Estimates of breed group means for height by age
(yr) are presented in Table 6. Cows of all breed groups
continued gaining stature through 7 yr of age, but at
very low rate at later ages. In fact, almost no increases
in height were found after 5 yr of age for any breed
group. The increases in stature (4 to 7 cm), in the 6-yr
periods reported here, accounted only for 3.1 to 5.4%
of the total stature through 7 yr of age in the different
breed groups, indicating that mature height is reachedearlier than mature weight. Heights at 2 yr of age could
be acceptable estimates of mature height for the breed
groups considered here. Breed groups tended to cluster
in two groups for height. British breeds (Hereford, Angus)
and crosses (HA reciprocal crosses and Jersey
crosses) tended to have similar heights, which were less
than the corresponding heights for crosses with South
Devon and continental breeds (Charolais, Limousin,
and Simmental). Rankings of breed groups for height
shifted across ages, but there was a general tendency for
heavier breed groups to be taller. Estimates of contrastsbetween breed groups and standard errors, as well as
significance levels are presented in Table 7. Cows with
Jersey sires were not different (P > 0.05) from HA cross
cows. Cows with sires of continental breeds had similar
heights and were taller (P < 0.01) than HA at every
age. The superiority of the cows with continental sires
over the HA cross cows ranged from 5.1 (Limousin at
7 yr) to 6.9 cm (Simmental at 2 yr). Reciprocal HA
cows were slightly taller than the average of purebred
(Hereford and Angus) cows at each age, but that superiority
was significant only when greater than 1 cm at 4yr (P > 0.05) and after 5 yr (P < 0.01). Consequently,
heterosis ranged only from 0.1 to 1.1% across ages.
Cows with Hereford dams were taller than cows with
Angus dams by differences that were significant (P <
0.01) when the difference exceeded 1 cmafter 2 yr of age.
Gregory et al. (1992; 1995) reported heights of purebred
and composite cows of 2 to 7+ yr of age from the
GPU project at MARC. Hereford cows were 1 to 2 cm
taller than Angus cows as in the present study, but the
means for height were greater in their study, ranging
from 124 to 129 cm for Hereford, and from 123 to 127
cm for Angus.