Contraceptive efficacy of an intra-uterine device in Brahman cattle

The contraceptive efficacy of an intra-uterine device was evaluated using 218 heifers and 212 cows on three north Australian cattle stations. The heifers were aged approximately 2 years and weighed 250-378 kg; the cows were aged 3-16 years and weighed 256-540 kg. All cattle were non-pregnant, non-lactating Brahmans. At the end of the monsoon (wet) season (April-June 1997), the cattle were allocated by stratified randomisation to the three treatments which were untreated controls (n=59), surgical ovariectomy (n=105), or implantation with a bovine intra-uterine device (BIUD; n=266). All cattle grazed and were managed as one group within each station. They were exposed to bulls (4 per 100 females) from soon after treatment until slaughter approximately 12 months later.The BIUD could not be implanted in 25% of heifers and 8% of cows due to narrow or twisted cervices. Correct placement of the BIUDs appeared to be achieved in 57% of heifers and 72% of cows. At slaughter, the devices were incorrectly positioned in 73% of heifers and 49% of cows into which BIUDs had been inserted and that remained non-pregnant. Uterine perforations by the BIUD were observed in 35 and 45% of these heifers and cows, respectively; most perforations appeared to occur during implantation. Low-grade endometritis was observed at slaughter in most BIUD-implanted animals; 2% had pyometra.BIUD animals did not have significantly different growth to that of control or ovariectomised animals, other than when ovariectomy suppressed growth following surgery. Most animals implanted with BIUDs appeared to have normal ovarian function and animals were observed mating. All ovariectomised animals remained non-pregnant. Over 80% of controls were pregnant within 8 months of exposure to bulls, except heifers at one station where pregnancy rate was restricted to 25% as a result of severe nutritional conditions. Pregnancy was diagnosed in 21% of heifers and 33% of cows with implanted BIUDs. The device remained correctly positioned and with no pregnancy diagnosed in the year following implantation in only 2% of heifers and 14% of cows originally allocated.Because of the difficulties of implanting BIUDs, the high frequency of associated uterine injury, the high pregnancy rate in implanted animals, and that growth was unaffected by the presence of a BIUD, it was concluded that the device had poor contraception efficacy and no growth-promotant effect in Brahman cattle.     

Pregnancy rate of Nelore females inseminated with male-sexed semen

Pregnancy rates of Nelore females inseminated with male-sexed semen and conventional semen from the same bulls were evaluated. The females included 433 heifers (2 years old) and 230 non-suckling cows, totaling 663 animals. Average body condition score was 3.5 (1-5 scale). Estrus was induced with prostaglandin F2α. The total pregnancy rate of females inseminated with male-sexed semen of bulls A, B and C was 38.8% (131/338) less (P<0.0001) than the total pregnancy rate observed for females inseminated with conventional semen from the same bulls (57.9% [188/325]). Pregnancy rates of non-suckling cows inseminated with male-sexed semen was 43.3% (49/113), which was similar (P≥0.05) to the values found for heifers inseminated with male-sexed semen from the same bulls (36.4% [82/225]). The pregnancy rate of females inseminated with male-sexed semen was less compared with females inseminated with conventional semen. In addition, there was no significant difference in the pregnancy rate of heifers versus non-suckling cows.     

