Neospora caninum and reproductive wastage in extensively managed Queensland beef herds.

OBJECTIVE: To compare reproduction in extensively managed, tropically adapted beef cows that were either seropositive or seronegative to Neospora caninum. DESIGN: Longitudinal study of cows within management groups. METHODS: Compare pregnancy with weaning outcomes for 502 seropositive and 3255 seronegative cows in 25 management groups. RESULTS: We found N. caninum in all herds, with an average of 20% of 2640 tested animals seropositive within management group; prevalence varied between 0% and 94%. At 7 of 10 sites assessed, there was evidence of horizontal transmission of N. caninum. There was no overall difference in pregnancy rate (79% vs 75%; P > 0.05), reproductive wastage after confirmed pregnancy diagnosis (11% vs 10%; P > 0.05) or weaning rate (67% vs 68%; P > 0.05) between seronegative and seropositive cows, respectively. In one herd where a combination of risk factors for N. caninum was present, a significant reduction in pregnancy rate occurred after the 6 months mating (85% vs 69%; P < 0.05). The fetal and calf losses observed were lowest in south-east Queensland (4.3% of 117 pregnancies), highest in north-west Queensland (15.5% of 413 pregnancies) and intermediate in north-east Queensland (10.2% of 1625 pregnancies). Other infectious agents that are known to cause reproductive wastage were endemic in many herds, though none appeared to cause significant fetal or calf loss in this study. CONCLUSION: Despite a high prevalence of N. caninum, there was no apparent effect on beef cattle reproduction, but there is potential to cause reproductive wastage if known risk factors to neosporosis are in effect.

The integrity of sperm chromatin in young tropical composite bulls.

Sperm chromatin fragmentation is associated with subfertility, but its relationship with age progression in young bulls is poorly understood. The objective was to assess sperm chromatin fragmentation during the early post-pubertal development of 20 tropical composite bulls, using a sperm chromatin structure assay (SCSA) and sperm-bos-halomax (SBH). Bulls were subjected to bull breeding soundness evaluation (BBSE) at mean ages of 13, 18, and 24 mo. Traits measured included liveweight (WT), body condition score (BCS) and scrotal circumference (SC). Semen samples were collected by electroejaculation and assessed for mass activity (MA), motility (Mot), concentration (conc), sperm morphology and chromatin fragmentation. Concentration (r=0.34, P=0.0076), Mot (r=0.36, P=0.0041) and percentage of morphologic normal sperm (percent normal sperm (PNS); r=0.31, P=0.0132) were positively correlated with age. The percentage of sperm with proximal droplets (PD) was negatively correlated with age (r=-0.28, P=0.0348), whereas neither SCSA nor SBH results were significantly correlated with age. The percentage of sperm with chromatin fragmentation using SCSA was correlated with PNS (r=-0.53, P<0.0001), the percentage of sperm with head abnormalities (r=0.68, P<0.0001) and the percentage of intact sperm (Int) with SBH (r=-0.26, P=0.0456). In summary, for assessment of sperm chromatin fragmentation, samples could be equally collected at 13, 18 or 24 mo of age, as results did not vary with age.

Genome-wide association studies of female reproduction in tropically adapted beef cattle.

The genetics of reproduction is poorly understood because the heritabilities of traits currently recorded are low. To elucidate the genetics underlying reproduction in beef cattle, we performed a genome-wide association study using the bovine SNP50 chip in 2 tropically adapted beef cattle breeds, Brahman and Tropical Composite. Here we present the results for 3 female reproduction traits: 1) age at puberty, defined as age in days at first observed corpus luteum (CL) after frequent ovarian ultrasound scans (AGECL); 2) the postpartum anestrous interval, measured as the number of days from calving to first ovulation postpartum (first rebreeding interval, PPAI); and 3) the occurrence of the first postpartum ovulation before weaning in the first rebreeding period (PW), defined from PPAI. In addition, correlated traits such as BW, height, serum IGF1 concentration, condition score, and fatness were also examined. In the Brahman and Tropical Composite cattle, 169 [false positive rate (FPR) = 0.262] and 84 (FPR = 0.581) SNP, respectively, were significant (P < 0.001) for AGECL. In Brahman, 41% of these significant markers mapped to a single chromosomal region on BTA14. In Tropical Composites, 16% of these significant markers were located on BTA5. For PPAI, 66 (FPR = 0.67) and 113 (FPR = 0.432) SNP were significant (P < 0.001) in Brahman and Tropical Composite, respectively, whereas for PW, 68 (FPR = 0.64) and 113 (FPR = 0.432) SNP were significant (P < 0.01). In Tropical Composites, the largest concentration of PPAI markers were located on BTA5 [19% (PPAI) and 23% (PW)], and BTA16 [17% (PPAI) and 18% (PW)]. In Brahman cattle, the largest concentration of markers for postpartum anestrus was located on BTA3 (14% for PPAI and PW) and BTA14 (17% PPAI). Very few of the significant markers for female reproduction traits for the Brahman and Tropical Composite breeds were located in the same chromosomal regions. However, fatness and BW traits as well as serum IGF1 concentration were found to be associated with similar genome regions within and between breeds. Clusters of SNP associated with multiple traits were located on BTA14 in Brahman and BTA5 in Tropical Composites.

