Veterinary Science - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemA novel approach to treating cows with postpartum anoestrus and associated risk factors( 2016)This project combines the findings of two distinct, but related research studies investigating postpartum anoestrus (PPA) in lactating dairy cows. These studies involved five seasonally calving commercial Victorian dairy herds. The first study was completed as a cross-sectional analysis of prevalence and risk factors for PPA on the five herds involved, and the second study was conducted as a clinical trial to investigate the differences in reproductive success of cows with PPA when treated with either a FTAI based protocol, or a protocol that relied on oestrus detection. Study one assessed data from 3,194 cows from within the five herds. Prevalence was calculated as the proportion of animals which were presented for PPA treatment to a veterinarian as a proportion of the total number of cows within herds at the start of each herd’s Mating start date (MSD). Risk factors that were assessed included age, days in milk (DIM), body condition score (BCS), and herd. The outcome of interest for the study was risk of PPA, and this was assessed in a multivariable logistic regression model. Risk factors were reported as adjusted odds ratios from the model, and because a significant interaction terms of BCS x DIM was discovered, these variables were referenced to each other in the model, and the combined variables were reported with predicted probabilities. The average prevalence of PPA across the herds involved was 44%. The association between age and the probability of being diagnosed as anoestrus was greatest for 2-year-old cows with the odds of anoestrus being 2.43 (95% CI 1.80 to 3.27) times greater in this group compared with cows aged 6 years and greater. For cows that were > 90 DIM at MSD, the odds of anoestrus for cows that were < 4.0 BCS was 8.05 (95% CI 3.20 to 20.2) times that of cows that were BCS 4.5 to 5.5. For cows that were > 90 DIM at MSD the odds of anoestrus for cows of BCS ≥ 4.0 to < 4.5 was 2.10 (95% CI 1.25 to 3.54) times that of cows that were BCS 4.5 to 5.5. The highest predicted probability of anoestrus was for cows of BCS < 4.0 that were DIM < 60 at MSD (0.89, 95% CI 0.85 to 0.93). Study two involved a clinical trial with 1,167 cows that were presented for treatment of PPA. This compared Ovsynch + P4 (FIXED) to a protocol that removed the final GnRH injection and instead relied on detection of oestrus (HEAT). The aim of this trial was to determine whether improvements in first service conception rate (FSCR) could be made with an alternative program to Ovsynch + P4, and to assess whether reductions could be made in the proportion of cows with prolonged return to oestrus interval after an unsuccessful insemination. The FSCR for the HEAT treatment group was numerically, but non-significantly, higher than the FIXED treatment group, however this did not result in superior reproductive performance overall, with 49-day pregnancy rates similar between treatment groups. The proportion of cows that had returned to oestrus by 24 days after an unsuccessful insemination differed across the two treatment groups, 46% versus 67% for FIXED and HEAT respectively (χ2 test statistic 25.19; df 1; P < 0.001). When assessed in a Cox proportional hazard regression model, for cows in the HEAT treatment group, the daily hazard of returning to oestrus was 1.75 (95% CI 1.40 to 2.19) times that of cows in the FIXED treatment group. Overall, the studies within this project provided new information regarding prevalence of PPA in seasonally calving Victorian dairy herds, as well as highlighting key risk factors. The clinical trial highlighted that alternative strategies may be implemented to further enhance the reproductive success observed in cows with PPA when treated with an alternative synchrony protocol that does not rely on FTAI.