Estrus synchronization in cattle using estradiol, melengestrol acetate and PGF

In Experiment 1, all cattle were fed MGA (0.5 mg/head/d) for 7 d (designated Days 0 to 6) and given PGF on Day 6. One-half were administered estradiol valerate (EV; 5 mg, im) on Day 0. At Location 1, a higher proportion (P < 0.005) of EV-treated heifers were detected in estrus and bred by AI between Days 7 and 13 than control heifers not receiving EV (27 of 33 versus 15 of 32), but the number of pregnancies (12 vs 10) was not significantly different. Eighty-three of 104 EV-treated and 89 of 106 control cows were inseminated, resulting in 50 and 45 pregnancies, respectively (not significant). At Location 2, cattle were similarly treated and exposed to bulls on Days 7 to 49. Fall pregnancy rate was higher (P < 0.015) for EV-treated than control heifers (44 of 48 vs 33 of 46), but was not significantly different for cows (22 of 26 vs 19 of 23). In Experiment 2, estradiol 17beta (E17beta; 5 mg, im) and progesterone (100 mg, im) were administered on Day 0 (instead of EV). In a third group (designated the PGF group), cattle were bred on Days 0 to 6, and PGF was administered on Day 6 to those not yet bred. For 213 cows, the percentage pregnant to a synchronized estrus was greater in the PGF group (72%) than in either the control group treated with MGA (49%; P = 0.005) or the group receiving MGA and E17beta (54%; P < 0.025). Fall pregnancy rates were 91, 89, and 96% for the 213 cows (not significant) and 89, 93, and 98% for 131 heifers (not significant) in the PGF, MGA and E17beta groups, respectively. In cattle without a functional CL, the average diameter of the largest follicle at Day 6 was 1 to 2 mm smaller in the E17beta + MGA group than in the MGA group (difference significant only in cows at Location 1). Combined for both locations, the synchronized pregnancy rate in heifers without a functional CL on Day 6 was higher (P < 0.05) in the E17beta + MGA group than in the MGA group (11 of 21, 52% versus 4 of 20, 20%). Estrogen treatment caused regression of ovarian follicles with emergence of a new follicular wave. Including estrogen in an estrus synchronization program utilizing MGA and PGF significantly increased fall pregnancy rate in heifers (at 1 location) and the synchronized pregnancy rate of heifers without a functional CL at the time of PGF treatment (combined for both locations).     

Pregnancy percentage following deposition of sex-sorted sperm at different sites within the uterus in estrus-synchronized heifers

Our objective was to assess the effect on heifer pregnancy rate of deposition at three sites within the uterus of frozen-thawed sex-sorted sperm at a fixed time after estrus synchronization. Estrus was synchronized in 209 heifers by administration of PGF2a 14 days apart. At 80-82 h after the second PGF2a injection, X-chromosomes bearing fractions of semen with 2.2 x 10(6) sperm in insemination dose were used for single insemination into the uterine body (UB-AI, n=91) or for intracornual deposition in the middle of the uterine horn (MH-AI, n=57) or close to the utero-tubal junction (UTJ-AI, n=61). The overall pregnancy rate was 43.1%. Pregnancy rates did not differ (P>0.05) among sites of sperm sperm deposition, between the two farms at which the heifers were kept or between the two bulls producing the semen. Within UB-AI, MH-AI and UTJ-AI treatments, pregnancy rates were 41.8%, 49.1% and 39.3%, respectively (P>0.05). Pooled across classes for deposition site, pregnancy rate was 25.1% higher (P<0.01) for heifers showing strong signs of estrus than for heifers showing weak signs of estrus (45.9 versus 20.8%, respectively). Embryonic and fetal loss from diagnosis of pregnancy to term and at calving equalled 5.6%. Of 88 calves of identified sex, 93.2% were female. In conclusion, pregnancy rates of heifers did not differ significantly following deposition of 2.2 x 10(6) sex-sorted sperm 80-82 h after the second PGF2a injection near the utero-tubal junction, in the middle of the horn or into the uterine body.     

