean | ||||
|---|---|---|---|---|---|---|---|
| 10th | 25th | 50th | 75th | 90th | |||
| 1. Are you a pig producer? | 147 | NA | NA | NA | NA | NA | NA |
| 2. How many farms are represented in your responses? | 147 | 1 | 1 | 2 | 10 | 60 | 96 |
| 3. Do you have breeding herd(s)? If yes, answer 4. | Yes (99) | NA | NA | NA | NA | NA | NA |
| 4. Do you house sows in pen gestation? If yes, answer 5-7. | Yes (65) | NA | NA | NA | NA | NA | NA |
| 5. Average No. of sows/pen | 64 | 4 | 8 | 12 | 43 | 65 | 30 |
| 6. Maximum No. of sows housed in each pen | 64 | 6 | 10 | 20 | 50 | 170 | 51 |
| 7. Minimum No. of sows housed in each pen | 64 | 1 | 2 | 6 | 10 | 48 | 17 |
| 8. Do you raise grower-finisher pigs? If yes, answer 9-14. | Yes (130) | NA | NA | NA | NA | NA | NA |
| 9. Average No. of grower- finisher pigs/barn | 129 | 30 | 500 | 1200 | 2000 | 2500 | 1333 |
| 10. Maximum No. of grower- finisher pigs/barn | 128 | 39 | 225 | 1200 | 2400 | 4000 | 1605 |
| 11. Minimum No. of grower- finisher pigs/barn | 123 | 14 | 30 | 550 | 1000 | 2120 | 748 |
| 12. Average No. of pens in a grower-finisher barn | 125 | 4 | 10 | 20 | 40 | 48 | 26 |
| 13. Maximum No. of pens in grower-finisher barns | 125 | 4 | 13 | 31 | 49 | 79 | 37 |
| 14. Minimum No. of pens in grower-finisher barns | 126 | 2 | 5 | 10 | 20 | 40 | 16 |
| Grower-finisher pigs/pen* | 124 | 11 | 25 | 43 | 94 | 146 | 75 |
| ≥ 2 areas | NC† | NE‡ | SC§ | SE¶ | W** | ||
| 15. In what area of the country do you have production? No. responses. | 7 | 77 | 19 | 10 | 6 | 15 | |
* Grower-finisher pigs per pen was calculated by dividing the individual responses of question 9 by question 12.
† North Central (NC) included IL, IN, IA, MN, NE, ND, SD, and WI.
‡ Northeast (NE) included CT, DE, MD, MA, MI, NH, NJ, NY, OH, PA, RI, VT, and WV.
§ South Central (SC) included AR, KS, LA, MO, OK, and TX.
¶ Southeast (SE) included AL, FL, GA, KY, MS, NC, SC, TN, and VA.
** West (W) included AZ, CA, CO, ID, MT, NV, NM, OR, UT, WA, and WY.
NA = not applicable.
A total of 73 swine veterinarians provided responses to the survey (Table 2). The median number of production sites personally overseen by veterinary respondents was 55; the median number of production sites serviced by the respondents’ clinics was 150. The majority of veterinarians (n = 54; 74.0%) worked with breeding herds that utilized pen gestation. Among these herds, the mean number of sows per pen was 37, with 10 sows/pen at the 10th percentile and 88 sows at the 90th percentile. The mean number of pigs per pen in nurseries, wean-to-finish, and grow-to-finish pig sites was 107, 94, and 82 pigs/pen, respectively. All survey participants (n = 73) used oral fluid sampling for disease surveillance.
