Cattle breed-variation in infestation by the horn fly Haematobia irritans

A study was carried out to assess the resistance of pure and cross-bred groups of cattle to the horn fly Haematobia irritans (Linnaeus) (Diptera: Muscidae) in northern Argentina. Pure-bred cattle were Criolla, Iberian Bos taurus Linnaeus (Artiodactyla: Bovidae) and Nellore, Bos indicus Linnaeus (Artiodactyla: Bovidae). Cross-bred cattle were Hereford, British B. taurus (34%) X Nellore (66%) and Hereford (66%) X Nellore (34%). All were heifers and animals were maintained in two groups, each containing a mixture of pure and cross-breeds. The lowest fly numbers were found on Criolla heifers and the highest on Hereford X Nellore cross-breeds. However, it could not be determined from this study whether this was a consequence of breed and/or size, as Criolla heifers were lighter than the corresponding Hereford X Nellore heifers. Fly numbers on the heifers followed an approximately negative binomial distribution. However, the ranking of individual animals in their level of infestation within subgroups was not consistent. Hence, culling the most infested heifers on any given date would at best give only a small improvement in H. irritans control.     

Estimate of the population of preantral follicles in the ovaries of Bos taurus indicus and Bos taurus taurus cattle

The number of oocytes recovered from Bos taurus indicus females subjected to ovum pick-up averaged two to four times greater compared to Bos taurus taurus females. The objective of the present study was to test the hypothesis that this difference in oocyte yield was due to more preantral follicles in the ovaries of Bos indicus females. Ovaries (n = 64) from Nelore (Bos indicus) fetuses (n = 10), heifers (n = 12), and cows (n = 10), and Aberdeen Angus (Bos taurus) fetuses (n = 10), heifers (n = 12), and cows (n = 10) were cut longitudinally into halves, fixed, and processed for histological evaluation. The number of preantral follicles was estimated by counting them in each histological section, using the oocyte nucleus as a marker and employing a correction factor. The average number of preantral follicles in the ovaries of Bos indicus vs Bos taurus was (mean ± SD) 143,929 ± 64,028 vs 285,155 ± 325,195 for fetuses, 76,851 ± 78,605 vs 109,673 ± 86,078 for heifers, and 39,438 ± 31,017 vs 89,577 ± 86,315 for cows (P > 0.05). The number of preantral follicles varied greatly among individual animals within the same category, as well as between breeds. In conclusion, we inferred that the higher oocyte yield from Bos indicus females was not due to a greater ovarian reserve of preantral follicles. Therefore, mechanisms controlling follicle development after the preantral stage likely accounted for differences between Bos indicus and Bos taurus females in number of oocytes retrieved at ovum pick-up.     

Reproductive response in supplemented heifers in the humid tropics of Costa Rica

To evaluate the effect of two supplementary diets to determine the consequence on productive and reproductive performance in heifers (Bos indicus x Bos taurus) averaging between 24 and 36 months of age and grazing tropical pastures, two trials were conducted. Thirty animals (initial BW 325.1+/-33.6 kg) were divided in two groups in the initial study: supplemented (SG) and control (CG); SG received a concentrate (5.5% CP and 2.85 Mcal/kg of DE dry matter basis) at 1% of body weight (BW). In the second study, 45 heifers (initial BW 332.6+/-29.3 kg) were assigned in two treatments, with the same amount of supplement (1% BW) but with a greater nutrient content (13% CP and 3.15 Mcal/kg of DE). The proportion of animals with a corpus luteum at the end of each study was greater in the supplemented groups (P<0.05). Ovarian follicular dynamics was similar between groups in the first study, but in the second study there were more heifers in the SG group with follicles larger than 9 mm in diameter (P<0.05). Pregnancy rate was similar for SG and CG (P>0.05). The response to a regimen of estrous synchronization in both trials was numerically superior in the SG group. No differences were observed in the length of estrus. Daily gain and body condition score were similar for supplemented and control groups (0.27 compared with -0.06 in the first study and 0.90 kg compared with 0.60 in the second study, respectively). Dietary supplementation improved the number of animals initiating estrous cycles and the expression of estrus when compared with unsupplemented control heifers. The dietary regimens imposed in these studies appear to be an adequate for the management of growing heifers destined to a reproductive program.     