Comparison of ovulation, fertilization and embryonic survival in low-fertility beef cows compared to fertile females

The objective was to determine physiological causes of low fertility in beef cows. Fertility was compared between low-fertility cows (34 British cows and 64 Brahman crossbred cows; cows that did not get pregnant when mated to fertile bulls in one or two previous breeding seasons); fertile cows (16 Brahman crossbreds; cows having a calf in several of the preceding breeding seasons), and virgin heifers (45 Brahman crossbreds, 2 yr of age). Females were mated to fertile bulls and killed 3 or 34 d after breeding to obtain reproductive tracts. There were no significant differences among groups in rates of ovulation or fertilization. Overall, 14% of females failed to ovulate and 24% that ovulated failed to undergo fertilization. The proportion of cows that were not detected in estrus before Day 34 of pregnancy was lower (P < 0.01) for low-fertility British cows (5 of 16 cows, 31%) than for other groups, including low-fertility Brahman crossbred cows (23/32, 72%), fertile cows (8/9, 89%), and heifers (21/24, 88%). All cows that did not return to estrus by Day 34 had an identifiable conceptus. The proportion of conceptuses recovered at Day 34 that were classified as normal (weight and length) was lower (P < 0.05) for cows with low fertility (British: 2/5, 40%; Brahman crossbred: 9/23, 39%) than for fertile cows (8/8, 100%) or heifers (18/21; 86%). Similarly, the proportion of cows in which a normal embryo was recovered (cows with normal embryos/number of cows mated) was lower (P < 0.001) for low-fertility British cows (2/16, 13%) and low-fertility Brahman crossbred cows (9/32, 28%) than for fertile cows (8/9, 89%) and heifers (18/24, 75%). In conclusion, cows that were infertile in previous breeding seasons did not experience reduced ovulation or fertilization rates, but had greater embryonic mortality. These data highlighted the importance of ovulation and fertilization failure and embryonic mortality as important determinants of pregnancy success. Moreover, increased embryonic loss after Day 34 contributed to infertility in low-fertility cows.     

Clitoral stimulation and the effect of age, breed, technician, and postpartum interval on pregnancy rate to artificial insemination in beef cattle

An experiment was conducted to assess the effect of clitoral stimulation on pregnancy rate to artificial insemination in 1,856 beef females. For unadjusted data, pregnancy rate to first service in nonstimulated females was 61% for cows and 59% for heifers, and that of females receiving clitoral stimulation was 69% for cows and 58% for heifers. After adjusting the data for the influences of age, breed, technician, and postpartum interval, clitoral stimulation increased first service pregnancy rate in cows (74 +/- 3% vs 59 +/- 3%, P<0.05), but not in heifers (53 +/- 5% vs 57 +/- 5%, P>0.10). Postpartum interval significantly influenced pregnancy rate, regardless of treatment, and clitoral stimulation was beneficial when applied to cows with relatively short postpartum intervals. Variation in the effect of clitoral stimulation on pregnancy rate was observed among technicians. Clitoral stimulation improved pregnancy rate more effectively in 3 to 4 year old cows than in cows 2 years old and cows 5 years of age or older. We concluded that clitoral stimulation at the time of artificial insemination was an effective means of increasing pregnancy rate in cows, but not in heifers.     

Calving difficulty and calf survival in beef cows fed two energy levels

Angus and Hereford yearling heifers (469) were bred to either Hereford or Angus bulls and fed either a high level (13.3 Mcal of ME) or a low level of nutrition (7.3 Mcal of ME) for the last 116 days of pregnancy. The level of calving difficulty did not differ between heifers receiving the high (H) and low (L) level of nutrition the first year (36% and 34%) but was significantly higher (P < .01) in heifers receiving H the second year (33% vs. 16%). Calves born to heifers receiving H weighed at birth 2.7 kg more (P < .01) both years than calves born to heifers receiving L. In the first year of the study the size of the pelvic opening was 10 cm larger 1 week before calving (P < .01) in heifers receiving H than in heifers receiving L. Calf survival was not related to level of nutrition, breed of dam or breed of sire either year of the study.     

