Resposta ovariana e taxa de prenhez em vacas Bos taurus de corte lactantes submetidas à associações de diferentes doses de ECQ e períodos de exposição à P4

Abstract

The high rate of animals in anestrous compromises the reproductive capacity of animals. TAI allows acyclic animals to be exposed to AI resulting in increased service and pregnancy rate. Although ovulation synchronization protocols are already considered to be established, adjustments can still be made to specific genetic groups, as well as to optimize results, minimize costs and allow flexibility in management. In this sense, the effect of different doses of equine chorionic gonadotropin (300 or 400 IU of eCG) on TAI protocols with 8 or 9 days of progesterone device (P4) stay length on the rate of estrus occurrence, rate of pregnancy and follicular growth in Bos taurus lactating dairy cows. There were 1171 cows with a body condition score (BCS) of 2.80 ± 0.04 (1 to 5, 1-lean to 5-obese), from six commercial farms located in three distinct regions of the state of Rio Grande do Sul/Brazil and maintained at pasture. At the beginning of the protocol the cows were divided into two treatments, homogeneously on the BCS: D9 (D0, n=591) or D8 (D1, n=580), with 24 hours of difference between groups, according to the stay length of the progesterone device (9 or 8 days respectively: P4, CIDR®, Zoetis) associated with 2mg of estradiol benzoate (IM; Gonadiol®, Zoetis). On day 9, CIDR was removed and given 12.5 mg of dinoprost tromethamine IM (Lutalyse®, Zoetis) and 1 mg of estradiol IM cypionate (E.C.P®, Zoetis). At this time, the cows were again distributed in two treatments: 300 IU (n=605) or 400 IU (n=566) of eCG (Novormon®, Zoetis), forming 4 treatments D8300IU (n=303), D8400IU (n=277), D9300IU (n=302) and D9400IU (n=289). TAI was performed 48 hours after P4 removal using frozen semen from 24 bulls. Two doses of semen from each batch were evaluated and only departures with minimal progressive motility of 30% after thawing were used. The females had the base of the tail painted with a marking pen (Raidl-Maxi, RaidexGmbH, Dettingen / Erms, Germany) at the time of removal of the P4 device, and the females did not present ink at the time of the TAI considered in estrus. Statistical analysis was performed by PROC GLIMMIX SAS®. The stay length time affected estrus occurrence rate, LF diameter at the time of removal of the P4 device, LF diameter at the time of TAI, (P=0.02); (P<0.0001); (P<0.0001), respectively, being lower in the groups with a stay length of 8 days when compared to the stay length of nine days. The pregnancy rate and the follicular growth rate were not affected by the stay length (P=0.62); (P=0.96), respectively. The pregnancy rate and the follicular growth rate were not affected by the stay length (P=0.62); (P=0.96), respectively. The occurrence of estrus and pregnancy of TAI (P/AI) of groups D8300IU, D8400IU, D9300IU and D9400IU were respectively (60.4%/40.3%, 66.8% / 45.8%, 70.8%) (P=0.01/0.01, P=0.0001/0.33, P <0.0001 / 0.26, P=0001/0.08), with the lowest estrous rate in the D8300IU group. The occurrence of estrus rate, P/AI, diameter of the LF at the time of removal of the P4 device, diameter of the LF at the time of the TAI and the rate of growth of the LF, was not affected by the dose of eCG [(P=0,77); (P=0.50); (P=0.65); (P=0.44); (P=0.17)]. For these same variables, no interaction between stay length time and eCG dose (P=0.06) was observed; (P=0.10); (P=0.73); (P=0.22); (P=0.24). Additionally the BCS affected the diameter of the LF D9 (P<0.0001); LF D11 (P<0.0001); rate of LF growth (P=0.03); estrus occurrence rate (P<0.0001) and pregnancy rate (P=0.004), with animals with BSC ≤2.75 presented lower results. In the study it was demonstrated that the stay length of the P4 device affects the estrus rate; the diameter of the FL at the time of P4 withdrawal, the follicular diameter at the time of the TAI, being lower in the D8 stay length group, however, the pregnancy rate is not affected in the treatment groups. Regarding the occurrence of estrus, the D8300IU group was inferior compared to the others, in sum, lower results were observed in animals presenting with BSC≤2,75. Thus, in TAI protocols lasting 8 days of exposure to P4, the use of 300 IU of eCG should be recommended in animals with BSC greater than 2.75.