Evaluation of two progestogen-based estrous synchronization protocols in yearling heifers of Bos indicus × Bos taurus breeding

Yearling Bos indicus × Bos taurus heifers (n = 410) from three locations, were synchronized with either the Select Synch/CIDR+timed-AI (SSC+TAI) or 7-11+timed-AI (7-11+TAI) treatments. On Day 0 of the experiment, within each location, heifers were equally distributed to treatments by reproductive tract score (RTS; Scale 1-5: 1 = immature, 5 = estrous cycling) and body condition score. The 7-11+TAI treatment consisted of melengestrol acetate (0.5 mg/head/d) from Days 0 to 7, with PGF_2α (25 mg im) on Day 7, GnRH (100 μg im) on Day 11, and PGF_2α (25 mg im) on Day 18. The SSC+TAI heifers received the same carrier supplement (without MGA) from Days 0 to 7, and on Day 11 they were given 100 μg GnRH and an intravaginal CIDR (containing 1.38 g progesterone). The CIDR were removed on Day 18, concurrent with 25 mg PGF_2α im For both treatments, estrus was visually detected for 1 h twice daily (0700 and 1600 h) for 72 h after PGF_2α, with AI done 6 to 12 h after a detected estrus. Non-responders were timed-AI and received GnRH (100 μg im) 72 to 76 h post PGF_2α. The 7-11+TAI heifers had a greater (P < 0.05) estrous response (55.2 vs 41.9%), conception rate (47.0 vs 31.3%), and synchronized pregnancy rate (33.5 vs 24.8%) compared to SSC+TAI heifers, respectively. Heifers exhibiting estrus at 60 h (61.7%) had a greater (P < 0.05) conception rate compared to heifers that exhibited estrus at ≤ 36 (35.3%), 48 (31.6%), and 72 h (36.2%), which were similar (P > 0.05) to each other. As RTS increased from ≤ 2 to ≥ 3, estrous response, conception rate, synchronized pregnancy rate, and 30 d pregnancy rate all increased (P < 0.05), irrespective of synchronization treatment. In conclusion, the 7-11+TAI treatment yielded greater synchronized pregnancy rates compared to SSC+TAI treatment in yearling Bos indicus × Bos taurus heifers.     

Animal and temporal effects on ovarian follicular dynamics in Brahman heifers

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 < 0.05). There were significant animal effects on a number of ovarian characteristics including number of dominant follicles per oestrous cycle (P < 0.001), with one heifer having four dominant follicles in more than a third of oestrous cycles observed. In addition, changes in daylength over the 10 month period were related to changes in duration of the interovulatory interval, persistence and maximum diameter of CL and size of ovulatory follicles. Liveweight change over the same period was related to changes in maximum diameter of the first dominant follicle.     

Patency of the cervical canal in cross-bred female Zebu x Brown Swiss selected for non-surgical recovery or transfer of embryos

A test to evaluate the degree of patency of the cervical canal was carried out on 260 Zebu x Brown Swiss cows and heifers that were assigned for non-surgical recovery or transfer of embryos. A total of 204 females (78.5%) were found to have a patent cervix (P <0.05), compared with 56 (21.5%) which had a blockage or obstruction when an attempt was made to pass a catheter through the cervical canal or the "cervical knot" of the purebred and cross-bred Zebu cows. A smaller percentage (12.89%) of non-patent cervices (P <0.05) were detected in multiparas, compared with 25 and 27.5% in primiparas and nullipara, respectively. The test for cervical patency is carried out on Bos indicus cattle to screen the animals before non-surgical embryo transfer manipulations and to attain better results in the application of non-surgical recovery and transfer of embryos.     

