Reproduction performances and conditions of group-housed non ...

- Paper III -
tions, αi is the fixed effect of parity group, βj is the fixed effect of layout, (αβ)ij is the interac-
tion between parity group and layout, xijklm is the back fat at weaning or three weeks after
mating, a is the regression parameter, Ak(j) and Bl(kj) are the normal distributed random ef-
fects of herd (within layout) and batch (within herd) respectively. For all continuous indica-
tors, Yijklm ~ N(E(Yijklm), σijklm 2 ) whereas for all categorical indicators, Yijklm ~ B(1, pijklm).
The covariate xijklm was only included in the model of the indicator back fat gain. Further,
when analysing the effect of herd and batch, βj and (αβ)ij were excluded. Effects with Pvalues
above 0.10 were eliminated from the model one by one and the analysis was repeated.
For all continuous indicators, the statistical analyses were performed with a linear mixed
model using the MIXED procedure (Littell et al., 1996) in SAS ® (SAS Institute Inc. 1990).
For all categorical indicators, the statistical analyses were performed with a generalized
linear mixed model using the glmmPQL function in the MASS package (Venables & Ripley
2002) of R (R Development Core Team 2004). Effect of herd and batch within herd
(random effects) were analysed by a Wald Z-test that provides an approximate test that the
variance components are zero. For all continuous indicators, this was done by including the
option COVTEST in the MIXED procedure (Littell et al., 1996), and a similar test was calculated
in R. The test for variance heterogeneity (test for different between-sow variation in
herds with group feeding and herds with individual feeding) for all back fat measurements
was performed by comparing a model with homogeneous covariance structure to a model
with heterogeneous covariance structure by means of the likelihood ratio test. For all continuous
indicators, the option GROUP in the MIXED procedure was used to specify heterogeneity
in the covariance structure (Littell et al., 1996).
The correlation between continuous indicators was calculated using the CORR procedure
(SAS Institute Inc. 1990). The calculations were based on data transformed to obtain normality
and corrected for herd and batch mean.
3. Results
For all continuous indicators, the overall averages and different measurements of the variation
between sows across all 14 herds are presented in Table 3. For all lesions- and aggressions
measurements, the 50% quantile was lower than the mean value, which indicates that
these variables were not normally distributed but skewed to the right.
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