Veterinary Science - Theses
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ItemAnthelmintic resistance in gastrointestinal nematode parasites of dairy cattle in the Macalister irrigation district of Victoria( 2016)Gastrointestinal nematode parasitism is one of the most production limiting diseases of pasture-based dairy enterprises in Australia, in terms of its effect on early lactation milk production and live weight gain of replacement heifers. Farmers are heavily reliant upon the use of anthelmintics to control parasitism, which raises concerns about the development of anthelmintic resistance. Studies in beef cattle in Australia and overseas have demonstrated very high levels of resistance to all available anthelmintic classes. However, there have been no substantial investigations of anthelmintic resistance on dairy cattle farms anywhere in the world. The objective of the first trial was to report anthelmintic resistance in gastrointestinal nematode parasites of cattle on commercial dairy farms in the Macalister Irrigation District of Gippsland, Victoria. Faecal egg count reduction tests (FECRTs) were used to assess anthelmintic resistance on 20 farms between May 2013 and June 2014. All three currently available anthelmintic classes for cattle nematodes in Australia were tested. Faecal samples were collected 10-14 days post-treatment for individual faecal egg counts (FEC) and larval differentiation. The arithmetic mean FEC for each treatment group was compared with an untreated control post-treatment. Resistance was defined as <95% reduction in FEC, with a lower 95% confidence interval <90% when the mean FEC of the control group, differentiated by genus, was greater than 25 eggs/g. Anthelmintic resistance was present on all 20 dairy farms involved in this study. Resistance to doramectin in at least one species was detected on 15/20 (70%) farms, fenbendazole on 16/20 (80%) farms and levamisole on 5/20 (25%) farms. On three farms, resistance by Ostertagia ostertagi to all three anthelmintic classes was detected. This is the first report of anthelmintic-resistant O. ostertagi on Australian dairy farms and is of concern, given the high pathogenicity of this species. There is universal agreement that reliance upon chemical-based parasite control must be addressed in order to ensure the long term sustainability of current and future anthelmintics. One option for reducing the selection pressure for anthelmintic resistance in dairy replacements is the concept of targeting anthelmintic treatments to only those animals that require it, rather than the whole group. The second trial aimed to compare whole group treatment applied at 6-weekly intervals (control) with a live weight gain-based targeted selective anthelmintic treatment (TST) regime, in terms of anthelmintic use and growth performance, using a non-inferiority study design. Four-hundred and thirty heifers were allocated to either a TST or control group according to odd or even ear tag number. Live weights and faecal egg counts were monitored three times over 12 weeks (day 0, week 6, week 12). Heifers in the control group were all treated on each occasion with a subcutaneous injection of moxidectin. Heifers in the TST group were treated only if they failed to achieve a predetermined live weight target calculated for each visit. An arbitrary non-inferiority margin of 10%, considered the largest commercially acceptable difference, was defined a priori and average daily gain (ADG; g/day) for each group was compared statistically. The results showed that ADG of heifers in the TST group were not inferior to whole group anthelmintic treatment and anthelmintic usage was reduced by 48%. This supports the case for a larger scale study of this strategy in Australian dairy herds; particularly since long-term TST in sheep has been shown to slow the development of anthelmintic resistance in the field.
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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.
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ItemThe effects of post-partum and pre-mating start date weight change and production level on reproductive performance in lactating cows in a seasonal calving, pasture-based system( 2013)Retrospective fertility data from a 600 cow, predominantly Holstein-Friesian, seasonal calving herd in south west Victoria was analysed to ascertain fertility trends on the farm over 10 years. Prospectively, daily body weights were collected and analysed with reproductive and production data to establish associations between weight change and production level both post-partum and pre-mating start date and reproductive performance. Body weights from automated walkover weighing scales at the exit to the dairy were collected, filtered and analysed to determine weight change trends post-partum (PP) and pre-mating start date (MSD). Herd reproductive parameters were established from calving dates, AI dates and early ultrasound pregnancy testing results. Production data from inline milk meters were collected and analysed to provide measures of PP and pre-MSD production level. The effects of PP and pre-MSD weight change and production variables on reproductive performance, together with the confounding variables age, time calved and calving induction, were statistically analysed to establish predictors of poor reproductive performance. ACCURACY OF THE SCALES Initially, a study was performed to determine the repeatability of the walkover scales and their agreement with static scales. Forty-six cows from the study herd were weighed once using a set of static scales and repeatedly using the automated walkover weighing system. The walkover weighing system showed substantial agreement with the static scales when assessed using Lin’s concordance correlation coefficient. This contrasted with limited agreement when assessed using Bland and Altman’s method, largely due to poor repeatability. Removal of a single outlier from the data set increased all measures of agreement and repeatability. Cow misbehaviour during walkover weighing accounted for many of the larger weight discrepancies. The limited agreement and poor repeatability suggest that the automated weighing system is inadequate for detecting small body weight differences in individual cows based on comparisons of single weights. Excluding outlier weights and comparing means of 7 consecutive daily weights may improve agreement sufficiently to allow meaningful assessment of small short-term changes in automated weights. WEIGHT CHANGE TRENDS Once an accurate weight dataset was established through data filtration, weight change trends over time were established. Linear regression lines were fitted through all filtered weights recorded between days 8 to 28 and 29 to 49 PP and days 42 to 22 and 21 to 1 pre-MSD. Positive slopes were assigned ‘Up’ trend and negative slopes were assigned ‘Down’ trend resulting in one of four possible double weight trends – Down Down (DD), Down Up (DU), Up Down (UD) and Up Up (UU) - for both PP and pre-MSD weight change. EFFECTS OF WEIGHT CHANGE AND PRODUCTION ON REPRODUCTIVE PERFORMANCE The most important factor affecting reproductive performance was duration of time calved. An increased time from calving to MSD resulted in improved measures of reproductive performance. PP weight change, age and yield at 100 days-in-milk were also significant predictors of submission by 3 weeks with PP DU cows having 1.7 times the odds of being submitted compared to DD cows. Three to 5 year olds had almost 3 times the odds of being submitted compared to 2 year old cows. Producing an extra 100L of milk by 100 days-in-milk resulted in 5% reduced odds of being submitted by 3 weeks. Pre-MSD weight change was a significant predictor of not being in-calf by 21 weeks. Pre-MSD DU, UD and UU cows had reduced odds of not being in-calf compared to DD cows. Cows producing an extra litre of milk daily in the 3 weeks prior to MSD had 5% reduced odds of submission by 3 weeks and 7% increased odds of becoming an NVO. In summary, reduced reproductive performance was observed in first calving heifers, animals not long calved, DD cows and high producing animals. The ability to identify these high risk animals prior to MSD could assist reproductive management, either through modified nutritional supplementation, prompt implementation of hormonal treatments or through alteration of insemination choices. Assessing the efficacy of these suggested management modifications at improving reproductive performance of high risk animals was beyond the scope of this study but provides suggestions for further research.