Plasma concentration of progesterone during normal estrous cycles and following prostaglandin F(2alpha) treatment of Bos indicus and tropic-adapted Bos taurus heifers

The introduction of the use of prostaglandin F(2alpha) (PGF(2alpha)) to synchronize estrus in cattle adapted to the tropics suggests a need to investigate the endocrine response to this treatment. Progesterone (P) concentrations in blood plasma of Bos indicus and tropic-adapted Bos taurus heifers during normal estrous cycles and following estrus synchronization were compared. After PGF(2alpha) administration, the heifers were divided into two groups on the basis of response to treatment. Mean P levels in heifers showing estrus after the first injection ranged from 1.0-3.0 ng/ml, decreasing to 0.2-0.4 ng/ml 24 to 48 hr after treatment. The second group exhibited estrus only after the second PGF(2alpha) injection and had low P (0.2-0.9 ng/ml) in plasma before the first injection. Mean peak P levels in both groups 8 to 12 d after the first injection in the periestrous period were not different from values in the same heifers at similar periods of the preceding control estrous cycle. Neither the tropical location nor breed affected the luteolytic effect of PGF(2alpha).     

Artificial insemination of subtropical commercial beef cattle following synchronization with cloprostenol (ICI 80996): II. Estrous response

Data collected from two controlled breeding field trials involving 561 Bos indicus x Bos taurus cows and heifers were analyzed for estrous and fertility response following a cloprostenol ICI-80, 996 (CLP) synchronization regime. Fertility data were discussed in a companion paper (1). In Trial 1, 128 animals were assigned to four treatments: 1) controls which were inseminated at the naturally occurring estrus; 2) Animals artificially inseminated at approximately 72 hr and 96 hr following a second CLP; 3) Animals artificially inseminated at approximately 72 hr following a second CLP; and 4) Animals artificially inseminated approximately 12 hr after detection of estrus post-second CLP. Trial II included 391 heifers distributed among six treatments: 1) Artificially inseminated between 70 and 90 hr post-second CLP; 2) Sham inseminated between 70 and 90 hr post-second CLP; 3) Processed with no manipulation of the genital tract between 70 and 90 hr post-second CLP; 4) Artificially inseminated approximately 12 hr after the detection of estrus following a second CLP; 5) Artificially inseminated at the naturally occurring estrus and 6) Non-treated heifers exposed to fertile bulls. Cloprostenol ICI-80996 was effective (P < .01) in synchronizing estrus in comparisons of treated vs non-treated controls in Trials I and II (82 vs 29%; 57 vs 19%, respectively). However, a significant reduction in the expression of estrus was observed following Timed AI when compared to heifers bred 12 hr after detection of CLP-induced estrus in Trial II (37 vs 54%, P < .05). The authors conclude that a single timed insemination of Brahman crossbred heifers suppresses the behavioral expression of estrus. Other evidence (1) indicates that the fertility during this period is similarly reduced.     

Ovarian follicular dynamics in Nelore breed (Bos indicus) cattle

The most common beef cattle raised in Brazil is the Nelore breed (Bos indicus). Information obtained by ultrasonography on follicular growth in Bos taurus cattle has been accumulating rapidly. However, there are few publications to date on follicular development in Bos indicus breeds. The follicular dynamics in Nelore heifers and cows during natural or prostaglandin (PG)-induced estrous cycle were studied. From the detection of estrus onward, all animals were examined daily by ultrasonography for one (n = 35) or two (n = 10) consecutive estrous cycles. The follicular dynamic in Nelore cattle was characterized by the predominance of 2 follicular waves in the cows (83.3%, n = 18, P < 0.05) and 3 waves in the heifers (64.7%, n = 16, P < 0.05). Most of the cattle observed over 2 consecutive estrous cycles presented the same pattern of follicular waves in the first and second cycle, and only 30% showed variation in the number of waves from one cycle to the other. Most of the follicular parameters analyzed were not affected by PG treatment or age but were altered by follicular waves. Consequently, data on cows and heifers were combined according to the number of follicular waves. The ovulatory follicle was larger than the other dominant follicles (P < 0.05), and the ovulatory wave was shorter than the preceding waves (P < 0.05). The interovulatory interval was longer in animals showing 3 waves than those exhibiting 2 waves (P < 0.05). Maximum diameter of the dominant follicle (around 11 mm) and of the corpus luteum (CL, approximately 17 mm) were smaller than those reported for European breeds. In conclusion, the results demonstrate that although the dominant follicle and corpus luteum are smaller than in European breeds, the follicular dynamics in Nelore cattle were similar to those observed in European breeds and were characterized by 2 or 3 follicular waves for cows and heifers, respectively, during the natural or prostaglandin-induced estrous cycle.     