Fertility-associated antigen on Nelore bull sperm and reproductive outcomes following first-service fixed-time AI of Nelore cows and heifers

The objective was to determine whether the presence of fertility-associated antigen (FAA) on sperm collected from Nelore (Bos indicus) bulls can be used to assess potential fertility of sperm for use at first-service fixed-time AI (TAI). Six Nelore bulls were selected based on FAA status (FAA-negative: N = 3; FAA-positive: N = 3) and the ability to produce neat semen with ≥ 70% morphologically normal sperm and 60% estimated progressive motility before cryopreservation. In Experiment 1, suckled multiparous Nelore cows (N = 835) were evaluated for body condition score (BCS) and received an intravaginal progesterone device (CIDR) and 2.0 mg of estradiol benzoate (Day 0). On Day 9 the CIDR was removed, 12.5 mg of PGF_2α and 0.5 mg of estradiol cypionate were administered, and calves were removed for 48 h. All cows received TAI on Day 11 (48 h after CIDR removal). Pregnancy per TAI (P/TAI) was not different between FAA-positive and FAA-negative bulls (41.5% vs. 39.3%, respectively). There was an effect of AI technician on P/TAI (36.0% vs. 43.9%; P < 0.05) and BCS tended to affect P/TAI (P = 0.09), as cows with BCS ≥ 2.75 were 1.4 times more likely to become pregnant compared with cows with BCS < 2.75. In Experiment 2, nulliparous Nelore heifers (N = 617) were evaluated for BCS and received a CIDR and estradiol benzoate (2.0 mg) on Day 0. On Day 7, all heifers received PGF_2α (12.5 mg). On Day 9, CIDR inserts were removed and all heifers received estradiol cypionate (0.6 mg) and 200 IU eCG. All heifers received TAI on Day 11 (48 h after CIDR removal). Pregnancy/TAI was different (P = 0.04) between FAA-positive and FAA-negative bulls (33.7% vs. 40.7%, respectively). Presence of FAA on sperm was unsuccessful in assessing the potential fertility of sperm for use in TAI.     

Effect of restricted suckling on pregnancy rates and calf performance in Brahman cows

Seventy-six Brahman cows and first-calf heifers were assigned to one of two groups: 1) normal suckling (34 cows) or 2) twice-daily suckling (45 minutes of suckling each time; 42 cows). Twice-daily suckling was carried out from 30 days postcalving until weaning (seven months). All animals were maintained under artificial insemination for a four-month breeding period. Mean pregnancy rate was 63.06 +/- 0.06% and was influenced by suckling group (P<0.01) and number of parturitions (P<0.05). The pregnancy rates were 33% higher in twice-daily suckled cows. Forty-four percent of the first calf heifers in the twice-daily suckling group became pregnant compared to 9% in the normal suckling group (P<0.01). Twice-daily suckling improved pregnancy rate without depressing preweaning calf performance.     

Embryo quality and andrological study of two subfertile bulls versus five control bulls with normal fertility

Andrological studies and embryo morphology evaluation of superovulated cows were performed on 2 randomly selected subfertile dairy bulls whose semen was used for artificial insemination and on 5 control bulls with normal fertility. Neither sperm motility studies, nor sperm morphology or testicular measurements differed between the subfertile and the control bulls. Altogether 315 ova were recovered from 41 superovulated cows inseminated with semen collected from either the subfertile or the normal control bulls. The spermatozoa of one of the 2 subfertile bulls was shown to have a decreased ability to fertilize superovulated ova, while the other subfertile animal, the bull with the lowest noreturn rate, was found by chromosome analysis to have a reciprocal translocation (60, XY, rcp 20:24), causing embryonic death. We suggest that subfertile bulls should not be used in commercial embryo transfer programs nor in artificial insemination and that andrological studies on subfertile bulls with good sperm motility should include evaluation of 6- to 7-day-old ova from superovulated cows to determine if the fertilization rate is normal or impaired. A chromosome analysis should also be performed when a subjertile bull has a normal fertilization rate of ova.     

