The effect of dexamethasone on immunological memory to tetanus toxoid in sheep.

One of two groups of sheep was immunosuppressed with the glucocorticoid, dexamethasone, at the time of the first but not of the second of two booster vaccinations with tetanus toxoid given at an interval of 28 days. Treatment with dexamethasone decisively reduced the anti-tetanus antibody response to the first booster vaccination and affected both IgM and IgG1 antibody. However, antibody titres increased after the second booster vaccination in the treated sheep and were similar in size to those in the untreated sheep which rose in stepwise fashion after each booster vaccination. The differences in response imply that processes involved in displaying an anamnestic response and recalling previously established memory are sensitive to glucocorticoids. Accordingly, they can be separated from the glucocorticoid-resistant processes that lead to the expansion of immunological memory following multiple exposures to an antigen.

Production of interspecies chimeric calves by aggregation of Bos indicus and Bos taurus demi-embryos.

Two experiments were conducted to produce interspecific chimeric calves by aggregation of B. indicus and B. taurus demi-embryos. In the first experiment, morulae, compacted morulae, and early blastocysts were collected nonsurgically from Brahman (B. indicus) and Friesian (B. taurus) donors. Embryos were bisected and one demi-embryo from each species was placed in a single zone pellucida. In the second experiment, Brahman (B. indicus) and Hereford-Shorthorn (HS) (B. taurus) demi-embryos were aggregated. The resulting 'chimeric embryos' were transferred nonsurgically to synchronous recipients immediately following microsurgery. Of 112 recipients of 112 chimeric embryos, 29 (26%) were pregnant at 60 days. Of these, 24 (83%) produced full-term calves and 5 (17%) aborted between 2 and 5 months' gestation. From the 24 full-term pregnancies, two sets of twins and 22 singleton calves were born. Of the 22 singletons, 15 were chimeric including six bull calves (one Brahman-Friesian and five Brahman-HS) that were overt chimeras. All the overt chimeras resulted from aggregation of halves of early morulae (precompaction).

Patterns of development of gonads, sex-drive and hormonal responses in tropical beef bulls.

The development of different traits was studied in tropical beef bulls of seven genotypes (Brahman, Africander, British and combinations of these) from approximately 500 to 910 d of age. Bulls were raised under pasture conditions without supplementation. At each examination, approximately 2 mo apart, bulls were weighed, palpated (including scrotal and testicular measurement), electroejaculated, and subjected to two libido tests with estrus-induced females. At alternate examinations, plasma luteinizing hormone (LH) was measured at 30 and 150 min post gonadotrophin releasing hormone (GnRH) injection (LH - 30 and LH - 150) and testosterone (T) was measured at 150 min (T - 150). In general, nutritional and environmental stressors appeared to impede bull reproductive development. Scrotal circumference increased nonlinearly, apparently influenced by puberty and average daily gain (ADG). Libido increased overall, albeit nonlinearly also. No apparent marked differences in development of either trait could be attributed to genotype differences, although Brahman bulls tended to display lower sexual interest. The LH-30 level was relatively high (>14 ng/ml) at 500 and 640 d of age, but then dropped markedly at 760 d followed by a slight recovery. The LH-150 level followed a similar pattern, although it was very low at 500 d of age. The T-150 level showed a reverse pattern, being lower initially and higher in the latter part of the study. No apparent genotype differences occurred. Possible contributory influences on these patterns, including the onset of puberty and sexual maturity, season and nutrition, are discussed herein.

Calving rates of Brahman and Brahman-cross cows to fixed-time insemination after treatment with pregnant mare serum gonadotrophin and intravaginal progesterone.

In 2 experiments involving 151 non-lactating and 107 lactating Brahman or Brahman-cross cows, the effects of progesterone treatments (PRID) of 2 durations (7 v 14 days) and injections of pregnant mare serum gonadotrophin (PMSG) at 3 doses (O v 375 v 750 IU) were examined. All cows were inseminated with frozen/thawed semen 54 to 58 and 70 to 74 hours after PRID removal. Calving rates of non-lactating cows (38%) were unaffected by breed or treatment but were higher in previously cyclic than acyclic cows (44% v 19%, P less than 0.025) and in cows which exhibited oestrus after treatment (52% v 23%, P less than 0.001). Calving rates of lactating cows were not affected by these factors but were affected by treatment; PMSG produced no significant effect after a 7-day PRID treatment (33% overall) but increased calving rates after 14-day PRID treatments (22% v 46% v 37% for 0, 375 and 750 IU respectively, P less than 0.10). Comparisons of calving rates of lactating cows treated with 14-day PRID and PMSG and artificially inseminated, and untreated naturally mated cows, showed that treatment significantly increased the proportion of cows conceiving in the first 35 days of the mating period (50% v 23%, P less than 0.025). The results show that treatment with PRID for 14 days and PMSG can overcome post-partum anoestrus in lactating Brahman and Brahman-cross cows leading to significant reductions in the calving to conception interval.

