Protocolos de inseminação artificial em tempo fixo à base de progesterona para vacas leiteiras

In the last 50 years, milk production increased in lactating dairy cows. In contrast, reproductive efficiency has dramatically decreased. Several causes may be involved, such as management and environmental factors, physiological and nutritional factors, disease challenges and others. Steroid hormone concentrations in high-producing lactating dairy cows are often at lower levels, due to high dry matter intake and increased liver blood flow and steroid hormones metabolism, which is associated with the compromised estrus expression and reduced oocyte quality, thus decreasing fertility. However, with the largely use of FTAI programs, fertility has turned the corner with current reports of increasing reproductive efficiency. Given the great number and variations on E2/P4-based FTAI protocols, three studies were performed involving different hormonal combinations and are presented in two chapters in this thesis. The first study aimed to compare the ovarian dynamics and fertility using two different treatments at the initiation of a P4-based FTAI protocol, GnRH vs. EB, combined with two different treatments at the end of the protocol, EB vs. ECP. For this study, 1,035 lactating cows were completely randomized into one of four treatments: GnRH-EB, GnRH-ECP, EB-EB, EB-ECP. Interactions and treatments at the end of the protocol did not affect fertility. However, GnRH rather than EB at the beginning tended to increase P/AI and greater proportion of cows regressed the CL when EB was used. The second study aimed to compare plasma P4 concentrations in non-lactating Holstein cows fitted implanted with new (New), or 8-days used intravaginal P4 implants previously autoclaved (Aut) or disinfected (Dis), and containing 1.9 or 1.0 g of P4. Using a 2x3 factorial arrangement of treatments, 24 cows were randomly assigned to two of six treatment groups (two replicates). Mean circulating P4 during 8 days with P4 implant were the following regarding treatments: 1.9 g > 1.0 g; 1.9 g: Aut > New > Dis; 1.0 g: Aut = New > Dis (P 1,0 g; 1,9 g: Aut > Novo > Des; 1,0 g: Aut = Novo > Des (P < 0.05). O terceiro experimento objetivou comparar as concentrações de P4, a dinâmica ovariana e a fertilidade durante o uso de implante Aut ou Des com 1,9 g de P4, em 349 vacas holandesas lactantes submetidas a um protocolo de IATF à base de E2/P4, combinado com GnRH no início do protocolo. Pequenas variações foram verificadas entre os tratamentos nas concentrações de P4, porém sem efeito na dinâmica folicular, na taxa de sincronização e na P/IA. Contudo, a ciclicidade ou a ovulação no início do protocolo influenciaram variáveis reprodutivas, tais como o momento e a sincronização da emergência da onda folicular, a proporção de vacas em cio ao final do protocolo e o tamanho do folículo ovulatório. Além disso, mais vacas sincronizadas ao protocolo ficaram gestantes