view - Department of Reproduction, Obstetrics and Herd Health

GENERAL DISCUSSION
(Hogeveen, 2012). Although, the effect of MD during the transition period on the level of
milk production has already often been described (Fourichon et al., 1999), studies
yielded contrasting results and estimations of short- and long-term effects are currently
missing for most MD. Moreover, statistical pitfalls hamper adequate interpretation of
the effect of MD on milk production:
In early lactation, dairy cows show a high variability in daily milk production as
cows proceed towards peak milk yield (Kessel et al., 2008; Quist et al., 2008;
Lovendahl et al., 2010). Hence, short term evaluation by comparing daily milk
production before and after clinical diagnosis (Detilleux et al., 1994; Detilleux et
al., 1997; Bareille et al., 2003) is hugely affected by the lactation stage. This
makes comparability among the different studies nearly impossible.
Monthly test day milk productions lag time before they are measured. Especially
lower producing diseased animals may have been culled before their first test
day (Bartlett et al., 1997) provoking bias and an underestimation of the effect of
MD on milk production (Fourichon et al., 1999).
Similarly, cows culled on 59, 99 or 304 DIM, are excluded from any analysis
when using milk production data within the first 60 (M60; Sheldon et al., 2004);
100 (van Werven et al., 1992) or 305 DIM (M305; Dubuc et al., 2011)
respectively.
Using first test day milk or M60 has the benefit of including more animals but
may not provide accurate predictions of the entire lactation (Leblanc, 2010).
Finally, differences in definition and severity of diseases (van Werven et al.,
1992), and interrelationships between the different transition diseases
(Ingvartsen et al., 2003) require controlling for bias (Rajala and Grohn, 1998;
Fourichon et al., 1999).
Lactation curve models have been used to model residuals between predicted
and observed milk yield in healthy and diseased animals (Lucey et al., 1986; Rowlands
and Lucey, 1986). Therefore, in Chapter 4.1, we demonstrated the use of the new
Milkbot model (Ehrlich, 2011) on a large dataset of 1,555 lactations from one herd to
estimate the effect of MD on the shape of the lactation curve. The proposed model can
be fitted to milk production data to summarize an individual lactation as a set of 4 fitted
parameter values, each corresponding to a specific aspect of the lactation curve with
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