Male reproductive traits and their relationship to reproductive traits in their female progeny: a systematic review.

The overall objective of one of the major research programs in the Co-operative Research Centre (CRC) for Beef Genetic Technologies is to 'Improve female reproductive performance' in tropical, northern Australian beef cattle herds. To address this overall objective, a quantitative genetics project focused on investigation of male reproductive traits was designed and linked to three female reproduction-focussed projects, (i) discovery of genes associated with post-partum re-conception and age at puberty; (ii) expression of genes associated with post-partum re-conception; and (iii) early predictors of lifetime female reproductive performance. During the initial planning of this male reproductive traits project, the CRC Scientific Review Committee recommended that the research team investigate and evaluate potentially new, early-life (i.e able to be measured before 2 years of age) predictors of both male and female reproductive performance. To address this recommendation, the following was carried out: (i) criteria for selection of traditional and candidate traits were established; (ii) methodology for tabulation of potential traits/phenotypes that define male and female reproductive function was developed; and (iii) a systematic scientific review of early-life predictors of male and female fertility was prepared. This review concluded that although factors that might be useful in predicting male reproductive performance have been studied for many years, there was relatively little useful information available to meet the objectives of this review. It was also concluded that the direction of future research should be guided not only by previous research which was scarce, but also by speculative hypotheses arising from an understanding of the physiological, endocrinological and genetic processes active in reproduction. A small number of new traits were recommended in addition to traditional sperm morphology, sexual behaviour, anatomical structure and growth traits. Potential additional traits include measurement of gonadotrophin-releasing hormone-stimulated luteinizing hormone (GnRH-stimulated LH); inhibin; several seminal plasma proteins (osteopontin, spermadhesin and seminal plasma proteins BSP30 and phospholipase A(2) could be used in an index); 11ß-hydroxysteriod dehydrogenase; and leptin. In addition, the potential also exists to screen animals for a number of genetic markers associated with age of puberty, follicular recruitment and ovulation rate and genes associated with bovine seminal plasma protein and testosterone production. Insulin-like growth factor-1 (IGF-1) measurements are included because of their association with growth parameters, and an additional analysis demonstrated associations with male and female reproductive traits. Some of these factors have been previously evaluated in small numbers of animals of various species under intensive management conditions. Therefore, there is a need to evaluate these factors in much larger numbers of beef cattle grazing semi-extensive tropical production systems in northern Australia to determine their value in improving beef cattle enterprise profitability through improved herd fertility.

A review of factors that impact on the capacity of beef cattle females to conceive, maintain a pregnancy and wean a calf-Implications for reproductive efficiency in northern Australia.

A review of factors that may impact on the capacity of beef cattle females, grazing semi-extensive to extensive pastures in northern Australia, to conceive, maintain a pregnancy and wean a calf was conducted. Pregnancy and weaning rates have generally been used to measure the reproductive performance of herds. However, this review recognises that reproductive efficiency and the general measures associated with it more effectively describe the economic performance of beef cattle enterprises. More specifically, reproductive efficiency is influenced by (1) pregnancy rate which is influenced by (i) age at puberty; (ii) duration of post-partum anoestrus; (iii) fertilisation failure and (iv) embryo survival; while (2) weight by number of calves per breeding female retained for mating is influenced by (i) cow survival; (ii) foetal survival; and (iii) calf survival; and (3) overall lifetime calf weight weaned per mating. These measures of reproductive efficiency are discussed in depth. Further, a range of infectious and non-infectious factors, namely, environmental, physiological, breed and genetic factors and their impact on these stages of the reproductive cycle are investigated and implications for the northern Australian beef industry are discussed. Finally, conclusions and recommendations to minimise reproductive inefficiencies based on current knowledge are presented.

