view - Department of Reproduction, Obstetrics and Herd Health

LACTATION CURVE ANALYSIS IN METABOLIC DISEASES
treated as independent variables in statistical models, along with the derived variables
time to peak milk (TPEAK), peak milk (MPEAK), cumulative 60 d milk yield (M60) and
cumulative 305 d milk yield (M305) which are easily calculated directly from MilkBot
parameter values (Ehrlich, 2011).
STATISTICAL ANALYSIS
Descriptive statistics were done using the MEANS and FREQ procedure in SAS
version 9.2 for Windows (SAS Institute, Inc., Cary, NC, USA, 2010). To evaluate the days
in milk to culling or death, a Kaplan-Meier survival graph was constructed using the
LIFETEST procedure in SAS version 9.2 for Windows (SAS Institute, Inc., Cary, NC, USA,
2010). To test equality over MD strata, the Peto and Wilcoxon test was used to evaluate
difference in the beginning of the survival curves, whereas the log-rank test was used to
evaluate difference in the tail of the curves (Hosmer and Lemeshow, 2008). Proportion
of surviving cows are reported with their 95 % confidence interval for the Kaplan-Meier
estimates. To evaluate the effect of MD on the lactation curve, all MilkBot parameters
and calculated production metrics were checked to approximate the Gaussian
distribution. Subsequently, each variable except TPEAK was analysed using PROC
MIXED from SAS version 9.2 for Windows (Institute, Inc., Cary, NC, USA, 2010). The
TPEAK analysis was performed using PROC GLIMMIX with the lognormal distribution
function from SAS version 9.2 for Windows (Institute, Inc., Cary, NC, USA, 2010). The
statistical models contained fixed effects for MD, parity, and the interaction between MD
x parity. Metabolic disease least square means and contrasts were computed using the
LSMEANS and LSMESTIMATE option. Data are reported as model least square means
with standard errors (LSM ± SE) except for the TPEAK. Results from the TPEAK model
were back-transformed and reported with the 95% confidence intervals using the ILINK
function. Significance and tendency were declared at P < 0.05 and 0.05 < P < 0.1
respectively.
RESULTS
In the first analysis, the overall proportion of cows that survived until 120 DIM
was 82% (80-84%) which differed between the MD strata (P < 0.0001). The proportion
of cows that survived until 120 DIM was estimated to 87% (85-89%) for H as opposed to
75% (71-79%) and 67% (60-73%) for MD and MD+ respectively (Figure 1).
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