Fertility traits

An overall 6-d shorter (non-significant) interval to first service was observed in the GP group when compared to the C group (GP= 91 d vs. C= 97 d) (Table 6.4).

Although the interval from calving to first service did not differ significantly among the cows having different ovarian activity by 28 days postpartum in both the GP and C groups (Table 6.5), the interval seemed to be reduced in cows forming the GP group in comparison to cows in the C group (84 d, 86 d, 97 d and 91 d vs. 100 d, 86 d, 113 d and

94 d, respectively). These numerical differences might have accounted for the overall earlier time to first service observed in the GP group.

The rate of conception to first service did not differ between C and GP groups (Table 6.4). However, conception rate to first service appealed or was lower for cows in the GP group that lacked a functional CL by 21 days postpartum, despite a detectable luteal activity by 28 days (33 % and 0 %, respectively) (Table 6.5). In contrast, conception to first service appealed higher for cows in the C and GP group in which CL regressed between 21 and 28 days postpartum (71.4 and 100 %, respectively). Similarly, the conception rate was higher (non-significant) for cows in the GP group in which a functional CL was present at the time of both GnRH and PGF2α treatments (60 %).

The rate of cows submitted for breeding within 100 days was numerically higher for the GP group than for the C group (72.9 vs. 61.7 %, respectively) (Table 6.4).

Among the different patterns of ovarian activity observed, the submission rate was highest for cows in the GP group which had a functional CL present at the time of both GnRH and PGF2α treatments (93.3 %). Similarly, higher rates were observed for cows in the C and GP group in which CL regressed between 21 and 28 days postpartum (85.7 and 75 %, respectively); as well as in GP cows lacking CL formation during the treatment period (80 %).

Postpartum interval to conception was non-significantly shorter for cows in the GP and the C group in which a functional CL regressed in between 21 and 28 days postpartum (86 d and 97 d, respectively) (Table 6.5). Likewise, cows in C group not showing active CL by 28 days postpartum also had shorter mean interval to conception (118 d). A shorter interval was observed for cows in the GP group that had an active CL throughout the treatment period (120 d). Overall, the mean period to conception seemed

shorter for the GP group in comparison to the C group (126 d vs. 131 d, respectively) (Table 6.4).

Average number of services per conception did not differ among the C and the GP groups (1.8 vs. 2.1). However, GP cows in which CL was not present at the time of either treatment apparently required 1.7 times more services to conceive than the overall average for this group (3.6 vs. 2.1, respectively).

By 150-d postpartum, the proportion of pregnant cows did not differ between the GP and the C groups (64.6 and 63.8%, respectively) (Table 6.4). However, the rate of conception was comparatively higher for cows in either group which had a functional CL by 21 days postpartum, irrespective of the presence of CL 7 days later (Table 6.5).

Likewise, the proportion was high for GP cows not having a CL by 21 days postpartum, but that form a CL by 28 days (66.7). Conception was similarly higher (non-significant) for cows in the C group without luteal function within 21 and 28 days postpartum.

6.4 Discussion

According to plasma P4 levels evaluation by 21 days postpartum, more than 50 % of the cows in both the C and the GP group did not have ovulated. This is in agreement and closely relates with previous results observed in high producing early postpartum cows in Japan (43, 89) and New Zealand (33). Moreover, the similarities in the proportion of cows in both groups with and without luteal activity by day 21 postpartum clearly show the degree of equality between both groups at the time of the start of treatment.

More cows in the GP-NCL group (82.8%) had functional CL 7 days following the treatment with GnRH when compared to the C group (29.6%). In agreement with

the results in the previous chapters; this result indicated that treatment with GnRH effectively induced ovulation in anestrous cows. Although the proportion of cows undergoing luteolysis between 21 and 28 days postpartum was small and did not differ significantly between the GP and the C groups, the proportion of cows that had functional CL on day 28 was more in the GP group than in the C group (83.3% vs.

