K.W. Entwistle

New clinical uses of GnRH and its analogues in cattle

Abstract Gonadotropin-releasing hormone (GnRH) and its analogues cause an acute secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH) such that concentrations in peripheral blood are elevated for a 3–5 h period. GnRH-induced alterations in the function of the corpus luteum (CL) or follicle appear to be indirect through alterations in LH and FSH secretion. Repeated injections of GnRH during diestrus or single injections late in diestrus will cause acute increases in plasma progesterone and a delay in CL regression. Injections or continual administration of GnRH during early phases of CL development appear to augment CL differentiation and alter subsequent CL function. These effects are attributable to induced increases in LH. Injections of GnRH during the estrous cycle will re-synchronize follicle development owing to ovulation or luteinization of the dominant follicle leading to subsequent recruitment and selection of a new dominant follicle during a 7 day period. Injection of GnRH followed by injection of prostaglandin (PGF 2α ) at 6 or 7 days is a system of estrous synchronization in which follicle development and CL regression are both synchronized and fertility at the induced estrus is good. Injection of GnRH during the luteal phase post-insemination (e.g. Days 12–14) or post-embryo transfer, to alter CL and/or follicular function, has not resulted in a consistent increase in pregnancy rates. An overall assessment of studies that injected GnRH at the time of insemination in first service postpartum cows or in repeat breeders is rather disappointing. Considerable variation existed among studies within both types of cow populations relative to significant differences, directions of pregnancy rate change ( + vs. −), and magnitude of pregnancy rate increases. Recent findings indicate that timing of GnRH injections closer to the onset of estrus may be beneficial in increasing the conception rate. Utilization of GnRH in combination with progesterone and PGF results in an acute treatment sequence to program follicular development, ovulation and a subsequent cycle in cows with follicular cysts. The use of GnRH, with or without PGF, as a reproductive management program in the early postpartum period has not shown a clear improvement in subsequent reproductive efficiency. Development of precise systems to control ovarian function and reproductive efficiency with GnRH and other pharmaceutical agents is possible. However, such advancements must be founded on a clear understanding of GnRH-induced physiological effects and ability to capture any advantage by good management of the farm unit.

Animal and temporal effects on ovarian follicular dynamics in Brahman heifers

Abstract The aims of the current study were to determine if the pattern of ovarian follicular growth and development in Bos indicus heifers is different to that reported in Bos taurus breeds, and to examine the factors that determine which dominant follicle will ovulate. In addition, the extent to which variation in follicular dynamics is attributable to variation between animals and over time was evaluated. The ovaries of 17 Brahman heifers were examined daily by transrectal ultrasonography using a 7.5 MHz transducer for a total of 117 interovulatory intervals over a period of 10 months. Size and position of individual follicles ⪖5 mm in diameter, and size of corpora lutea (CL) were recorded. Circulating progesterone concentrations were determined from plasma samples obtained twice weekly. Although size of dominant follicles and CL within the ovaries of Bos indicus heifers were smaller than reported for Bos taurus breeds, the overall patterns of dominant follicle growth were similar. There were significant correlations between number of dominant follicles occurring prior to ovulation and time of appearance of the second dominant follicle, duration of detection of CL and size of the ovulatory follicle in the preceding oestrous cycle ( P P

Synchronization of estrus and ovulation and associated endocrine changes in Bos indicus cows

