The effect of strain of Holstein-Friesian cow and feeding system on reproductive performance in seasonal-calving milk production systems

Three strains of Holstein-Friesian (HF) cows: high production North American (HP), high durability North American (HD) and New Zealand (NZ) were assigned, within strain, to one of three pasture-based feeding systems : Moorepark (MP), high concentrate (HC), and high stocking rate (HS). The effects of strain of HF, feeding system and parity on milk production, body condition score (BCS), live weight, energy balance and reproductive performance were studied using a repeated measures model with a factorial arrangement of strain of HF and feeding systems. Associations between these variables and conception to first service (CONCEPTl), conception to first and second service (CONCEPT1_2), pregnancy rate at 6 weeks (PREG6) and overall pregnancy rate (PREG) were assessed using logistic regressions. When treatment means were compared, the NZ strain had a shorter gestation length and a higher CONCEPT1_2 than both the HP and HD strains. Similarly, the NZ strain had a higher PREG6 and PREG than the HP strain. Feeding system had no significant effect on reproductive performance. The HP strain had the highest milk yield at first AI and peak milk yield, the NZ strain had the lowest milk yield while the HD strain was intermediate. The energy balance of the NZ strain was higher than that of the HP and HD strains. The NZ strain had the lowest live weight and highest BCS; the HD strain had the highest live weight and the HP strain had the lowest BCS. The results show that dairy cows with superior genetic merit for fertility traits have better reproductive performance.