| Survey questions | Responses | Percentiles | |||||
|---|---|---|---|---|---|---|---|
| 10th | 25th | 50th | 75th | 90th | Mean | ||
| 1. Are you a veterinarian currently in practice? | Yes (73) | NA | NA | NA | NA | NA | NA |
| 2. No. of production sites serviced by your vet clinic? | 73 | 10 | 43 | 150 | 320 | 960 | 326 |
| 3. How many of these sites do you personally oversee? | 54 | 5 | 20 | 55 | 150 | 275 | 109 |
| 4. Do your sow herds use pen gestation? If yes, answer 5-8. | Yes (54) | NA | NA | NA | NA | NA | NA |
| 5. Percent of sow herds that use pen gestation | 53 | 10 | 23 | 40 | 75 | 100 | 49 |
| 6. Average No. of sows/pen | 54 | 10 | 15 | 25 | 40 | 88 | 37 |
| 7. Maximum No. of sows/pen | 54 | 20 | 32 | 60 | 100 | 240 | 86 |
| 8. Minimum No. of sows/pen | 54 | 4 | 5 | 10 | 15 | 39 | 16 |
| 9. Percent of sow herds that wean into nursery? | 72 | 8 | 10 | 50 | 89 | 100 | 48 |
| 10. Average No. of weaned pigs/pen in the nursery | 72 | 20 | 25 | 30 | 50 | 100 | 107 |
| 11. Percent of sow herds that use W-F | 64 | 10 | 40 | 70 | 90 | 95 | 62 |
| 12. Average No. of weaned pigs/pen in the W-F | 64 | 27 | 50 | 75 | 120 | 215 | 94 |
| 13. Average No. of pigs/barn in grow-to-finish | 72 | 360 | 1000 | 1200 | 2150 | 2400 | 1418 |
| 14. Average No. of pens/barn in grow-to-finish | 72 | 11 | 20 | 26 | 40 | 48 | 29 |
| Grower-finisher pigs/pen* | 72 | 21 | 26 | 50 | 85 | 125 | 82 |
| 15. Do you use oral fluid sampling for disease detection? | Yes (73) | NA | NA | NA | NA | NA | NA |
* Grower-finisher pigs/pen was calculated by dividing the individual responses of question 13 by question 14.
W-F = wean-to-finish; NA = not applicable.
Discussion
Swine producers achieved major gains in efficiency and sustainability over the course of the 20th century while, simultaneously, the industry underwent a major demographic shift. For example, in 1987, 8% of US pigs were on production sites housing ≥ 5000 head vs > 72% in 2017 (Table 3).12-15 The most remarkable period of change occurred between 1992 and 2009 when, as a result of the growth of multi-site production and improvements in productivity, there was a > 850% increase in the number of hogs sold or removed from production sites.16
| Farm inventory | No, (%) of farms and pigs by year 12-15 | |||||||
|---|---|---|---|---|---|---|---|---|
| 2017 | 2007 | 1997 | 1987 | |||||
| Farms | Pigs | Farms | Pigs | Farms | Pigs | Farms | Pigs | |
| 1-24 | 46,475 (70.0) | 278,691 (0.4) |
45,047 (59.7) |
260,154 (0.4) |
56,092 (44.9) |
381,729 (0.6) |
86,621 (35.6) |
743,251 (1.4) |
| 25-49 | 3759 (5.7) |
122,915 (0.2) |
4292 (5.7) |
146,672 (0.2) |
9411 (7.5) |
325,329 (0.5) |
26,895 (11.0) |
939,637 (1.8) |
| 50-99 | 1889 (2.8) |
122,090 (0.2) |
3182 (4.2) |
215,206 (0.3) |
9334 (7.5) |
639,493 (1.1) |
29,881 (12.3) |
2,058,524 (3.9) |
| 100-199 | 1220 (1.8) |
160,882 (0.2) |
2590 (3.4) |
354,203 (0.5) |
10,364 (8.3) |
1,417,039 (2.3) |
32,293 (13.3) |
4,426,492 (8.5) |
| 200-499 | 1451 (2.2) |
454,960 (0.6) |
4524 (6.0) |
1,467,383 (2.2) |
16,539 (13.2) |
5,194,768 (8.5) |
40,156 (16.5) |
12,334,432 (23.6) |
| 500-999 | 1305 (2.0) |
905,123 (1.3) |
3588 (4.8) |
2,488,234 (3.7) |
10,378 (8.3) |
7,104,689 (11.6) |
17,878 (7.3) |
11,924,290 (22.8) |
| 1000-1999 | 2016 (3.0) |
2,741,382 (3.8) |
4013 (5.3) |
5,527,798 (8.2) |
6597 (5.3) |
8,794,666 (14.4) |
6865 (2.8) |
8,870,231 (17.0) |
| 2000-4999 | 4724 (7.1) |
14,893,679 (20.6) |
5356 (7.1) |
16,532,918 (24.4) |
4323 (3.5) |
12,752,495 (20.8) |
2403 (1.0) |
6,733,228 (12.9) |
| ≥ 5000 | 3600 (5.4) |
52,701,285 (72.8) |
2850 (3.8) |
40,793,750 (60.2) |
1851 (1.5) |
24,577,941 (40.2) |
406 (0.2) |
4,241,035 (8.1) |
| TOTAL | 66,439 | 72,381,007 | 75,442 | 67,786,318 | 124,889 | 61,188,149 | 243,398 | 52,271,120 |
