Five organic piglet-producing farms fulfilled the criteria for being included in the study and five conventional commercial farms were chosen as comparison. The criteria to be fulfilled by the farms, in order for them to be included in the study were: the farms should have more than 30 sows in production, they should use the computerized performing monitoring system PigWin Sugg (Svenska Pig) and, obviously, they should have owners who were willing to share their herd data. The organic farms adhered to the Swedish requirements for organic certified production .
The certified organic farms were the five among the 21 piglet producing organic farms serviced by the Swedish Animal Health Service that fulfilled the criteria above. Five conventional piglet-producing farms were chosen as a comparison to the five organic farms. To each of the organic farms, the owner of the conventional farm geographically located the closest and using PigWin Sugg was asked to participate. In the conventional farms, the sows and their piglets were housed in individual pens throughout the nursing period. In both farm types cross-fostering was applied in order to even out the sizes of the litters.
The data extracted from the herds’ computerized performance monitoring system (PigWin Sugg, Svenska Pig), consisted of information on farrowings in the period April 2007 to March 2008. Also, data of the sows’ next farrowing, after this time period, was captured from the data in order to calculate farrowing interval. Data was transferred to the SAS software (ver. SAS 9.2; SAS Institute Inc., Cary, NC, USA) for editing and analysis. The number of farrowings per organic farm ranged from 72 to 392 and the corresponding number per conventional farm ranged from 217 to 1288. The total number of farrowings, both categories, during the 12 months was 4697, 830 in organic herds and 3867 in conventional herds, giving a total number of 60786 piglets born. In both types of herds, the sow was crossbred between Landrace and Yorkshire, and the piglets were sired by Hampshire AI boars. Notably, the genetic background for pigs in organic production in Sweden is the same as in conventional pig production .
Data was analysed using analysis of variance (PROC MIXED) in the SAS software (ver. SAS 9.2; SAS Institute Inc., Cary, NC, USA). The analysed traits were for each litter: total born, live born and stillborn piglets, and number piglets weaned, weaning age and following farrowing interval. Farrowing intervals longer than 300 days were in the analyses regarded as missing observations. The statistical model included the fixed effects of herd type (organic or conventional), herd nested within herd type, two-month periods of farrowing and parity number (1, 2, 3, 4+). The statistical model also included the random effect of sow, nested within herd and herd type. Least squares means were calculated, and pair-wise comparisons were made using Student’s t-test.