Bull selection and use in northern Australia. 1. Physical traits.

A systematic breeding soundness examination, including assessment of serving capacity was conducted on 5/8 Brahman, Brahman and Belmont Red bulls, most aged 2-4 years, at 12 different cattle properties across northern Australia. A subset of bulls (n=235) were subsequently mated in various groups, as multiple-sires, to cows and heifers at bull:female ratios of 2.5-6%. The number of calves sired by individual bulls (calf output) was determined by DNA typing for paternity. Overall, the incidence of physical abnormalities that were judged likely to result in reduced reproductive performance was low (5.6-12% of bulls). Measures of key physical traits scrotal circumference, sheath depth (vertical distance from ventral abdominal wall to preputial orifice), and sheath score were moderately to highly repeatable. Except in Belmont Red bulls, liveweight was positively correlated with scrotal circumference (r=0.36-0.78; P<0.01), and both traits increased with age. In 2-year-old Brahman bulls only, percent normal spermatozoa was correlated with scrotal circumference (r=0.34; P<0.05). However, 12 and 15% of Santa Gertrudis and Brahman bulls, respectively, with greater than or equal to the recommended threshold values for scrotal circumferences, had less than 50% morphologically normal spermatozoa. Size and conformation of the umbilicus was associated with conformation of the sheath, and influenced mating ability in 2-year-old Brahman and 3-year-old Santa Gertrudis bulls. In 2-year-old Brahman bulls umbilical cord thickness was positively related (r=0.36; P<0.05) to sheath depth, and negatively related (r=-0.65; P<0.05) to number of mounts and serves in a serving capacity test. In 3-year-old Santa Gertrudis bulls navel (external umbilical scar) score was negatively related to the number of serves (r=-0.53; P<0.01) in a serving capacity test. None of the physical traits measured were consistently included in the final regression model for calf output for each breed.

Bull selection and use in northern Australia. Part 2. Semen traits.

Detailed semen evaluations were carried out on approximately 363 Santa Gertrudis, 5/8 Brahman and Brahman bulls on 12 different properties across northern Australia, as part of systematic breeding soundness examinations. A subset of bulls (n=245) were subsequently mated in groups, to cows and heifers at bull:female ratios of 2.5-6.0%, with the paternity of resulting calves being determined by microsatellite DNA testing. Motility traits of semen and spermatozoa were moderately repeatable and correlated with each other, but were unrelated to calf output. The percentage of morphologically normal spermatozoa in ejaculates was moderately to highly repeatable (e.g. r=0.10-0.64). The most common morphological abnormalities seen were mid-piece abnormalities, in particular, distal mid-piece reflex associated with a cytoplasmic droplet. Semen quality, particularly percent normal spermatozoa, was consistently related to calf output. In general, bulls with <50% normal spermatozoa sired few calves while bulls with the highest calf outputs had >70% normal spermatozoa. The presence or absence of heparin binding proteins in semen did not influence calf output. Semen from 93% of tested bulls was positive for heparin binding proteins. These results confirm that examination of semen, in particular, evaluation of percent morphologically normal spermatozoa, should be included in the breeding soundness examination of bulls.

Bull selection and use in northern Australia. 3. Serving capacity tests.