The shift toward larger swine operations justified investment in technology and technological innovations. These advancements, in turn, reduced the cost of production and labor. For example, between 1992 and 2015, production costs were estimated to have decreased by 59% to achieve 100 pounds of weight gain.16 In the same period, labor declined by 83% to produce 100 pounds of weight gain.16 Over a slightly longer time frame (1960 to 2015), the US swine industry achieved major improvements in efficiency and sustainability: feed conversion went from 4.5 to 2.8 pounds of feed per pound of gain, weaned pigs per litter increased from 7 to 10, land use (99% of which is used for feed production) was reduced by 75.9%, water use decreased by 25.1%, global warming potential decreased by 7.7%, and energy use decreased by 7%.17
The remarkable improvements that were achieved in productivity and sustainability were largely made possible by the technification and efficiencies made possible by economies of scale. Large, specialized hog operations increased production efficiency and sustainability, but the shift in infrastructure also impacted disease. Some infections essentially disappeared as a direct consequence of housing pigs in confinement, eg, Toxoplasma gondii and Ascaris suum infections.18 However, other infectious agents have thrived in confined swine populations, eg, the agents of the porcine respiratory disease complex.19 Recognizing that the evolution from outdoor to indoor production had caused a fundamental shift in disease ecology, Schwabe20 promoted systematic on-farm data collection and analysis as the best approach for understanding the processes impacting livestock health and production. This data-driven approach for understanding causality and evaluating the effects of management decisions on swine health is the basis of today’s population medicine.
An important part of population medicine is to understand how production practices affect productivity and health. For example, influenza A virus infections in the small herds of times past were seasonal, with herd immunity rapidly acquired and the infection eliminated. In today’s large herds, influenza A virus circulates throughout the year.21 Similarly, Rotolo et al22 showed that porcine reproductive and respiratory syndrome virus moves non-uniformly within and between wean-to-finish barns on the same site. Thus, infrastructure impacts disease and disease spread, but there is essentially no information on housing designs currently used by US producers. This study showed that the swine industry, in fact, is still diverse in terms of total inventory and housing design. Interestingly, producers may house from a few to several hundred pigs per pen. Future research should address the impacts of pen and barn inventory on disease ecology and disease surveillance.
Implications
Under the conditions of this study:
- The industry is moving to group housing for gestating sows and larger pens for growing pigs.
- Disease surveillance must fit contemporary production systems.
- All veterinary respondents reported using oral fluids for surveillance.
Acknowledgments
We thank all veterinarian and producer respondents for their time to take the survey and provide valuable information. Thank you to the American Association of Swine Veterinarians and the National Pork Board for distributing the survey. This work was partially funded by a cooperative agreement with the USDA, Center for Epidemiology and Animal Health. The findings and conclusions in this document are those of the authors and should not be construed to represent any official USDA or US Government determination or policy.
Conflict of interest
None reported.
Disclaimer
Scientific manuscripts published in the Journal of Swine Health and Production are peer reviewed. However, information on medications, feed, and management techniques may be specific to the research or commercial situation presented in the manuscript. It is the responsibility of the reader to use information responsibly and in accordance with the rules and regulations governing research or the practice of veterinary medicine in their country or region.
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