The effects of 4 estrus synchronization treatments on intervals to and synchrony of estrus and ovulation, on timing of the preovulatory LH surge and associated changes in plasma progesterone, LH, FSH, and 17beta-estradiol (E(2)) were investigated in 48 Bos indicus cows. Treatment 1 consisted of 2 injections of PGF(2alpha) 14 d apart (n = 12); Treatment 2 of a subcutaneous 3-mg norgestomet implant and an intramuscular injection of 3 mg of norgestomet and 5 mg estradiol valerate, with the implant removed 10 d later (n = 12; norgestomet-estradiol); Treatment 3 of norgestomet-estradiol, with a subcutaneous injection of PMSG given at time of implant removal (Day 10; n = 12); and Treatment 4 of norgestomet implant (as for Treatments 2 and 3) inserted for 10 d, with an intramuscular injection of PGF(2alpha) given at the time of implant removal (n = 12). The experiment was conducted in 2 replicates (24 cows/replicate, 6 cows/group). Estrus, ovulation and timing of the preovulatory surge of LH varied less in cows treated with norgestomet-estradiol and PMSG than in cows in Treatments 1 and 4 (P < 0.008). Treatment with PMSG reduced variation in ovulation times and timing of the LH surge in cows treated with norgestomet-estradiol (P < 0.02). Concentrations of E(2) were higher in cows in Treatments 2 and 3 on the final day of treatment and at about 6 h post ovulation compared with cows in Treatments 1 and 4 (P < 0.05). Different methods for synchronizing estrus did not alter sequential endocrine and behavioral changes in relation to the timing of the LH peak, and the results were consistent with current recommendations for insemination times in Bos taurus cattle.

Effect of season on follicular dynamics and plasma concentrations of estradiol-17β, progesterone and luteinizing hormone in lactating Holstein cows

Lactating Holstein cows (n=16), averaging 64.1 d in milk, were utilized over 4 replicate months (April, June, August and November) in a shade management system to examine the effects of season on follicular dynamics and plasma concentrations of estradiol (E2), progesterone (P4) and luteinizing hormone (LH). Cows were synchronized to estrus using a combination of Buserelin (GnRH, 8 ug) and prostaglandin F2α (PGF2α, 25 mg) given 7 d apart. Follicular development was monitored daily by ultrasonography, and plasma concentrations of E2, P4 and LH measured by radioimmunoassay. The replicate month had no detectable effects on estrus interval (3.1 ± 0.3 d) or percentage of cows (78.1 ± 9.4%) that expressed estrus following GnRH and PGF2α treatment. Preovulatory follicles grew at faster rates (P<0.01) in June (2.0 ± 0.6 mm/d), than in April (1.1 ± 0.6 mm/d), August (1.0 ± 0.6 mm/d) or November (1.2 ± 0.6 mm/d). First wave dominant follicles were consistently larger in April than in June, August and November. The larger and more persistent size of the first wave dominant follicle in April was associated with an earlier regression of the largest subordinate follicle and a sharper decrease in the number of medium size follicles (6 to 9 mm) by Day 9 of the estrous cycle. Conversely, growth of the first wave dominant follicle was slower and the largest subordinate follicle was more persistent in August than in April, June or November. The proestrous rise in plasma E2 occurred faster (P<0.01) in August (10.1 ± 2.1 pg/d) than in April (4.6 ± 2.1 pg/d), June (5.3 ± 2.1 pg/d) or November (5.9 ± 2.1 pg/d). Concentrations of P4 in plasma increased and reached higher (P<0.01) luteal values in August (15.1 ± 0.6 ng/ml) and November (16.0 ± 0.6 ng/ml) than in April (6.1 ± 0.6 ng/ml) and June (10.6 ± 0.6 ng/ml). There was no detectable effect of month on LH pulse characteristics 48 h post-PGF2α. The maximum size of the corpus luteum (CL) was greatest in November and was related positively to diameter of the ovulatory follicle of the preceding cycle. Results indicated that ovarian follicular development and dominance may be altered during summer months. However, it is uncertain whether these changes can be related to the well-documented low breeding efficiency during warmer months of the year in subtropical environments.

The effect of scrotal insulation on spermatozoal morphology and the rates of spermatogenesis and epididymal passage of spermatozoa in the bull.

In five beef bulls spermatogenic cells labelled with (3)H-thymidine were used to estimate the duration of one cycle of the seminiferous epithelium (S.E.) and epididymal passage (E.P.) of spermatozoa after 10 or 20 hr of scrotal insulation designed to increase testicular temperature. From six autoradiographic estimates in three bulls, the mean value for 1 S.E. cycle was 13.4 days (range 12.9 to 13.6 days). Two estimates of E.P. were both 13.5 days. Scrotal insulation altered both semen quality (lowered percentage live and motile spermatozoa, raised percentage abnormal spermatozoa and lowered concentration per ml spermatozoa) and the histological appearance of the seminiferous epithelium. However, the rate of spermatogenesis and epididymal transport was unchanged.

