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The effect of genetic merit and concentrate proportion in the diet on nutrient utilization by lactating dairy cows

Published online by Cambridge University Press:  01 June 1999

C. P. FERRIS ,
F. J. GORDON ,
D. C. PATTERSON ,
M. G. PORTER  and
T. YAN
C. P. FERRIS
Affiliation:
Agricultural Research Institute of Northern Ireland, Hillsborough, Co. Down BT26 6DR
F. J. GORDON
Affiliation:
Agricultural Research Institute of Northern Ireland, Hillsborough, Co. Down BT26 6DR The Queen's University of Belfast, Newforge Lane, Belfast BT9 5PX Agricultural and Environmental Science Division, Department of Agriculture for Northern Ireland
D. C. PATTERSON
Affiliation:
Agricultural Research Institute of Northern Ireland, Hillsborough, Co. Down BT26 6DR The Queen's University of Belfast, Newforge Lane, Belfast BT9 5PX Agricultural and Environmental Science Division, Department of Agriculture for Northern Ireland
M. G. PORTER
Affiliation:
Agricultural Research Institute of Northern Ireland, Hillsborough, Co. Down BT26 6DR
T. YAN
Affiliation:
Agricultural Research Institute of Northern Ireland, Hillsborough, Co. Down BT26 6DR
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Abstract

Sixty Holstein/Friesian dairy cows, 28 of high genetic merit and 32 of medium genetic merit, were used in a continuous design, 2 (cow genotypes)×4 (concentrate proportion in diet) factorial experiment. High and medium merit animals had Predicted Transmitting Abilities for milk fat plus protein yield, calculated using 1995 as the base year (PTA95 fat plus protein), of 43·3 kg and 1·0 kg respectively. Concentrate proportions in the diet were 0·37, 0·48, 0·59 and 0·70 of total dry matter (DM), with the remainder of the diet being grass silage. During this milk production trial, 24 of these animals, 12 from each genetic merit, representing three animals from each concentrate treatment, were subject to ration digestibility, and nitrogen and energy utilization studies. In addition, the efficiency of energy utilization during the milk production trial was calculated.

There were no genotype×concentrate level interactions for any of the variables measured (P>0·05). Neither genetic merit nor concentrate proportion in the diet influenced the digestibility of either the DM or energy components of the ration (P>0·05). When expressed as a proportion of nitrogen intake, medium merit cows exhibited a higher urinary nitrogen output and a lower milk nitrogen output than the high merit cows. Methane energy output, when expressed as a proportion of gross energy intake, was higher for the medium than high merit cows (P<0·05), while urinary energy output tended to decrease with increasing proportion of concentrate in the diet (P<0·05). In the calorimetric studies, neither heat energy production, milk energy output and energy retained, when expressed as a proportion of metabolizable energy intake, nor the efficiency of lactation (kl), were affected by either cow genotype or concentrate proportion in the diet (P>0·05). However when kl was calculated using the production data from the milk production trial the high merit cows were found to have significantly higher kl values than the medium merit cows (0·64 v. 0·59, P<0·05) while k l tended to fall with increasing proportion of concentrate in the ration (P<0·05). However in view of the many assumptions which were used in these latter calculations, a cautious interpretation is required.

Type
Research Article
Information
The Journal of Agricultural Science , Volume 132 , Issue 4 , June 1999 , pp. 483 - 490
Copyright
© 1999 Cambridge University Press

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