The influence of dairy cow genetic merit on the direct and residual response to level of concentrate supplementation

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.