Cp. Ferris et al., The influence of dairy cow genetic merit on the direct and residual response to level of concentrate supplementation, J AGR SCI, 132, 1999, pp. 467-481
Twenty-eight high genetic merit and 32 medium genetic merit Holstein/Friesi
an dairy cows with Predicted Transmitting Abilities for milk fat plus prote
in yield, calculated using 1995 as the base year (PTA(95) fat plus protein)
of 43.3 and 1.0 kg respectively, were used in a continuous design, 2 (cow
genotypes) x 4 (concentrate proportion in diet) factorial experiment consis
ting of eight treatments. Concentrate proportions in the diet were 0.37, 0.
48, 0.59 and 0.70 of total dry matter (DM), while the forage component of t
he diet was grass silage. Diets were offered ad libitum in the form of a co
mplete diet. Animals remained on these concentrate regimes for a mean of 84
.7 days before completing a standard 98-day grazing period. At pasture, cow
s received either 5.0 or 6.0 kg concentrate daily according to turnout date
. There were no significant genotype x nutrition interactions for any of th
e variables examined during either the indoor feeding or post-turnout grazi
ng periods (P < 0.05). Outputs of milk, fat, protein and fat + protein were
greater for high merit cows than for medium merit animals (P < 0.001). Mil
k output and milk protein output responses to increasing concentrate propor
tion were linear for both cow genotypes (P < 0.001), while high merit anima
ls showed a linear response in terms of milk fat plus protein output (P < 0
.01) with these responses being statistically parallel for both merits. Hig
h merit cows had a significantly higher DM intake than medium merit animals
(P < 0.01). With high merit animals, concentrate proportion had little eff
ect on body tissue reserves, while medium merit animals showed a tendency f
br increased condition score and backfat thickness with increasing concentr
ate inclusions (P < 0.05). In terms of the output of milk and milk constitu
ents during the grazing period, there were no significant residual effects
from winter concentrate feed rate (P > 0.05), while high merit cows continu
ed to have higher outputs of milk and milk constituents (P < 0.001). Body t
issue reserves of both genotypes changed little during the grazing period.
It is likely that the higher milk yields of the high merit cows can be attr
ibuted both to their higher DM intakes and their ability to partition a gre
ater proportion of the nutrients consumed into milk rather than body tissue
reserves. Although statistically both high and medium merit cows responded
in a similar manner to an increasing proportion of concentrate in the diet
, high merit cows did exhibit a 49% greater fat plus protein yield response
during the indoor period, compared to animals of medium genetic merit, per
haps hinting at the existence of a genotype x nutrition interaction.