Effect of early luteolysis in progesterone-based timed AI protocols in Bos indicus, Bos indicus x Bos taurus, and Bos taurus heifers

The objective of this study was to evaluate the effects of treatment with an intravaginal progesterone-releasing device (CIDR) and estradiol benzoate (EB) on follicular dynamics in Bos indicus (n=23), Bos taurus (n=25), and cross-bred (n=23) heifers. To assess the influence of reduced serum progesterone concentrations during 8 days of treatment with a progesterone-releasing device on follicular dynamics, half of the heifers received PGF at CIDR insertion (Day 0; 3 x 2 factorial design). Mean (+/-S.E.M.) serum progesterone concentrations during CIDR treatment varied (P<0.05) among genetic groups: B. indicus (5.4+/-0.1 ng/mL), B. taurus (3.3+/-0.0 ng/mL), and cross-bred (4.3+/-0.1 ng/mL). Maximum diameter of the dominant follicle (DF) was smaller (P<0.01) in B. indicus heifers (9.5+/-0.5 mm) than in cross-bred (12.3+/-0.4 mm) or B. taurus heifers (11.6+/-0.5 mm). B. indicus experienced lower (P<0.01) ovulation rate (39.1%) than did B. taurus (72.7%) and cross-bred (84.0%). Heifers treated with PGF on Day 0 had lower (P<0.05) serum progesterone concentrations during progesterone treatment. The PGF treatment on Day 0 increased (P<0.01) the diameter of the DF (11.9+/-0.4 mm vs. 10.5+/-0.4 mm). Moreover, greater (P=0.02) ovulation rates (78.8 vs. 54.0%) occurred in heifers treated with PGF on Day 0. In summary, B. indicus heifers had greater serum progesterone concentrations, smaller DF diameter, and a lower ovulation rate compared to B. taurus heifers. Prostaglandin treatment on the day of CIDR insertion reduced serum progesterone during treatment, and resulted in increased maximum DF diameter and ovulation rate.     

Estradiol regulation of luteinizing hormone secretion in heifers of two breed types that reach puberty at different ages

The working hypothesis was that 17 beta-estradiol (E(2)) negative feedback on the hypothalamic-pituitary axis in regulation of LH secretion decreases during peripuberty in heifers of 2 different genotypes. We investigated whether Bos indicus heifers had a period postpuberty, as compared with prepuberty, of greater E(2) inhibition of LH secretion at a time when heifers of this genotype have been reported to have a period of anestrus. Prepubertal heifers 9 mo of age of 2 genotypes (B. indicus and B. taurus) were assigned to 3 groups (6 animals/group) to either remain intact (control), be ovariectomized, or be ovariectomized and implanted with E(2). Variables evaluated from 10 to 28 mo of age were circulating concentrations of progesterone (P(4)), presence of corpora lutea, and pulsatile pattern of LH release. Results confirmed that B. taurus heifers attained puberty at younger ages (P < 0.001) and at lower live weights (P = 0.015) than did B. indicus heifers (507 +/- 37 days of age vs. 678 +/- 7 days of age; 259 +/- 14 kg vs. 312 +/- 11 kg; respectively). There was cessation of E(2) inhibition of LH pulses coincident with the onset of puberty in heifers of both breed types but at a much younger age in B. taurus heifers. There was no evidence of enhanced negative feedback of E(2) on LH secretion subsequent to puberty in B. indicus heifers nor was there cessation of estrous cycles in control heifers of either breed type after puberty.     

