where y is the observation vector for TD of MY, FAT or PRT
content as weighted mean of morning and evening milking,
where weighting is according to the corresponding
milk yield; b is the vector of fixed effects that includes
herd year season (HYS 60 classes: three herds, 10 years and
two seasons of birth from January to June and from July to
December); herd test day (HTD 273 classes of test day
within herd); age is the time-independent fixed effect of
the age at first lambing class (four classes: 1 when first
lambing occurred at 10–14 mo of age, 2 at 15–19 mo of
age, 3 at 20 to 24 mo of age and 4 at 25–29 mo of age);
when age at first lambing was missed, such as when ewes
were recorded from a second or later lactation, the age at
first lambing was approximated by considering a fixed
interval between parities equal to 365 d; Legendre polynomial
of order five (op: from 1 to 5), as used by Tolone
et al. (2007) in the same dairy sheep breed to fit the
lactation curve within parity class; a is the vector of direct
additive genetic effects; pe1 and pe2 are the vectors of
permanent environmental effects within and between