44.7%). Due to the nature of the experiment, it is uncertain to know the characteristics of the ovarian structures of the cows in the GP group prior to and following PGF2α

treatment. However, the overall high proportion of cows in the GP with CL on 28 day s postpartum and the apparently normal steroidogenic function of CL indicate that the PGF2α treatment had a high possibility to induce luteolysis followed by ovulation.

Improved synchronization and fertility have been reported for cows having presence of CL with normal P4 production at the time of PGF2α treatment in dairy cows (99)

Significant differences in the metabolic status of the cows were not detected during the synchronization period. Moreover, the biochemical parameters analyzed were within the normal ranges (47). These results indicate that the metabolic status was not likely a factor causing differences in the ovarian status by the time of the start of the protocol.

Calving-to-first service interval was reduced, on average, 6 days in the GP group but a significant difference could not be observed. When examining cows according to the changes in ovarian status between days 21 to 28 postpartum, the analysis revealed that 4 out of the 4 subdivision groups in the GP group had, on average, their first AI or ET before 100 days postpartum when compared to 2 out of 4 in the C group. Based on these results, it could be speculated that the treatment combination induced normal cycles at least in those cows having an active CL (83%) prior to PGF2α

treatment. Similarly and irrespective of the changes in ovarian status within the treatment protocol, numerically more cows were submitted for breeding within 100 days postpartum in the GP group when compared to the C group (72.9% vs. 61.7%, respectively). Together, the improvements in the calving-to-first service interval and the submission rate may indicate some advantage in the required conditions that could improve the fertility in the treated cows. The submission rate, together with other parameters of fertility has effect on the distribution of calvings in dairy herds (31). The practices for the management of reproduction in this herd, namely the increase in efforts to breed cows after 60 days postpartum may have limited the potential for early breeding in the GP group thus limiting the improvements in several parameters of fertility. The results in Chapter 4 together with the results of previous research (5) indicate that the frequency of ovulations increase before day 60 in cows that respond to treatment with GnRH and PGF2α in the early postpartum.

Conception to first service was improved in a limited number of cows in both C and GP that had a CL on day 21 regardless of a spontaneous luteolysis as observed by 28 days postpartum. This is in agreement with statements awarding better fertility to cows that ovulate early and subsequently have more ovulations during the postpartum (17, 43, 86, 93).

The absence of an overall significant difference between C and GP in conception rate to first service was apparently caused by the low conception in cows of the GP group that did not have ovulated by 21 days postpartum. In comparison to other groups with different ovarian activity, both calving-to-first service and the rate of submission to breeding by 100 days in these cows were optimal. Moreover, the mean P4

level prior to PGF2α treatment in the cows that apparently ovulated after GnRH was

normal (5.2 ng / ml). The low conception rate may find explanation on the production characteristics reported for these groups. In cows failing to have an early first ovulation, the rate of increase in milk production from soon after parturition to the 3rd postpartum week and/or to the week of peak milk yield is greater than in cows that ovulate early (44). High milk production decreases conception to first service (10, 27, 91), and increases embryo mortality (27). Conception to first and / or to any other service is the result of fertilization, early embryonic, late embryonic and fetal development (27).

Similarly to conception rate to first service, mean days from calving to conception and the number of services per conception were increased whereas conception rates by 100-days were decreased in non-treated cows that had a functional CL on both 21 and 28 days postpartum, as well as in cows that did not have ovulated by the time when the treatment with GnRH and PGF2α started. The reasons for such results are not clear and can not be easily explained due to the nature of the experiment.

Probably and in agreement with the better performance reported for early ovulating cows (17, 43, 93) and with reports on cows treated with PGF2α following GnRH (20), luteolysis prior to and / or after the treatment with PGF2α in GP cows that had CL by 21days, or luteolysis prior to 28 days postpartum in non-treated cows with CL on day 21 may have improved fertility.

In conclusion, the treatment with GnRH and PGF2α as early as 21 days postpartum to commercially managed dairy cows is able to improve the pre-required conditions for the early establishment of conception. However, management and / or other factors affecting conception and embryo-fetal development may play a limiting role on the fertility of hormonally treated cows.