A field investigation of the effects of bovine viral diarrhea virus infection around the time of insemination on the reproductive performance of cattle

During a study of methods of synchronizing estrus in Bos indicus cattle, blood was collected from 169 heifers and 38 cows 2 to 3 days prior to artificial insemination (AI), and then again at Day 51 and Day 210 after AI to determine the incidence of infection with bovine viral diarrhea (BVD) virus. Prior to insemination 53 and 68% of the cows and heifers, respectively, were seronegative to the BVD virus. At Day 51 after AI, 70 and 32% of the seronegative cows and heifers, respectively, had seroconverted; but between Day 51 and Day 210, only 17 and 3% of the seronegative cows and heifers, respectively, had seroconverted. The Day- 51 pregnancy rate of cows which were immune (seropositive) to BVD virus infection at the time of AI was similar to the rate of the cows which became infected around the time of AI. However, the pregnancy rate of the immune heifers (44%, n=54) was significantly (P=0.04) greater than the rate of the heifers which became infected around the time of AI (24%, n=37). It was concluded that infection of susceptible females with BVD virus around the time of AI may significantly lower the pregnancy rate.     

Comparison of four synchronization protocols for fixed-time bovine embryo transfer in Bos indicus x Bos taurus recipients

The objective was to evaluate the effects of 400 IU of eCG given on Days 5 or 8 of an estrus synchronization protocol with progesterone-releasing intravaginal devices (PRID) and estradiol benzoate (EB), in recipients for fixed-time embryo transfer. A secondary objective was to determine the effects of injectable progesterone (given concurrent with EB treatment). Three-hundred-and-four crossbred Bos taurus x Bos indicus beef heifers were randomly assigned to one of four treatment groups (2 x 2 factorial design). At unknown stages of the estrous cycle (Day 0), all heifers received a progesterone-releasing intravaginal device (PRID), plus 2mg of EB i.m., with or without a concurrent treatment of 50mg of progesterone i.m. Heifers were further subdivided to receive 0.15 mg of d-cloprostenol (PGF) i.m. and 400 IU of eCG i.m. on Days 5 or 8. In all heifers, intravaginal devices were removed on Day 8 and 1mg of EB was given i.m. on Day 9 (Day 10 was arbitrarily considered the day of estrus). On Day 17, all heifers with >1 CL or a single CL with a diameter > or =18 mm (based on ultrasonographic examination), received an in vitro produced (IVP) embryo by non-surgical transfer. On Day 17, there was an effect of day of eCG administration on the number of CL (1.35 +/- 0.08 versus 1.13 +/- 0.04, for Day 5 versus Day 8, respectively; P = 0.02) and (in a subset of 154 heifers) mean (+/-S.E.M.) plasma progesterone concentrations (2.41 +/- 0.26 versus 1.74 +/- 0.19 ng/mL; P = 0.03). Although the proportion of recipients transferred/treated and pregnant/transferred did not differ among groups, the proportion of recipients pregnant/treated tended (P = 0.1) to be higher in heifers treated with eCG on Day 5 versus Day 8 (47.0% versus 40.7%, respectively). Progesterone treatment had no significant effect. In conclusion, treatment with eCG (and D-cloprostenol) on Day 5 significantly increased the number of CL and plasma progesterone concentrations and tended to increase pregnancy rates, although progesterone treatment had no significant effect.     

Efficacy of either a single or split treatment of PGF2alpha after a 14 day melengestrol acetate treatment to synchronize estrus and induce luteolysis in Bos indicus x Bos taurus heifers

