| Abstract |
Numerous prediction equations have been developed
based on mid-infrared (MIR) spectra, and some could be
potentially used as biomarkers of heat stress. However,
practical experience shows that confusion can easily occur
between the effect of heat stress and other effects,
such as lactation stage or feeding variation over the year.
On this basis, the objective of this study was to identify
potential milk components predicted by MIR as biomarkers
of heat stress based on a 2-step approach allowing
correction for those effects. The first step consisted in
the estimation of residuals from test-day random regression
models on DIM to remove systematic lactation stage
effects. These models also contained, among others,
general (i.e., month of production) or specific (i.e., herd
× test-day) fixed effects related to feeding and management.
During the second step, means and variances of
residuals by temperature-humidity index (THI) classes
were studied. The models were applied to 611,063 records
from 97,042 primiparous Holstein cows from 2015
to 2022 in the south of Belgium. The MIR-predicted
milk components with the highest deviations from the
mean with increasing THI were protein percentage, casein
concentration, magnesium concentration, and (to a
lesser extent) PUFA concentration. Concerning residual
variances, the highest heteroscedasticity with THI was
obtained for milk MIR MUFA, C18:1 cis-9, and citrate
concentrations. Conversely, a relative homoscedasticity
of variance with increasing THI was observed for several
milk MIR components including protein percentage and
casein concentration. Based on the criteria of the good
biomarkers guidelines, milk protein percentage seems to
be the most promising trait of this study, followed by
Mg concentration. However, in the context of genetic
evaluation, which requires variability, milk MIR MUFA,
C18:1 cis-9, or citrate concentration variations, if they
are heritable, could be of great interest. Finally, an increase
in milk MIR citrate concentration variance could
be an early warning for the detection of heat stress in the
frame of DHI. |
| Authors |
Lemal, P., Grelet, C., Dehareng, F., Soyeurt, H., Schroyen, M., Gengler, N. |
