Variação da dose e do momento de administração da PGF2α em um protocolo de Inseminação Artificial em Tempo Fixo (IATF) em vacas Nelore (Bos taurus indicus) em anestro pós-parto

Abstract

Long periods of post-partum and nutritional anestrus have been directly associated as the main causes of reduced profitability in the national livestock industry, to which both are considered

to be responsible for a decrease in annual calving rates. The use of fertility programs for fixed- time artificial insemination is one of the main tools utilized to timely reestablish cyclicity in

cows. However, fertility programs that disregard the use of intravaginal releasing devices tend to not produce satisfactory results in Nelore cows. This is mainly due to the large proportion of females in anestrus. Based on results from the Animal Reproduction Laboratory of UFMT, Sinop Campus, obtained through the development of a new fertility program provisionally named NSynch, which includes the administration of equine chorionic gonadotropin (eCG) followed by the Ovsynch protocol, we developed two hypothesis: (1) will reducing time to emergence of a new follicular wave to the moment of PGF2α administration produce ovulation rates greater than 95% after the second GnRH injection of the NSynch protocol? (2) will the increase in dose of PGF2α augment luteolysis rates on both fifth and sixth days after ovulation of a follicle or creation of an accessory CL? Thus, the aim of this study was to compare the efficiency of two doses of cloprostenol (500μg or 1000μg) on two different days (D7.5 or D8.5) on ovulation rates after the first and second GnRH injections towards the development of a new FTAI fertility program (NSynch) for Nelore cows in anestrus. 52 cows in post-partum anestrus were used in this study, being 46 multiparous and 6 primiparous at 80.01 ± 35.84 of age, body condition score (BCS) of 3.64 ± 0.47, and weighting 536.54 ± 52.34 kg in body weight with a voluntary waiting period (VWP) of 31.58 ± 7.17 days. Cows were randomly assigned into 4

treatments: GI (D0: 300 UI of eCG, D2.5: 10.5μg of buserelina acetate, D7.5: 500μg of D- cloprostenol, and D9.5: 10.5μg of buserelina acetate); G II (D0: 300 UI of eCG, D2.5: 10.5μg

of buserelina acetate, D8.5: 500μg of D-cloprostenol, and D10.5: 10.5μg of buserelina acetate); G III (D0: 300 UI of eCG, D2.5: 10.5μg of buserelina acetate; D7.5: 1000μg of D-cloprostenol, and D9.5: 10.5μg of buserelina acetate); G IV (D0: 300 UI of eCG; D2.5: 10.5μg of buserelina acetate, D8.5: 1000μg of D-cloprostenol, and D10.5: 10.5μg of buserelina acetate). Follicular dynamics was assessed on days 0, 2.5, 5, 7.5, 8.5, 9.5, 10.5, 11.5, and 12.5. Blood samples for serum levels of progesterone were collected on days 0 (groups G I, G II, G III, and G IV); 7.5 (G I and G III); D8.5 (G II and G IV); D9.5 (G I and G III), and D10.5 (G II and G IV). Ovulation rates to the first GnRH injection on groups GI, GII, GIII, and GIV were 100%; 71%; 77%, and 92%; and 62%; 71%; 46%, and 58% after the second GnRH injection, respectively (P >0.05). Luteolysis rates were 69.23%, 85.71%; 84.62%, and 100% for groups GI, GII, GIII, and GIV, respectively (P>0.05). Luteolysis was determined by serum levels of progesterone <1.0 ng/mL on the day of the second injection of GnRH. We concluded that reducing time to emergence of a new follicular wave to the moment of PGF2α administration (D7.5 vs. D8.5) did not increase ovulation rates above 95% after the second GnRH injection of NSynch. Additionally, the increase in dose of PGF2α (500μg vs. 1000 μg) did not augment luteolysis rates on both fifth and sixth days after creating a new CL.