FERTILITY IN DAIRY-COWS MANAGED FOR CALVING INTERVALS OF 12, 15 OR 18MONTHS

Effects on reproduction in a total of 135 dairy cows managed for calvi ng intervals of 12, 15 or 18 months (72, 38 and 25 cows respectively) were studied. The cows were of the Swedish Red and White Breed (SRB) a nd the Swedish Freisian Breed (SLB) and were housed in 2 different her ds with 3 different management systems (tied, loose, and tied but milk ed in a milking parlour; mixed). The cows in one of the herds (48 cows ) were assigned for milking either 2 times or 3 times a day. When comp aring conception rate at 1st insemination (AI) and the percentage of c ows finally pregnant, we found no significant differences between the 3 calving interval groups, however, a tendency for a higher conception rate with a 15 months' interval compared with a 12 months' interval w as found in one of the herds (50% vs 41.5%). The percentage of finally pregnant animals varied between 81% and 100%, but this variation was mainly attributed to the herd rather than calving interval group. A si gnificantly higher percentage of cows was treated for anoestrous in th e 12-month group than in the 15-month group in one of the herds (28.6% vs. 5.3%). The frequency of ovulations with external heat signs incre ased with ovulation number up to the 4th ovulation and thereafter rema ined stable. No significant difference was found in number of AIs requ ired per conception with respect to calving intervals, breeds, or milk ing frequency groups. However, cows milked 3 times a day had a signifi cantly longer interval from the 1st Al to conception compared with cow s milked 2 times a day (45.8 days vs 17.6 days, p<.01). Cows kept loos e exhibited ist ovulatory oestrous, approximately 2 weeks earlier (55. 9 days vs 69.7 days, p<.05) than their herd dates kept tied. In conclu sion, our study shows that lengthening the calving interval to 15 or 1 8 months may have a positive influence on reproduction in terms of les s need for treatments of ovarian disorders and higher conception rates . Our results also indicate that milking 3 times a day may have negati ve effects, and keeping cows in a loose-housing management system may have positive effects on ovarian function.