Embryo production with sex-sorted semen in superovulated dairy heifers and cows

The aim of this study was to examine the effect of sex-sorted semen on the number and quality of embryos recovered from superovulated heifers and cows on commercial dairy farm conditions in Finland. The data consist of 1487 commercial embryo collections performed on 633 and 854 animals of Holstein and Finnish Ayrshire breeds, respectively. Superovulation was induced by eight intramuscular injections of follicle-stimulating hormone, at 12-hour intervals over 4 days, involving declining doses beginning on 9 to 12 days after the onset of standing estrus. The donors were inseminated at 9 to 15–hour intervals beginning 12 hours after the onset of estrus with 2 + 2 (+1) doses of sex-sorted frozen-thawed semen (N = 218) into the uterine horns or with 1 + 1 (+1) doses of conventional frozen-thawed semen (N = 1269) into the uterine corpus. Most conventional semen (222 bulls) straws contained 15 million sperm (total number 30–45 million per donor). Sex-sorted semen (61 bulls) straws contained 2 million sperm (total number 8–14 million per donor). Mean number of transferable embryos in recoveries from cows bred with sex-sorted semen was 4.9, which is significantly lower than 9.1 transferable embryos recovered when using conventional semen (P ≤ 0.001). In heifers, no significant difference was detected between mean number of transferable embryos in recoveries using sex-sorted semen and conventional semen (6.1 and 7.2, respectively). The number of unfertilized ova was higher when using sex-sorted semen than when using conventional semen in heifers (P < 0.01) and in cows (P < 0.05), and the number of degenerated embryos in cows (P < 0.01), but not in heifers. It was concluded that the insemination protocol used seemed to be adequate for heifers. In superovulated cows, an optimal protocol for using sex-sorted semen remains to be found.     

Use of GnRH agonist implants for long-term suppression of fertility in extensively managed heifers and cows

The ability of gonadotrophin releasing hormone (GnRH) agonist implants to suppress ovarian activity and prevent pregnancies, long-term, was examined in heifers and cows maintained under extensive management. At three cattle stations, heifers (2-year-old) and older cows (3- to 16-year-old) were assigned to a control group that received no treatment, or were treated with high-dose (12 mg, Station A) or low-dose (8 mg, Station B and Station C) GnRH agonist implants. The respective numbers of control and GnRH agonist-treated animals (heifers + cows) at each station were: Station A, 20 and 99; Station B, 19 and 89; Station C, 20 and 76. Animals were maintained with 4% bulls and monitored for pregnancy at 2-monthly intervals for approximately 12 months. Pregnancy rates for control heifers and control cows ranged from 60-90% and 80-100%, respectively, depending on the study site. The respective number of animals (heifers + cows) treated with GnRH agonist that conceived, and days to first conception, were: Station A, 9 (9%) and 336 +/- 3 days; Station B, 8 (10%) and 244 +/- 13 days; Station C, 20 (26%) and 231 +/- 3 days. Treatment with high-dose GnRH agonist prevented pregnancies for longer (approximately 300 days) than treatment with low-dose GnRH agonist (approximately 200 days). In the majority of heifers and cows treated with GnRH agonist, ovarian follicular growth was restricted to early antral follicles (2-4mm). The findings indicate that GnRH agonist implants have considerable potential as a practical technology to suppress ovarian activity and control reproduction in female cattle maintained in extensive rangelands environments. The technology also has broader applications in diverse cattle production systems.     