Effects of suckling on oestradiol benzoate induced LH release in post-partum, ovariectomised cows.

Twelve suckling and 12 non-lactating post-partum cows were treated with a progesterone-releasing pessary for 7 or 14 d followed by an injection of 500 micrograms oestradiol benzoate (ODB) 24 h after pessary removal or no injection. Suckling had no significant effect on plasma progesterone concentrations (plasma P4) or plasma luteinising hormone concentrations (plasma LH) during pessary insertion. After pessary removal plasma LH rose in response to ODB. The extent of the rise was similar in suckling and non-lactating cows treated with progesterone for 14 d but was significantly lower in non-lactating than suckling cows (P less than 0.025) treated for 7 d. These findings indicate that suckling increases the responsiveness of plasma LH in post-partum cows following progesterone and oestrogen treatment compared to non-lactating cows and that extended treatment with progesterone can remove this difference.

The effects of duration and concentration of plasma progesterone on the fertility of post-partum cows treated with pregnant mare serum gonadotrophin and intravaginal progesterone.

The effects of duration of insertion of progesterone releasing devices (PRIDs) on the concentrations of plasma progesterone (plasma P4) and the relationship of these with fertility were examined respectively in ovariectomised and intact Bos taurus cattle. In experiment 1, ovariectomised cows (n = 14) were treated with PRIDs to produce high and low plasma P4 for 7 or 14 days giving 4 treatment groups designated H7, H14, L7 or L14. Mean plasma P4 in the H14 and L14 groups were 6.6 and 6.0 ng/ml respectively, during the first week and 5.4 and 3.8 ng/ml respectively, during the second. Mean plasma P4 of 6.3 and 4.2 ng/ml were observed in H7 and L7. In experiment 2, suckled cows (n = 171) received the same PRID treatments as cows in experiment 1, plus either 375 or 750 iu pregnant mare serum gonadotrophin, and subsequent calving rates were recorded. These were significantly lower after L7 than the other treatments (12% v 38%, P less than 0.025). These results showed that fertility after oestrus was closely related to plasma P4 observed before oestrus and indicated that treatment with progesterone for as little as 1 week can produce normal fertility providing plasma P4 attain a critical threshold during treatment.

The use of progesterone administered intravaginally and pregnant mare serum gonadotrophin given by injection in controlled breeding programs in beef and dairy cattle.

Five experiments involving 1,244 cows and heifers were carried out to investigate the factors which might influence the calving performance to fixed-time artificial insemination following intravaginal administration of progesterone (PRID) and intramuscular injection of pregnant mare serum gonadotrophin (PMSG). Factors examined were duration of PRID treatment, time of treatment after calving, time and dose PMSG and lactational status. Experiments 1, 2 and 3 were carried out on milked Friesian cows, experiment 4 on Friesian heifers and experiment 5 on suckled and dry Herefords. All cows were inseminated once with frozen/thawed semen 54 to 58 h or 46 to 50 h after completion of PRID treatment. Overall there was a progressive increase in calving rates with an increase in the duration of treatment from 12 to 14 to 16 days but there was little or no effect of time after calving (4 v 7 weeks) at which treatment was commenced. The poorer calving performance of cows treated for 12 to 14 days was associated with relatively high peripheral levels of plasma progesterone at the time of the PRID removal, suggesting the presence at the end of treatment of residual secretory luteal tissue. There was an effect on calving performance of PMSG given at the time of PRID removal but its effect varied according to the duration of PRID treatment. After 12 days treatments (experiments 1 and 5) PMSG had little effect, whereas after 14 days treatments, 0, 500 and 750 IU PMSG gave calving rates of 27%, 40% and 46% in experiment 2 and 5%, 24% and 38% in experiment 5.(ABSTRACT TRUNCATED AT 250 WORDS)

Fertility indices for beef bulls.

