ABSTRACT
Data used in the present study included 1,095,980
first-lactation test-day records for protein yield of
154,880 Holstein cows housed on 196 large-scale dairy
farms in Germany. Data were recorded between 2002
and 2009 and merged with meteorological data from
public weather stations. The maximum distance between
each farm and its corresponding weather station
was 50 km. Hourly temperature-humidity indexes (THI)
were calculated using the mean of hourly measurements
of dry bulb temperature and relative humidity. On the
phenotypic scale, an increase in THI was generally
associated with a decrease in daily protein yield. For
genetic analyses, a random regression model was applied
using time-dependent (d in milk, DIM) and THIdependent
covariates. Additive genetic and permanent
environmental effects were fitted with this random
regression model and Legendre polynomials of order
3 for DIM and THI. In addition, the fixed curve was
modeled with Legendre polynomials of order 3. Heterogeneous
residuals were fitted by dividing DIM into 5
classes, and by dividing THI into 4 classes, resulting in
20 different classes. Additive genetic variances for daily
protein yield decreased with increasing degrees of heat
stress and were lowest at the beginning of lactation
and at extreme THI. Due to higher additive genetic
variances, slightly higher permanent environment variances,
and similar residual variances, heritabilities were
highest for low THI in combination with DIM at the
end of lactation. Genetic correlations among individual
values for THI were generally >0.90. These trends from
the complex random regression model were verified by
applying relatively simple bivariate animal models for
protein yield measured in 2 THI environments; that is,
defining a THI value of 60 as a threshold. These high
correlations indicate the absence of any substantial
genotype × environment interaction for protein yield.
However, heritabilities and additive genetic variances
from the random regression model tended to be slightly
higher in the THI range corresponding to cows’ comfort
zone. Selecting such superior environments for progeny
testing can contribute to an accurate genetic differentiation
among selection candidates.
Key words: heat stress , protein yield , random regression
model