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

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

Fertility-associated proteins in Nelore bull sperm membranes

The present study was undertaken to evaluate the protein composition of the sperm membranes (SM) of Nelore bulls, assessing protein markers associated with bull fertility, and whether these markers can be used for predicting bull fertility. Samples were obtained of 20 Nelore bulls, with fertility ranked and divided into three groups (greater, normal and least). To rank the bull's fertility weighted classification was used (according to the number of pregnant cows, number of AI cows and number of herds, considering three different breeding seasons), using the PROC GENMOD as a statistical model, with 99% significance. A total of 7897 Nelore cows, randomly distributed among 28 different farms, were considered in the statistical analyses. The bulls were divided into three fertility groups (pregnancy rates): greater (%F > 80), normal (79 < %F > 71) and least (< 68%F) with 3, 13 and 4 bulls, respectively. Two-dimensional gel electrophoresis (2DE) of sperm membranes indicated in 27 spots (SM40, SM53, SM69, SM93, SM102, SM111, SM137, SM138, SM189, SM196, SM201, SM202, SM204, SM225, SM236, SM237, SM239, SM241, SM246, SM247, SM275, SM283, SM342, SM346, SM355, SM372, SM391) was prevalent in the higher fertility group, and just one spot (SM244) was prevalent in the lower fertility group. Spots SM244 and SM239 had their identification defined by PMF/MALDI-MS, as BSP-A3 and aSFP, respectively. Both these proteins showed a great potential for predicting bull's fertility. The amount of aSFP was 8.5 times greater in the sperm membrane protein profile of the higher fertility groups of Nelore bulls. Besides that, the BSP-A3 was 2.5 times greater in the lower fertility group. For the other spots potentially associated with fertility not yet identified, additional tests will be necessary, but it is clear that the 2D electrophoresis of the sperm membrane can be used for a new approach to predict Nelore bull fertility.     

Double freezing of bovine semen

Fertility of bull spermatozoa cryopreserved in large volume by directional freezing technique, thawed, repackaged in straws and refrozen over liquid nitrogen vapor (double freezing, DF) was compared to conventional single freezing in straws (CF). Semen was collected from 6 bulls, 4 of which were selected for the field trial. Each semen collection was split into two parts, one frozen by CF and the other by DF. In vitro semen evaluations included motility (fresh, upon thawing and after 3 h incubation at 37 °C), viability and acrosome integrity. A total of 3610 cows and heifers were randomly inseminated by either CF or DF at about equal numbers. In vitro sperm analysis indicated no difference between CF and directional freezing in large volume and both were superior to DF (P < 0.001). Between-bull variations in fresh semen and in their reaction to CF or DF were apparent. Logistic regression analysis revealed that freezing method, bull, parity and inseminating technician, all had significant effect on pregnancy outcome (P ≤ 0.001 for all). Conception rate (CR) was 32.98% for CF and 28.05% for DF. Only in one bull conception rate by CF was significantly superior to DF (P < 0.05). When divided into heifers, primi- and pluriparous cows, only the difference in CR between the pluriparous cows was significant (P = 0.005). In conclusion, acceptable CR can be achieved by DF technique. These can be improved by selecting suitable bulls. The DF technique can be utilized in storage, sperm sexing and genome resource banking.     

Effect of reproductive tract scoring on reproductive efficiency in beef heifers bred by timed insemination and natural service versus only natural service