Young (16- to 30-month-old) beef bulls of 6 different genotypes were assessed for production and reproduction traits at different ages and intervals from single-sire mating. Fertility indices, in the form of multiple regression equations using pregnancy rate as the dependent variable, were derived from these assessments using non-orthogonal analyses of variance and covariance. "Among" and "within" genotype fertility indices showed significant correlations with pregnancy rate. "Within" genotype fertility indices showing significant multiple correlations (p less than 0.01) at 11 (r = 0.75), 8 (r = 0.89), 6 (r = 0.86) and 2 (r = 0.80) months prior to mating. It was found that the most important traits to include in the fertility indices were peripheral LH levels following GnRH stimulation, testicular volume, libido and body weight. In general, the fertility indices showed good correlations with bull reproductive performance and were not significantly affected by bull genotype.

Relationships among bovine male and female reproductive traits.

No significant relationship (p greater than 0.05) was found between age at puberty in heifers and the age and scrotal circumference at puberty in related bulls. There was a significant effect (p less than 0.01) of genotype and sire on age at puberty of heifers and a significant effect (p less than 0.05) of genotype on weight at puberty in heifers. There was a significant effect of genotype on age (p less than 0.01) and weight (p less than 0.05) at puberty of bulls. A significant difference (p less than 0.05) in age at puberty of bulls was found between the 2 methods of assessing puberty. It is possible that the assessment of puberty of heifers at 2-month intervals may not have been precise enough to detect such a relationship and/or that the variation in genotypes and ages in this study were too great to establish such a relationship.

The intensity of resistance by mature Merino ewes against Haemonchus contortus and Trichostrongylus colubriformis in single-species and combined-species infection.

Three-year-old, non-lactating and non-pregnant Merino ewes, raised on pasture under a program of strategic treatment with anthelmintic and found to be extremely resistant to "trickle" infection with Haemonchus contortus, were given single-dose infections with either H. contortus or Trichostrongylus colubriformis or both species together. The purpose was to ascertain the intensity of protective immunity against the 2 parasites in sheep with immunity acquired from a presumably slight exposure to infection. To provide a criterion, some infected ewes were immunosuppressed with corticosteroid, dexamethasone. Untreated ewes were extremely resistant to challenge infection with either 15,000 or 150,000 H. contortus or 15,000 T. colubriformis. Surprisingly, when mixed infection was given, egg counts for H. contortus were significantly elevated compared with infection by that species alone. Antibody to antigens from infective larval and adult H. contortus was measured in serum by enzyme-linked-immunosorbent assay (ELISA) during the course of infection. Serum titres against larval antigens were significantly depressed when infections with either H. contortus or T. colubriformis were permitted by immunosuppression with dexamethasone, whereas those against adult antigen were depressed when infection with T. colubriformis was permitted.

Effects of progesterone, oestradiol benzoate and cloprostenol on luteal function in the heifer.

The effects of treatment with intravaginal progesterone (PRID) oestradiol benzoate and cloprostenol at various stages of the oestrous cycle were examined in 2 experiments. In Exp. 1, PRIDs were inserted for 14 days commencing on Day 3, 10 or 17 of the oestrous cycle and half the animals in each group received 5 mg oestradiol benzoate at PRID insertion. Plasma samples were collected daily from the time of PRID insertion for 16 days, then every 4 days for a further 24 days. Samples were analysed for plasma progesterone concentration. In Exp. 2, heifers were treated on Day 10 as in Exp. 1, but half of each treatment group received 750 micrograms cloprostenol at PRID insertion. In Exp. 1, treatment with PRID alone appeared to inhibit endogenous progesterone production when treatment began on Day 3, but not when treatment began on Days 10 or 17. Treatment with oestradiol benzoate at the time of PRID insertion depressed progesterone levels, from about 2-5 days after injection. When treatment began on Day 10, the fall in progesterone levels after oestradiol benzoate was preceded by a marked rise in levels. In Exp. 2, treatment with cloprostenol markedly reduced peripheral concentrations of progesterone and also removed the effect of oestradiol. We suggest that oestradiol benzoate had a luteolytic effect, depressing luteal progesterone production when given on Days 3, 10 or 17 and that when given on Day 10 there was a transitory luteotrophic effect which preceded its luteolytic action.