Strategic test-day recording regimes to estimate lactation yield in tropical dairy animals.

BACKGROUND: In developing dairy sectors, genetic improvement programs have limited resources and recording of herds is minimal. This study evaluated different methods to estimate lactation yield and sampling schedules with fewer test-day records per lactation to determine recording regimes that (1) estimate lactation yield with a minimal impact on the accuracy of selection and (2) optimise the available resources. METHODS: Using Sahiwal cattle as a tropical dairy breed example, weekly milk records from 464 cows were used in a simulation study to generate different shaped lactation curves. The daily milk yields from these simulated lactation curves were subset to equally spaced (weekly, monthly and quarterly) and unequally spaced (with four, five or six records per lactation) test-day intervals. Lactation yield estimates were calculated from these subsets using two methods: the test-interval method and Wood's (Nature 216:164-165, 1967) lactation curve model. Using the resulting lactation yields, breeding values were predicted and comparisons were made between the sampling regimes and estimation methods. RESULTS: The results show that, based on the mean square error of prediction, use of Wood's lactation curve model to estimate total yield was more accurate than use of the test-interval method. However, the differences in the ranking of animals were small, i.e. a 1 to 5% difference in accuracy. Comparisons between the different test-day sampling regimes showed that, with the same number of records per lactation (for example, quarterly and four test-days), strategically timed test-days can result in more accurate estimates of lactation yield than test-days at equal intervals. CONCLUSIONS: An important outcome of these results is that combining Wood's model for lactation yield estimation and as few as four, five or six strategically placed test-day records can produce estimates of lactation yield that are comparable with estimates based on monthly test-day records using the test-interval method. Furthermore, calculations show that although using fewer test-days results in a decrease in the accuracy of selection, it does provide an opportunity to progeny-test more sires. Thus, using strategically timed test-days and Wood's model to estimate lactation yield, can lead to a more efficient use of the allocated resources.

Red foxes (Vulpes vulpes) and wild dogs (dingoes (Canis lupus dingo) and dingo/domestic dog hybrids), as sylvatic hosts for Australian Taenia hydatigena and Taenia ovis.

Foxes (n = 499), shot during vertebrate pest control programs, were collected in various sites in the Australian Capital Territory (ACT), New South Wales (NSW) and Western Australia (WA). Wild dogs (dingoes (Canis lupus dingo) and their hybrids with domestic dogs) (n = 52) captured also as part of vertebrate pest control programs were collected from several sites in the ACT and NSW. The intestine from each fox and wild dog was collected, and all Taenia tapeworms identified morphologically were collected and identified to species based on the DNA sequence of the small subunit of the mitochondrial ribosomal RNA (rrnS) gene. Taenia species were recovered from 6.0% of the ACT/NSW foxes, 5.1% of WA foxes and 46.1% of ACT/NSW wild dogs. Taenia ovis was recovered from two foxes, 1/80 from Jugiong, NSW and 1/102 from Katanning, WA. We confirm from rrnS sequences the presence of T. ovis in cysts from hearts and diaphragms and T aenia hydatigena in cysts from livers of sheep in Australia. T. ovis was not recovered from any of the wild dogs examined but T. hydatigena were recovered from 4(8.3%) wild dogs and a single fox. With foxes identified as a definitive host for T. ovis in Australia, new control strategies to stop transmission of T. ovis to sheep need to be adopted.

Reliability of the bulk milk somatic cell count as an indication of average herd somatic cell count.

Bulk milk somatic cell count (BMSCC) is a frequently used parameter to estimate the subclinical mastitis prevalence in a dairy herd, but it often differs considerably from the average SCC of all individual cows in milk. In this study, first the sampling variation was determined on 53 dairy farms with a BMSCC ranging from 56 000 to 441 000 cells/ml by collecting five samples on each farm of the same bulk tank. The average absolute sampling variation ranged from 1800 to 19 800 cells/ml. To what extent BMSCC represents all lactating cows was evaluated in another 246 farms by comparing BMSCC to the average herd SCC corrected for milk yield (CHSCC), after the difference was corrected for the sampling variation of BMSCC. On average BMSCC was 49 000 cells/ml lower than CHSCC, ranging from -10 000 cells/ml to 182 000 cells/ml, while the difference increased with an increasing BMSCC. Subsequently, management practices associated with existing differences were identified. Farms with a small (<20%) difference between BMSCC and CHSCC administered intramuscular antibiotics for the treatment of clinical mastitis more often, used the high SCC history when cows were dried off more frequently and had a higher number of treatments per clinical mastitis case compared with farms with a large (20%) difference. Farms feeding high-SCC milk or milk with antibiotic residues to calves were 2.4-times more likely to have a large difference. Although sampling variation influences the differences between BMSCC and CHSCC, the remaining difference is still important and should be considered when BMSCC is used to review the average herd SCC and the subclinical mastitis prevalence.