The objective was to determine the effects of reproductive tract score (RTS) on reproductive performance in beef heifers bred by timed artificial insemination followed by natural service (AI-NS) or by natural service only (NSO). Angus cross beef heifers (n = 2660) in the AI-NS group were artificially inseminated at a fixed time (5- or 7-day CO-Synch + controlled internal drug release protocol) once, then exposed to bulls 2 weeks later (bull-to-heifer ratio = 1:40–1:50) for the reminder of the 85-day breeding season. Angus cross beef heifers (n = 1381) in NSO group were submitted to bulls (bull-to-heifer ratio = 1:20–1:25) for the entire 85-day breeding season. Heifers were reproductive tract scored from 1 (prepubertal) to 5 (cyclic) 4 weeks before, and were body condition scored (BCS) from 1 (emaciated) to 9 (obese) at the beginning of breeding season. Pregnancy diagnosis was performed 70 days after AI for AI-NS group and 2 months after the end of breeding season for both groups. Heifers in both groups were well managed and of similar age (14.9 ± 0.4 [AI-NS] and 14.7 ± 0.8 [NSO] months). Pregnancy rates (PRs) and number of days to become pregnant were calculated using PROC GLIMMIX and PROC LIFETEST procedures of SAS. Adjusting for BCS (P = 0.07), expressed estrus (P < 0.05), year (P < 0.05), and BCS by year interaction (P < 0.05), the AI-PR was greater for heifers in AI-NS group with higher RTS (P < 0.0001; 40.7%, 48.3%, 57.6%, and 64.6% for RTS of 2 or less, 3, 4, and 5, respectively). Controlling for BCS (P < 0.05), year (P < 0.05) and the breeding season pregnancy rates (BS-PRs) were greater for heifers in the AI-NS group with higher RTS (P < 0.01; 81.2%, 86.5%, 90.4%, and 95.2% for RTS of 2 or less, 3, 4, and 5, respectively). Similarly, adjusting for BCS, year (P < 0.05), the BS-PR was greater for heifers in NSO group with higher RTS (P < 0.01; 79.7%, 84.3%, 88.4%, and 90.2% for RTS of 2 or less, 3, 4, and 5, respectively). Heifers with higher RTS in both groups became pregnant earlier in the breeding season compared with heifers with lower RTS (log-rank statistics: P < 0.0001). Heifers in the AI-NS group become pregnant at a faster rate compared with those in the NSO group (P < 0.01). The BS-PR for heifers with RTS 5 was different between AI-NS and NSO groups (P < 0.0001). In conclusion, the RTS influenced both the number of beef heifers that became pregnant during the breeding season and the time at which they become pregnant. Furthermore, irrespective of RTS, heifers bred by NSO required more time to become pregnant than their counterparts in herds that used timed AI. The application of RTS system is reliant on the use of synchronization protocol. The application of RTS for selection may plausibly remove precocious females with lower RTS. On the contrary, application of RTS would help select heifers that will become pregnant earlier in breeding season.     

The time required for the presence of bulls to alter the interval from parturition to resumption of ovarian activity and reproductive performance in first-calf suckled beef cows

This experiment was designed to determine 1) if exposure of firstcalf suckled beef cows to mature bulls in the first 30 days, after 30 days, or continuously post partum reduces the postpartum anestrous period and 2) if exposure to bulls alters the first service pregnancy rate. Postpartum first-calf suckled crossbred (Angus x Hereford; Hereford x Angus) cows were randomly assigned to be 1) exposed continuously to mature, epididectomized bulls (BE; n = 18); 2) exposed to bulls for the first 30 days post partum (BE/NE; n = 17); 3) exposed to bulls after the first 30 days post partum (NE/BE; n = 16); or not exposed to bulls (NE; n = 18). Blood samples were collected weekly to be assayed for progesterone to determine resumption of ovarian cyclic activity. All the cows were bred for 21 days by AI while under their respective treatment regimens and were then exposed to fertile bulls for an additional 35 days. The postpartum interval to resumption of ovarian cyclic activity did not differ (P>0.10) among the 3 (BE, BE/NE, and NE/BE) treatment groups, but it was 15.4 d shorter (P<0.05) than for cows in the NE group. The overall pregnancy rates did not differ (P>0.10) among the treatment groups. The AI pregnancy rates for the BE/NE and NE/BE treatment groups were higher (P<0.05) than for the NE group. The AI pregnancy rates for the BE and NE treatment groups did not differ (P>0.10). The results showed that all three treatments (BE, BE/NE and NE/BE) similarly decrease the postpartum interval and that exposure to bulls may improve the reproductive performance of first-calf suckled beef cows.     

