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The reproductive performance of beef cattle in 5 herds mated all year round in north-west Queensland was examined from 1970 to 1973. Conception rates, conception patterns and branding rates were used to assess annual reproductive performance. The level of loss between the times of confirmed pregnancy and branding was defined. The mean annual conception rate was 83.1% with individual property means ranging from 75.4 to 90.9%. Conception rates for heifers, first calf cows and mature cows were 83.9%, 80.6% and 84.7% respectively. Conception patterns were influenced by seasonal conditions, 68.1% of conceptions occurring in the wet season, between December and May. In this region, conception patterns were highly correlated with rainfall during the previous month. Losses between the times of confirmed pregnancy and branding ranged from 5.9 to 27.7%. Body condition of animals varied and was dependent upon lactation, seasonal and parity effects.

Sperm chromatin status was assessed in 565 Zebu and Zebu crossbred beef bulls in extensive tropical environments using the sperm chromatin structure assay (SCSA). The SCSA involved exposure of sperm to acid hydrolysis for 0.5 or 5.0 minutes, followed by flow cytometry to ascertain relative amounts of double-stranded (normal) and single-stranded (denatured) DNA, which was used to generate a DNA fragmentation index (%DFI). With conventional SCSA (0.5-minute SCSA), 513 bulls (91%) had15 %DFI, 24 bulls (4%) had 15 to 27 %DFI, and 28 bulls (5%) had27 %DFI. In 5.0-minute SCSA, 432 bulls (76%) had15 %DFI, 68 bulls (12%) had 15 to 27 %DFI and 65 bulls (12%) had27 %DFI. For most bulls, the SCSA was repeatable on two to four occasions; however, because most bulls had15 %DFI, repeatability of the SCSA will need to be determined in a larger number of bulls in the 15 to 27 %DFI and27 %DFI categories. The %DFI was negatively correlated with several bull semen parameters and the strongest negative correlation was with normal sperm. There was a strong positive correlation between %DFI and sperm head abnormalities. Based on these findings, most Zebu beef bulls in extensive tropical environments had relatively stable sperm chromatin. Based on the apparent negative correlations with conventional semen parameters, we inferred that the SCSA measured a unique feature of sperm quality, which has also been suggested for other species. Further studies on the relationships between sperm chromatin stability and fertility are required in beef bulls before chromatin status can be used as an additional predictor of the siring capacity of individual bulls in extensive multiple-sire herds.

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.

To compare reproduction in extensively managed, tropically adapted beef cows that were either seropositive or seronegative to Neospora caninum.Longitudinal study of cows within management groups.Compare pregnancy with weaning outcomes for 502 seropositive and 3255 seronegative cows in 25 management groups.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%; P0.05), reproductive wastage after confirmed pregnancy diagnosis (11% vs 10%; P0.05) or weaning rate (67% vs 68%; P0.05) 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%; P0.05). 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.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 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 (P0.001) 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 (P0.001) in Brahman and Tropical Composite, respectively, whereas for PW, 68 (FPR = 0.64) and 113 (FPR = 0.432) SNP were significant (P0.01). 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.

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); 11β-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 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; P0.01), and both traits increased with age. In 2-year-old Brahman bulls only, percent normal spermatozoa was correlated with scrotal circumference (r=0.34; P0.05). 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; P0.05) to sheath depth, and negatively related (r=-0.65; P0.05) 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; P0.01) 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.

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- andor =4-year-old Belmont Red bulls, respectively, completed 1 or more serves. This compared with 50, 50 and 66% of 2-, 3- andor =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; P0.05) with umbilicus thickness and in 3-year-old Santa Gertrudis bulls, where serves (r=-0.53; P0.01) and mounts+serves (r=-0.49; P0.01) 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.

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 with50% normal spermatozoa sired few calves while bulls with the highest calf outputs had70% 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.

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.

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 (P0.05) 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 (P0.05) 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 (P0.01) 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 (P0.05), and mounts (P0.01), but not number of serves, whilst in Brahman bulls, libido score was positively related to calf output (P0.05). The models only explained 35-57% of the variation in calf output.

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. (c) 2012 Elsevier Inc. All rights reserved.

Bos indicus-crossbred calves exposed to natural Babesia bovis infection in wet and dry tropical environments in northern Queensland were tested for antibodies using the indirect haemagglutination (IHA) test. There was evidence of maternal antibodies suggesting endemicity of B. bovis in the 2 environments. Within one year 35.3 to 94.6% of the calves had acquired B. bovis infection. The highest titres and most rapid conversion rates were observed just after the wet season when the tick population was at its peak. Clinical disease was not observed. A third group of Bos indicus-cross cattle of varying ages in a dry tropical environment of north Queensland was examined for antibodies against B. bovis and Anaplasma marginale using the IHA and complement fixation (CF) tests, respectively. B. bovis and A. marginale antibodies were detected on the farm but clinical disease did not occur.

SUMMARY 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.

SUMMARY Five experiments with Leptospira interrogans serovar hardjo vaccine were carried out over a 6-year period in pregnant Brahman-cross and Sahiwal-cross cows in the dry tropics of northern Queensland. The numbers ranged from 127 breeders aged 2 to 5 years in 1972 to 344 breeders aged 2 to 9 years in 1977. Half of the cows were vaccinated twice in midpregnancy except for 1977, when they were vaccinated once. In 1975–1977 inclusive, half of the heifers were given an additional dose of vaccine at commencement of mating. Vaccination caused a significant (P

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.

The prevalence, stages and causes of foetal and calf wastage were determined in Brahman (1/2 B, 3/4 B) and Sahiwal (1/2 San, 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.

In a northern Queensland beef herd where there was serological evidence of L. hardjo infection, lactation failure or wastage between pregnancy diagnosis and branding was 11.9% in animals vaccinated with a single dose of a L. hardjo vaccine and 19.4% in unvaccinated controls. This difference was statistically significant (P less than 0.05). Vaccination caused a significant (P less than 0.05) reduction in rate of lactation failure in heifers but not in other classes of cows.

Forty-six bulls between 17 and 23 months old were used in an investigation of testicular hypoplasia in Bos indicus strain bulls. Testes diagnosed as either unilaterally or bilaterally hypoplastic or normal were obtained at castration and were examined macroscopically and by quantitative histological methods. Estimations of sperm per gram of testis (spg) were made on these 46 bulls and two others. Testis weight was greatest in the 'normal of unilateral' group (106 g). The mean epididymis:testis weight ratio was 0.12 and did not differ significantly between groups. Increased basement membrane thickness, increased vacuolation and giant cells were not features of testicular hypoplasia per se. Seminiferous tubule diameter varied from 118 micrometer in bilaterally hypoplastic testes to 171 micrometer in normal testes. A mean tubule diameter of about 150 micrometer was considered useful for differentiating hypoplastic from normal testes in young post-puberal B indicus crossbred bulls. There was a significant correlation between tubular diameter and spg (r = 0.68, P less than 0.001) suggesting that tubular diameter measurements in histological sections could be used to predict sperm production. Although there was good agreement between clinical diagnosis and spg values, high spg values were found in several small testes which were considered hypoplastic on clinical examination. Examination of both unilaterally and bilaterally hypoplastic testes showed an interesting variation in the ratio of nucleus to cytoplasm in the interstitial cells. This was significantly (p less than 0.05) lower in the testes of normal bulls than in the other clinical groups.