Serving capacity studies were conducted on Santa Gertrudis, 5/8 Brahman, Brahman and Belmont Red bulls at 10 sites. Modifications, such as providing prior sexual experience, using females in oestrus and comparing restrained with unrestrained females, were made to the standard Bos taurus serving capacity test in a series of experiments with Santa Gertrudis bulls. Providing sexual experience to virgin bulls prior to testing improved the number of serves and libido scores in a serving capacity test. In serving capacity tests that used restrained females, expressions of sexual behaviour of bulls were not improved by having females in oestrus compared with non-oestrus females. There were more sexual behaviours, particularly mounts, in serving capacity tests involving restrained rather than unrestrained females. Summary statistics were collated from over 1100 serving capacity tests, in the 2 months prior to mating, across the 10 sites. In serving capacity tests using restrained females, in bulls of the same age, there were more expressions of sexual behaviours (except interest) in Belmont Red than in Brahman or 5/8 Brahman bulls with Santa Gertrudis bulls intermediate. The percentage of bulls displaying serves tended to increase with age; 82, 83 and 86% of 2-, 3- and > or =4-year-old Belmont Red bulls, respectively, completed 1 or more serves. This compared with 50, 50 and 66% of 2-, 3- and > or =4-year-old Santa Gertrudis bulls and 31% of 2-year-old Brahman bulls. Between 2 and 9% of bulls did not display any sexual behaviour in serving capacity tests with restrained females. Across genotypes, there were few and inconsistent relationships between either mounts or serves and liveweight, condition score, scrotal circumference, testicular tone and sheath measurements. The exceptions were in 2-year-old Brahmans where mounts were negatively correlated (r=-0.61; P<0.05) with umbilicus thickness and in 3-year-old Santa Gertrudis bulls, where serves (r=-0.53; P<0.01) and mounts+serves (r=-0.49; P<0.01) were negatively correlated with navel score. Measures of sexual behaviour were unable to consistently predict calf output of bulls in multiple-sire mating groups in extensive pastoral conditions. The main value of the serving capacity test in these genotypes is to identify whether a bull is capable of natural service.

Bull selection and use in northern Australia. 4. Calf output and predictors of fertility of bulls in multiple-sire herds.

On 10 northern Australian properties, the number of calves sired (calf output) by individual bulls in multiple-sire matings was measured by DNA typing for paternity. There were 235 bulls (92 Santa Gertrudis, 25 5/8 Brahman and 119 Brahman) from 37 multiple-sire mating groups. Number of bulls in groups ranged from 2 to 25 and ages of bulls ranged from 2 to 5 years. Mating periods were for 3-12 months and bull mating percentages were 2.5-6%. In all, there were 4251 calves tested and the resolution of paternity ranged from 92.5 to 100% and averaged 97.7% across all sites. This included 9.9% of calves with no potential sires in any of the mating groups. Of the 235 bulls mated, 58% sired 10% or less calves in each of their respective mating groups with 6% not siring any calves. In contrast, 14% sired over 30% of the calves in each of the respective mating groups. When bulls were mated in groups of 8-24, the maximum percent of calves sired by individual bulls was 26+/-7% (mean+/-S.D.) with a range 11-36%. However, when bulls were mated in groups of 2-7, the maximum percent of calves sired by individual bulls was 59+/-19% with a range 24-94%. Calf output of bulls was moderately repeatable across years at four of five sites. Multiple regression models relating pre-mating measures of physical, seminal and behavioural traits to calf output were developed for the three breed groups. In all, only 138 of the 235 bulls were included in the models (40 Santa Gertrudis, 24 5/8 Brahman and 74 Brahman). Sheath and testicular traits, such as scrotal circumference and testicular tone, were generally not related to calf output, the exceptions being sheath depth in Brahman bulls which was negatively related (P<0.05) and scrotal circumference in 5/8 Brahmans which was positively related (P=0.08) to calf output. Dominance was only included in the 5/8 Brahman model but there was no significant relationship between dominance hierarchy and calf output. Semen motility was only related (P<0.05) to calf output in 5/8 Brahmans. However, measures of semen quality based on spermatozoa morphology were important contributors to calf output in the Santa Gertrudis and Brahman models where percent normal spermatozoa was positively related (P<0.01) to calf output. In Santa Gertrudis and Brahman bulls, measures of sexual behaviour in the serving capacity test were related to calf output. In Santa Gertrudis, these were for the number of displays of sexual interest (P<0.05), and mounts (P<0.01), but not number of serves, whilst in Brahman bulls, libido score was positively related to calf output (P<0.05). The models only explained 35-57% of the variation in calf output.

Bull selection and use in northern Australia. 5. Social behaviour and management.