Two experiments evaluated a modified delivery of prostaglandin F2alpha (PGF2alpha) after a melengestrol acetate (MGA) treatment in Angus and Bos indicus x Bos taurus (BI) heifers. Experiment 1 was replicated three times with yearling BI heifers (n = 695). Heifers received MGA (0.5 mg head(-1) day(-1)) for 14 days. In Replications 1 and 2, heifers received either 25 mg of PGF2alpha im 19 days after MGA (single) or 12.5 mg of PGF2alpha im 19 and 20 days after MGA (split). In Replication 3, heifers received the same treatments, with PGF2alpha initiated either 18 or 19 days after MGA. Estrus was detected for 72 h after PGF2alpha, with AI commencing 8-12 h after a detected estrus. Heifers not observed in estrus by 72 h were timed-AI concomitant with GnRH (100 microg im). Heifers from Replication 2 (n = 146) had blood samples collected at the initial PGF2alpha and at timed-AI to determine corpus luteum (CL) regression by evaluating plasma progesterone concentrations. The interval from MGA withdrawal to PGF2alpha did not have a significant effect on any variable in Replication 3 and there were no treatment by replication effects for any variables, therefore data were pooled. Modifying the PGF2alpha treatment from a single treatment to two treatments on consecutive days increased (P < 0.05) 72 h estrous response (43.2% versus 50.1%), timed-AI (23.9% versus 33.5%) and total-AI pregnancy rates (34.5% versus 42.5%), and CL regression (79.1% versus 92.5%), respectively. In Experiment 2, yearling Angus (n = 66) and 2-year-old BI (n = 68) heifers were synchronized as per Experiment 1 (with the initial PGF2alpha 19 days after MGA). Neither breed nor PGF2alpha treatment effected (P > 0.05) 72 h estrous response, total-AI pregnancy rate, or CL regression rate. In conclusion, treating yearling BI heifers with split treatments of PGF2alpha (given on two consecutive days) improved estrous response and pregnancy rates by increasing PGF2alpha-induced luteolysis.     

Influences of diet during gestation on potential postpartum reproductive performance and milk production of beef heifers

The influences of nutritional protein and energy during early and mid pregnancy on milk production and postpartum reproductive parameters were determined in 70 beef heifers of two composite breeds (Bos indicus X Bos taurus). At artificial insemination (AI), heifers were divided into four dietary treatment groups identified by the level of protein, and to a lesser extent energy, fed during the first and second trimesters: high/high (HH), high/low (HL), low/high (LH), and low/low (LL). Milk production was lower in the heifers receiving high treatment in first trimester than that in heifers receiving the low treatment (P = 0.01). Milk production was negatively associated with dam body condition score (BCS; P = 0.01), nonesterified fatty acids (P = 0.001), and leptin (P = 0.02) and positively associated with urea (P < 0.001) concentrations during lactation. Increased dietary protein in the first trimester increased or decreased concentrations of colostral protein dependent upon genotype (P = 0.03). Colostral protein was positively associated with bovine pregnancy associated glycoprotein from late gestation (P = 0.007). Milk fat was negatively associated with BCS (P = 0.007) and influenced by genotype (P = 0.003). Dietary treatment did not affect the postpartum reproductive performance of beef heifers. Gestation length (P < 0.001) and the postpartum interval to first estrus (PPI; P = 0.02) were positively associated with calf size. Placental size was negatively associated with placental expulsion time (P < 0.01). Prepartum BCS of the heifers was negatively associated with PPI (P = 0.01). Overall, high levels of nutrition during early gestation are detrimental to milk production in beef heifers.     

Morphological characterization of follicle deviation in Nelore (Bos indicus) heifers and cows

Follicle diameter deviation is defined as the beginning of the differential change in growth rates between the largest and next largest follicles subsequent to wave emergence and is considered a key component of follicle selection. Follicle selection has been extensively studied in European breeds of cattle (Bos taurus) but has not been critically studied in Zebu breeds (Bos indicus). The objectives of the present study were to determine and compare the morphological characteristics of deviation associated with the first post-ovulatory wave (Wave 1) of the estrous cycle in Nelore heifers (n=8) and nonlactating cows (n=11). Beginning on the day of ovulation (day 0), the three largest follicles (F1-F3, respectively) were individually tracked every 12 h for 6d using transrectal ultrasonography. In individual animals, deviation was determined graphically using visual inspection of the diameter profiles of F1, F2 and sometimes F3 (observed deviation) and mathematically using segmented regression analysis of the diameter differences between F1 and F2 or sometimes F3 (calculated deviation). Mean day of emergence of Wave 1 when F1 reached >3 mm (approximately 1 d after ovulation) and growth rate of F1 during deviation (approximately 1.4 mm/d) were not significantly different between heifers and cows. The results of determining the beginning of deviation within heifers and cows using the observed and calculated methods were not significantly different. Averaged over both methods, diameter deviation occurred 2.8 d after ovulation when F1 reached 5.7 mm in heifers, and 2.4 d after ovulation when F1 reached 6.1 mm in cows. In conclusion, the emergence of Wave 1 and growth rates and diameters of the future dominant follicles at the beginning of deviation were similar in Nelore heifers and nonlactating cows, regardless of the methods used to determine deviation. Relative to Holstein cattle, emergence of Wave 1 appeared to occur about 1 d later and diameter of the future dominant follicle at the beginning of deviation was about 2 mm smaller in Nelore.     

