Extended lactations in a seasonal-calving pastoral system of production to modulate the effects of reproductive failure.

This study was conducted to determine whether extending the calving interval (CI) to 24 mo would be an alternative to culling and replacing cows that had failed to become pregnant. Forty-six nonpregnant lactating cows were assembled in November 2004 and assigned to receive either 3kg (low) or 6kg (high) of concentrate supplement and a basal diet of grass silage and maize silage over the winter period (13 wk). Cows returned to pasture in late March and received 1kg of concentrate/d until dry-off (milk yield <5 kg/d). Cumulative milk production was calculated from calving to the end of November 2004 (12-mo CI) and from the start of December 2004 until dry off in 2005 (extended lactation part of 24-mo CI). High winter feeding resulted in greater milk production over the winter confinement (20.0+/-0.3 vs. 17.8+/-0.3 kg/d for high and low winter feeding, respectively) and had a carryover effect during the remainder of the 24-mo CI period (5,177 vs. 4,686kg; SEM=173kg). At the end of the study, cows were ranked on cumulative milk solids and separated into 3 groups (R1, R2, and R3). During the 24-mo CI, milk yields were 7,287, 6,267, and 5,273kg (SEM=308kg) in yr 1, and 5,738, 4,836, and 4,266 (SEM=241kg) in yr 2 for R1, R2, and R3, respectively. Eighty-five percent of the cows became pregnant during the breeding season of yr 2, with a conception rate to first service of 52%. An economic analysis of different ranks with a 12-mo CI, a 24-mo CI, and an annualized herd effect, which compared an efficient spring calving system with 30% recycled cows in R1 and 10% recycled cows in R3, was carried out. Farm profit was reduced by 60% and 65% at a milk price of 22.3 euro-cents (c)/L with the corresponding values of 17% and 30% for a milk price of 30 c/L, respectively, when R1 and R3 systems were compared with an efficient spring milk (12-mo CI) production system. Within a spring system where 30% and 10% of R1 and R3 animals were subjected to extended lactations, the profit difference was reduced compared with an efficient spring system, The results indicated that lactations with a 24-mo CI may be a viable alternative to culling nonpregnant cows and be economically more suited to higher producing cows.