Calf output of bulls was derived using DNA typing for paternity following multiple-sire mating at two sites in northern Australia. At Swan's Lagoon Beef Cattle Research Station, 12, mixed-age, Brahman cross bulls were continuously mated with an average of 325 females in a 22km2 open-savannah paddock. Water was available in two troughs. Behaviour of the bulls and location of cows were monitored. At Kamilaroi Station, 2- to 2.5-year-old Brahman bulls were introduced to the study. Twenty-four bulls (HIGH%) were mated in an 84km2 paddock for 3.5 months to 411 heifers in 1995/1995 and for 4.5 months to 350 heifers and 320 first-lactation cows in 1995/1996. A second group of 10 bulls (LOW%) selected on reproductive soundness was mated concurrently in a neighbouring 60km2 paddock to 411 heifers in 1995/1995 and to 350 heifers and 298 first-lactation cows in 1995/1996. In each paddock in both years, 300-350 females were expected to cycle during mating. Both paddocks were flat and semi-forested and water was available only at troughs. At both sites, detailed physical and reproductive examinations of all bulls were conducted prior to and post-mating.Calf output of individual bulls was highly variable but repeatable (r=0.6-0.7) between years. Up to 90% of the 270-380 calves resulting from each mating were sired by between 6 and 8 bulls. Reducing from 3.7 to 2.8% bulls:females at Swan's Lagoon did not delay conceptions. At Kamilaroi, reproductively sound bulls achieved an estimated 5-6 conceptions per week over the peak mating period when sufficient cycling females were available. Differences in pregnancy rates between paddocks appeared due to differences in nutrition and it appeared that conceptions were not delayed with LOW% vs. HIGH% bulls. Variance between bulls in calf output was substantially lower when fewer bulls were used. Bull attrition occurred each year in the HIGH% paddock but not in the LOW% paddock. Calf output was unrelated to body condition of bulls. Seven of the 12 bulls in one 2-year period at Swan's Lagoon appeared to restrict their movement range to 250-750ha for 90-100% of the time. These ranges expanded when the bull:female ratio was reduced. Only one of the nine bulls remained within a 500ha home range for at least 85% of the time during peak mating in 1998 at 2.8% bulls:females. In previous years with 3.7% bulls:females, up to eight of the 12 bulls had remained within a 500ha home range for 85% of the time. Bull behaviour related to high calf output included restricted movement range, grazing with females at a majority of observations, stable social behaviour, and social dominance. These observations demonstrate that multiple-sire mating of reproductively sound Brahman and Brahman-derived bulls at 2.5% of cycling females will not jeopardise herd fertility under extensive management in northern Australia.

Effects on reproduction of estrous cycle variations, rectal temperatures and liveweights in mated Brahman cross heifers.

Estrous cycle variations and the association of rectal temperature with reproductive measurements and liveweight were examined in 25-month-old 1/2 and 3/4 Brahman heifers (n = 88). The mean cycle length was longer in the 1/2 Brahmari (24.3 days) than in the 3 4 Brahman heifers (21.3 days) due to the length of estrus-metestrus, but the overall difference was not statistically significant. Cycle length was not influenced by cycle number or liveweight. Cycles were classified into 6 types: normal, short, long, anovulatory and those involving embryonic mortality and prolonged diestrus. Only 33.6% of 1/2 Brahman cycles and 36.1% of 3/4 Brahman cycles were of normal duration (18 to 24 days), and 13.3% of 1/2 Brahman and 11.6% of 3/4 Brahman cycles were classified as embryonic mortality cycles. On an individual animal basis, 25.0% and 31.8% of 3 4 Brahman heifers, respectively, had cycles in which embryonic mortality was suspected. Heifers that became pregnant were significantly (P < 0.01) heavier throughout mating and had significantly (P < 0.05) lower mean rectal temperatures. Heifers in which embryonic mortality had occurred were lighter and had significantly (P < 0.01) higher rectal temperatures than heifers in which embryonic mortality had not occurred. Correlations between rectal temperature and ambient temperature were nonsignificant after eliminating the effect of genotype, but rectal temperature was significantly (P < 0.01) negatively correlated with liveweight.

Class I antigens of the bovine major histocompatibility system and resistance to the cattle tick (Boophilus microplus) assessed in three different seasons.

