Reproductive efficiency in dairy cattle calving during autumn or spring : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Animal Science, Institute of Veterinary, Animal and Biomedical Sciences, Massey University

Reproductive performance of cows is a key element in the productivity and efficiency of a dairy system. Therefore, the aim of this study was to evaluate and compare the reproductive performance of cows calving in d ifferent seasons (autumn or spring) in three dairy farm systems: 1 00 % spring calving (S) , 1 00 % autumn calving (A) and 50 % I 50 % spring and autumn calving (AS) . Data recorded at No. 1 Dairy Farm (Massey University) during 1 996-1 999 was used for the present study. Also, mi lk samples collected during 1 998-1 999 from the S and A farmlets were used to determine some metabolites (urea, progesterone and f3-hydroxybutyrate) in order to assess possible causes of d ifferences in reproductive function . During the three-year period, the AS farmlet showed a more concentrated calving pattern than the A and S farmlets . The planned start of calving to median calving date was 1 5, 1 7 and 1 9 days for the AS, A and S farmlets, respectively. In l ine with its calving pattern, S farmlet had a higher proportion of induced calvings ( 1 0 %) (P < 0.0 1 ) than A (3 %) and AS farm lets (5 %) and a lower proportion of cows showing heats before planned start of mating (PSM) (P < 0.01 ) (59 % vs 69 and 70 % for the A and AS cows, respectively) . Also S cows displayed a higher incidence of metabolic disorders (9 %) than cows in the other farmlets (4 and 3 % for the A and AS farmlets, respectively) . Despite a simi lar submission rate at 28 days for al l farmlets (92, 94 and 90 % for the A, S and AS farmlets, respectively) , cows in the S farmlet tended to show a lower conception rate to first service (CR1 st) (47 %) than cows in the A (54 %) and AS (53 %) farm lets. Therefore, cows in the S farmlet tended to have a longer PSM to conception interval (35 days) (P < 0. 1 ) than cows in the A (30 days) and AS (3 1 days) farm lets. Regardless of farmlet, anoestrus cows were less l ikely to be submitted for artificial insemination (odds ratio (OR) = 0.43) and to conceive (OR = 0.64) than their cycling herdmates. Simi larly, cows calving later in the season, had reduced probabilities of being submitted for artificial insemination (AI) and conceiving than cows calving earlier. The probabi l ity of conception either at first service or during the whole mating period was lower for cows experiencing a metabolic d isorder (OR = 0.27 and 0.64 for conception to first service and during the whole mating period, respectively) than cows with no events recorded. Simi larly, cows h aving two or more lameness episodes were less l ikely to conceive (OR = 0.61 ) than cows with no episodes. During 1 998-1 999, the effects of milk urea concentration (M U) and energy balance, assessed by changes in body condition (BC) and concentrations of f3hydroxybutyrate i n mi lk (8-0H) , upon fertility (CR1 st) of cows in the A and S farm lets were explored. Despite h aving a higher M U (P < 0.05) (8.5 vs 6.6 mmoi/L for the A and