Postpubertal changes in gonadal and extragonadal sperm reserves in Bosindicus strain bulls

Abstract Testes and epididymides were collected from 136 young (1.5 to 3.5 years of age) and 33 mature Bos indicus strain bulls, reared under extensive range conditions in the tropics. Testicular and epididymal sperm reserves and weights increased at different rates in the young bulls. Daily sperm production per gram of testis (DSPG) increased significantly before and epididymal sperm reserves after 2.5 years of age. Testicular and epididymal weights increased linearly with bodyweight. Sahiwal cross bulls had higher testicular and epididymal weights, and greater sperm reserves than did Brahman cross bulls at the same age. Testicular and epididymal weights, daily sperm production and epididymal sperm reserves for mature Brahman bulls were 670g, 61g, 4.9 billion and 25.5 billion respectively. Testis weight was highly correlated with total daily sperm production (DSP) and epididymal weight.

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.

Effects of supplementary feeding and suckling intensity on postpartum reproductive performance of small East African Zebu cows

Small East African Zebu cows (n = 120) which calved during the dry season were used to determine the effects of postpartum nutritional supplementation and suckling intensity on body weight, body condition score, reproductive performance and on calf mortality and preweaning growth performance. Cows were allocated based on calving body weight to either receive a supplementary feed of molasses blocks containing 10% urea or to serve as controls and to be grazed only. Within nutritional groups, cows were further assigned either to continuous suckling or to restricted suckling twice daily. Intact bulls were introduced 21 days after the first cow had calved, and data were collected for 8 months. Calving body weight and body condition score of dams ranged from 186 to 192 kg and from 3.8 to 4.2 and did not differ among the treatment groups. Supplementary feeding significantly (P<0.05) reduced calf mortality from 40 to 20%, while restricted suckling reduced mortality by 13% from 36 to 23%. Body weight and body condition score did not vary between groups during the supplementation period. Over the 8-month period, 65% of supplemented and 53% of control cows exhibited estrus (P>0.05). A significantly (P<0.05) karger percentage of restricted suckled cows showed estrus than of continuously suckled cows (74 vs 42%). The postpartum estrus interval was shorter by 54 days in supplemented than in control cows and by 13 days in restricted than in continuously suckled cows. The pregnancy rate was higher in supplemented than in control cows (37 vs 33%; P>0.05) and in restricted than in continuously suckled (46 vs 22%; P>0.05) cows.

Fetal development and gestational changes in Bos taurus and Bos indicus genotypes in the tropics

Weights of the gravid uterus and fetus as well as the fetal measurements were determined at slaughter for 107 Bos taurus cows grazed on improved pastures and for 70 Bos indicus cows grazed on native pastures in northern Australia. The stage of gestation was assessed from palpation per rectum in early-to-mid gestation and at slaughter and from fetal development characteristics at slaughter. The age and breed of the cow and the sex of the fetus did not significantly affect any of the uterine components or fetal measurements. Growth curves had dominant, positive linear components but negative quadratic ones, which improved the fit, particularly for the later stages of gestation. Uterine components and fetal measurements were highly correlated (0.94 to 0.99). For Bos taurus cows, there were higher estimates at birth for weights of the gravid uterus and the fetus, but estimates for other fetal measurements were similar to those for Bos indicus cows. Major fetal growth occurred during the third trimester, with the length of the foreleg tending to change relatively slowly and the head width quite fast during the first trimester. Correction factors for cow liveweight to adjust to commonality for non-pregnancy were 5, 7, 10, 14, 20, 29, 43 and 65 kg for Bos taurus and 2, 4, 6, 10, 15, 23, 35 and 51 kg for Bos indicus at 2 to 9 months of gestation.