Two consecutive calf crops consisting of 141 three-quarters Brahman/one-quarter Shorthorn cattle were assessed for resistance to the Australian cattle tick Boophilus microplus in May, July and October 1983. Although the level of expressed resistance to artificial infestation varied considerably between seasons, the animals maintained very similar rankings for resistance in all three seasons, and the repeatability of tick resistance ranged from 0.59 to 0.82. The cattle were typed for 30 bovine class I lymphocyte antigens. Antigens W6 and CA31 were associated with susceptibility to artificial tick infestation but none of the other lymphocyte antigens showed strong associations with resistance or susceptibility.

Epidemiological studies of bovine spumavirus.

Bovine spumavirus (BSV) infection is shown to be endemic in some herds in north Queensland. The virus was readily isolated from leucocytes of the majority of mature cattle which were BSV antibody positive (BSV reactors) in the agar gel precipitin test (AGPT). Calves born to dams which were BSV reactors showed no BSV antibody or circulatory leucocyte-associated BSV (CLAB) at birth, but became BSV reactors following ingestion of colostrum, and maintained such evidence of passive immunity until 3-5 months of age. Experimental infection of dams with BSV at 5-7 months of gestation did not cause foetal infection. In groups of young animals at pasture, segregated by age and sex, no evidence of BSV infection occurred, following loss of passive immunity, until 18-24 months of age. At this time, occasional members of some groups (both male and female) became BSV reactors and showed CLAB. At approximately 24 months old, following mixing in groups with older cows, and single-sire mating, 34% of heifers became BSV reactors within 10 weeks. A herd survey indicated 85% of 2-3-year-old cows to be BSV reactors; thereafter, the percentage of reactors declined gradually with increasing age. Using BSV grown in cell culture, transmission of infection by throat spray was the most successful route, 7 of 7 (100%) of susceptible cattle becoming BSV reactors within 8 weeks of spraying. Using the intravenous route, only 2 of 5 (40%) susceptible cattle became BSV reactors, and swabbing of cell culture virus into the prepuce or vagina did not result in infection of 4 BSV-susceptible animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Foetal and calf wastage in Bos indicus cross beef genotypes.

The prevalence, stages and causes of foetal and calf wastage were determined in Brahman (1/2 B, 3/4 B) and Sahiwal (1/2 Sah, 3/4 Sah) and high grade Sahiwal (HGSah) genotypes over a 12-year period. The mean total loss from pregnancy being first diagnosed to weaning was 13.5% (range 9.4 to 19.0%) and was not influenced by cow age. Across all age groups total losses were significantly lower in 1/2 B (11.8%) and 3/4 B (11.7%) than in 1/2 Sah (16.3%) and 3/4 Sah (19.4%) which were in turn significantly lower than in HG Sah (26.5%). Mean prenatal, perinatal and postnatal losses were 3.5%, 4.4% and 5.6%, respectively. For prenatal losses, 42.4% occurred in early-, 34.8% in mid- and 23.0% in late gestation, the prevalence being influenced only by year effects. Perinatal losses were lowest in 3/4 B (2.9%) and highest in 3/4 Sah (8.3%) with other genotypes intermediate. Postnatal losses were lowest in 1/2 B (4.1%) and 3/4 B (5.0%), intermediate for 1/2 Sah (7.2%) and 3/4 Sah (6.2%) and highest in HG Sah (15.5%). In Brahmans, important causes of perinatal and postnatal losses were from unknown causes, and in Sahiwals from bottle teats, unknown causes and factors influenced by the cow. Bottle teats were mostly in cows older than 2 years with the majority being in Sahiwals. A high proportion (44%) of the postnatal losses occurred by 14 days after birth.

Post-weaning coccidiosis in beef calves in the dry tropics: experimental control with continuous monensin supplementation via intra-ruminal devices and concurrent epidemiological observations.

Post-weaning Eimeria zuernii coccidiosis in beef calves was treated prophylactically with monensin from intra-ruminal continuous release devices. The evaluation of four treatments during three consecutive years also allowed observations on the epidemiology of this disease to be made. Monensin was an effective prophylactic and the intra-ruminal devices were a convenient method of continuous administration over a one to two month period. The three years of the trial included two characteristically very dry and one unusually wet weaning period. Coccidiosis was more severe in the dry years suggesting that oocyst challenge was less important than the immunodepressive effect of weaning and dietary stress in precipitating disease. Severe disease also occurred in calves subsequent to elimination of oocyst shedding by monensin during the initial treatment trial. The occurrence of disease in conditions inimical to oocyst survival and development and after treatment suggested a role for arrested endogenous stages.

