Change in lameness risk estimates in piglets due to the modelling of herd-level variation

In a previous study (Christensen, 1996. Prev. Vet. Med. 26, 107-118), an effect parameter changed from positive to negative depending on the model used. The study considered lameness in suckling piglets and the dataset included 7632 litters from 35 herds from the Health and Production Surveillance (HEPS) system database. To further investigate the change in the effect parameter, we performed a simple test to demonstrate the presence of herd-level variation. Finding substantial herd-level variation, we have re-analysed the data with special attention to the herd-level variation, and how to account for it. When herd-level variation was not accounted for in the model, a detrimental effect of prior treatment of the sow was seen but this turned into a beneficial effect when we accounted for the herd-level variation. The treatment of sow refers to any therapeutic treatment given to the sow after farrowing but before lameness or weaning occurred. This is, therefore, an example where not only the variances but also the effect parameters changed when we accounted for herd-level variation.