Pubertal traits and seasonal variation of the sexual activity in Brahman, Hereford and crossbred heifers

Age and weight at puberty and the pattern of ovulatory activity from puberty to 26 mo of age were studied in a total of 55 Brahman (B), Hereford (H), 5/8B-3/8H (B(5)H(3)) and 5/16B-11/16H (B(5)H(11)) heifers. The trial was conducted over two years. Heifers were kept during the whole experimental period on a paddock of native pasture at a stocking rate of 0.5 animals per hectare. Vasectomized bulls were used for estrus detection and blood was collected from the heifers twice weekly to monitor ovulatory activity through progesterone level as determinated by RIA. In addition to age and weight at puberty, the weights and dates at the time of cessation and resumption of ovulatory activity were also considered. There were significant effects (P<0.05 or P<0.01) of genetic group (purebreds vs crossbreds) for age and weight at puberty; the difference being 60 d and 18 kg, respectively, favoring the crossbred groups. After puberty attainment, an anestrous period was detected in all Brahman (118.6 d) and B(5)H(3) (113.4 d) heifers; anestrus was observed in only 14% of B(5)H(11) heifers and for a shorter (43.5 days) period. Hereford heifers did not exhibit interruption of ovulatory activity. In Brahman and B(5)H(3) heifers, the pattern of interruption and resumption of estrus and ovulatory activity correlated closely (0.89 and 0.95 for Brahman and B(5)H(3) heifers, respectively) with the photoperiod curve, suggesting the probable influence of this environmental factor on the regulation of estrous cycle activity in heifers with a higher proportion of Bos indicus genes. Thus, the genetic factor needs be taken into account when establishing programs in reproductive management.     

Superstimulation of ovarian follicular growth with FSH oocyte recovery, and embryo production from Zebu (Bos indicus) calves: effects of treatment with a GnRH agonist or antagonist

The capacity of heifer calves of a late sexually maturing Zebu (Bos indicus) genotype to respond to superstimulation with FSH at a young age and in vitro oocyte development were examined. Some calves were treated with a GnRH agonist (deslorelin) or antagonist (cetrorelix) to determine whether altering plasma concentrations of LH would influence follicular responses to FSH and oocyte developmental competency. Brahman calves (3-mo-old; 140 +/- 3 kg) were randomly assigned to 3 groups: control (n = 10); deslorelin treatment from Day -8 to 3 (n = 10); and cetrorelix treatment from Day -3 to 2 (n = 10). All calves were stimulated with FSH from Day 0 to 2, and were ovariectomized on Day 3 to determine follicular responses to FSH and to recover oocytes for in vitro procedures. Before treatment with FSH, heifers receiving deslorelin had greater (P < 0.001) plasma LH (0.30 +/- 0.01 ng/ml) than control heifers (0.17 +/- 0.02 ng/ml), while plasma LH was reduced (P < 0.05) in heifers treated with cetrorelix (0.13 +/- 0.01 ng/ml). Control heifers had a surge release of LH during treatment with FSH, but this did not occur in heifers treated with deslorelin or cetrorelix. All heifers had large numbers of follicles > or = 2 mm (approximately 60 follicles) after superstimulation with FSH, and there were no differences (P > 0.10) between groups. Total numbers of oocytes recovered and cultured also did not differ (P > 0.05) for control heifers and heifers treated with deslorelin or cetrorelix. Fertilization and cleavage rates were similar for the 3 groups, and developmental rates to blastocysts were also similar. Zebu heifers respond well to superstimulation with FSH at a young age, and their oocytes are developmentally competent.     