Epidemiological studies of parvovirus infections in calves on endemically infected properties.

Bovine parvovirus serology and virus excretion were monitored in calves located on three endemically infected North Queensland properties. Maternally derived serum antibody to bovine parvovirus was found to have a half-life of 19 days. On all three properties, calves developed intestinal bovine parvovirus infection with seroconversion soon after weaning. This occurred more promptly where the environment was subject to heavier bovine parvovirus contamination due to management practices. The concurrent presence of moderate levels of residual serum antibody had only minor influence on the onset of the infection. On one beef cattle property, onset of intestinal bovine parvovirus infection was associated with an outbreak of post-weaning diarrhoea. Anthelmintic treatment trials indicated that this syndrome was unrelated to helminth burdens, though coccidiosis appeared responsible for occasional subsequent cases of dysentery. It was considered that bovine parvovirus may have significantly contributed to the development of the diarrhoea syndrome, in conjunction with substantial weaning stresses.

Patterns of puberal development in Sahiwal and Brahman cross bulls in tropical Australia. I. Growth and semen characteristics.

Reproductive development was monitored in 13 1 2 Brahman (BX), 20 3 4 BX, 8 1 2 Sahiwal (SX) and 11 3 4 SX crossbred bulls between one and two years of age. Liveweight increased during wet season (0.7 kg/day) and decreased during dry season conditions (-0.25 kg/day). Scrotal circumference (SC) increased from 18.6 to 25.0 cm, but decreased slightly during the later stages of the dry season. First ejaculates containing 50 x 10(6) spermatozoa were obtained from 25% of the bulls at 15.3 months of age and from 84% by two years of age. Sperm motility and morphology showed no consistent pattern, but number of sperm per ejaculate increased and seminal fructose concentrations decreased with advancing age. Significant genotype differences were noted for SC and seminal characteristics. Estimated age and liveweight at puberty were 570, 563, 578 and 542 days, and 251, 265, 247 and 256 kg for 1 2 BX, 3 4 BX, 1 2 SX and 3 4 SX bulls, respectively.

Patterns of puberal development in Sahiwal and Brahman cross bulls in tropical Australia II. LH and testosterone concentrations before and after GnRH.

Plasma LH and testosterone (T) concentrations were measured before (basal) and two hours after (peak) GnRH stimulation in 52 Bos indicus strain bulls between one and two years of age. The animals comprised 13 1 2 Brahman, 20 3 4 Brahman, 8 1 2 Sahiwal and 11 3 4 Sahiwal cross bulls and samples were collected at approximately seven week intervals. Basal- and peak-T concentrations increased between one and two years of age, and basal LH concentrations decreased; no changes in peak LH were noted over time. Peak-T concentrations were significantly correlated with scrotal circumference (SC), sperm per ejaculate and seminal fructose. Significant genotype differences were noted, Sahiwal cross bulls had higher peak-T concentrations at puberty than Brahman cross bulls.

Coccidiosis associated with post-weaning diarrhoea in beef calves in a dry tropical region.

The roles of helminths and coccidia in post-weaning diarrhoea in beef calves in the dry tropics were investigated. Diarrhoea occurred approximately one month after weaning in both anthelmintic treated and untreated calves. The highest numbers of coccidial oocysts were seen 29 days after weaning and 3 necropsies confirmed Eimeria zuernii coccidiosis. In the absence of wet overcrowded conditions, disease may have been precipitated by environmental interactions leading to suppression of host immunity. Other Eimeria identified were E. bukidnonensis, E. wyomingensis, E. bovis, E. auburnensis, E. cylindrica, E. ellipsoidalis and E. subspherica. The last 5 species are believed not to have been previously documented in Australia. The presence of E. canadensis was strongly suspected.

Tick resistance and the major histocompatibility system.

199 3/4 Brahman 1/4 Shorthorn cattle, whose resistance to the cattle tick Boophilus microplus had been determined by clinical trial, were tested for 19 bovine lymphocyte antigens. Of eleven antigens detected at a frequency greater than 5%, two were found to be associated with tick resistance and a third was associated with susceptibility to ticks. However, none of these antigens accounts for more than 2.1% of the variance, indicating that these associations are not strong.