Follicle deviation and ovulatory capacity in Bos indicus heifers

The objectives of Experiment 1 were to determine the interval from ovulation to deviation, and diameter of the dominant follicle (DF) and largest subordinate follicle (SF) at deviation in Nelore (Bos indicus) heifers by two methods (observed and calculated). Heifers (n = 12) were examined ultrasonographically every 12 h from ovulation (Day 0) to Day 5. The time of deviation and diameter of the DF and largest SF at deviation did not differ (P>0.05) between observed and calculated methods. Overall, deviation occurred 2.5+/-0.2 d (mean +/- S.E.M.) after ovulation, and diameters for DF and largest SF at deviation were 6.2+/-0.2 and 5.9 +/- 0.2 mm, respectively. Experiment 2 was designed to determine the size at which the DF acquires ovulatory capacity in B. indicus heifers. Twenty-nine heifers were monitored every 24 h by ultrasonography, from ovulation until the DF reached diameters of 7.0-8.4 mm (n=9), 8.5-10.0 mm (n=10), or >10.0 mm (n=10). At that time, heifers were treated with 25 mg of pLH and monitored by ultrasonography every 12 h for 48 h. Ovulation occurred in 3 of 9, 8 of 10, and 9 of 10 heifers, respectively (P<0.05). In summary, there was no significant difference between observed and calculated methods of determining the beginning of follicle deviation. Deviation occurred 2.5 d after ovulation when the DF reached 6.2 mm, and ovulatory capacity was acquired by DF as small as 7.0 mm.     

Superovulatory response of dairy cattle (Bos taurus ) in a tropical environment

Dairy (Bos taurus) heifers and cows (n = 40) in a tropical environment were treated during mid-luteal phase using either SUPER-OV(R) or OVAGEN to induce superovulatory response after synchronization of the superovulatory estrus with a synthetic progestagen and cloprostenol (PG). Estrous cattle were inseminated twice using frozen-thawed semen, and embryos were recovered nonsurgically, on-farm, 7 d later. Between initiation of gonadotrophin treatment and recovery of embryos, 4 blood samples per animal were collected from 26 animals for determination of plasma progesterone (P4) concentration. Two (5%), 28 (70%) and 10 (22%) of the animals were observed in estrus 1.5, 2 and 2.5 to 3 d after PG, respectively. There was no difference (P = 0.7) in the number of palpable CL between animals treated with SUPER-OV (7.6 +/- 1.0; n = 18) and those treated with OVAGEN (7.9 +/- 1.1; n = 22). There was also no significant difference (P > 0.05) between Jersey vs Ayrshire breeds or heifers vs cows in the ovarian response as estimated by the number of palpable CL. However, a higher proportion of Ayrshire cattle and donors treated with OVAGEN yielded a higher total number and viable/transferable embryos than Jersey and SUPER-OV-treated cattle. There was a significant (P < 0.05) correlation between the number of CL and total number of embryos (r = 0.65); the number of transferable embryos was also significantly related to the total number of embryos per recovery (r = 0.85; P < 0.05). For 15 animals with normal P4 profiles, the mean (+/-SEM) plasma P4 concentration was 14.4 +/- 0.8, 0.5 +/- 0.2, 5.4 +/- 1.1 and 39.4 +/- 3.0 nmol L at initiation of gonadotrophin treatment, superovulatory estrus and Days 3 and 7, respectively. The mean (+/-SEM) interval between a PG injection given after embryo recovery and the induced estrus was 7.1 +/- 0.7 d (range 3 to 14 d) and the length of the superovulatory cycle was 24.1 +/- 3.2 d (range 12 to 35 d).