A comparison of natural service fertility of yearling and two-year-old bulls on pasture

The natural service fertility of yearling bulls on pasture was compared with that of 2-year-old bulls in a study (2 experiments) which involved a total of 748 composite breed cows over 3 breeding seasons. Other factors such as age of the cows, breed, year effects, and the sequence in which bulls of a particular age were assigned to cows were also evaluated. Pregnancy and calving rates and calving date were used as criteria for assessing fertility. The age of the bull, breed, year, or bull sequence did not significantly affect any of the traits studied. The age of cow was important in all the traits studied, with mature cows (5 years and older at breeding) calving earlier (P < 0.05) than the youngest group (2 years old at breeding) of cows, in both experiments. The pregnancy rate was also lower in the younger cows than in the mature cows in the first experiment. The results suggest that the age range of the cow herd is more important in determining herd fertility than the age of the bull or any of the parameters studied.     

Economics of selecting for sex: the most important genetic trait

Over 20,000 calves have resulted from artificial insemination (AI) of cattle with sexed, frozen/thawed sperm in the course of experimentation in several countries, and from commercial sales in the United Kingdom. This technology likely will become commercially available in many countries within a few years. Accuracy of the process is about 90% for either sex, and resulting calves appear to be no different from non-sexed controls in birthweight, mortality, rate of gain, and incidence of abnormalities. The main determinants of the extent of use of sexed sperm will be pregnancy rate and cost. Fertility of low doses (1.5 x 10(6)-2 x 10(6)) of sexed, frozen sperm for AI of heifers usually has been in the range of 70-80% of unsexed sperm at normal doses (10 x 10(6)-20 x 10(6) sperm) in well managed herds; it has been lower in poorly managed herds, and likely will be lower with lactating dairy cows. It is expected that fertility of sexed sperm will increase significantly due to very recent improvements in the hydrodynamics of the sexing process and potential improvements in cryopreservation procedures. It is unclear how sexed sperm will be priced; the cost of sexed sperm for cattle will likely be more than double the cost of unsexed sperm in most markets. The economic benefit of using sexed sperm also will depend on the baseline fertility of the herd since at lower fertility, it takes more doses of semen per calf produced. It is noted that for a small percentage of elite cattle, the economics of using sexed sperm do not depend primarily on increased production or efficiency of producing meat or milk, but rather on factors such as scarcity, tradition, cattle show winnings, and biosecurity during herd expansion. Until sorting efficiencies improve and costs decline, sales likely will be limited primarily to these niche markets. With near normal fertility and a premium for sexing in the range of US$ 10 per insemination dose, sexed sperm likely would become economically and environmentally beneficial for many, if not most populations of cattle being bred by AI.     

Effects of biostimulation and nutritional supplementation on pubertal age and pregnancy rates of Nelore heifers (Bos indicus) in a tropical environment

To determine effects of biostimulation (BIO) and dietary supplementation (BIO+S) on pubertal age and pregnancy rates, Nelore heifers (n=392) were randomly assigned to one of four treatment groups (n=98/group). All animals were in tropical environmental conditions, in the middle-west region of Brazil, grazing in pastures of Brachiaria brizantha, cv. Marandu; Panicum Maximum, cv. Tanzania and Brachiaria humidícula. The heifers of the BIO group were kept in the presence of bulls while being maintained on pasture; the animals in the BIO+S group were kept in the presence of bulls while being managed on pasture and were fed a diet with greater energy and protein content to produce 0.49 kg of BW gain/day; the animals in control group (the NBIO) were kept away from bulls and under pasture conditions; and the animals in the NBIO+S group were kept away from bulls, were maintained on pasture, and were fed the same diet as the BIO+S group. Heifers were bred at 22-23 months of age, and pregnancy diagnosis was made 45 days after the end of the breeding season. There were differences (P<0.05) between groups regarding pubertal heifers up to 19 months (NPH), final body weight (FBW) and pregnancy rates (P<0.01), with an advantage for the animals in the BIO and BIO+S groups. Although the effect of a diet with greater protein and energy content was not clear in this experiment, the exposure of heifers to a male during the prepubertal period decreased age at the first breeding season, resulting in a significant reduction in age of first pregnancy in Nelore heifers kept under extensive management systems in a tropical environment.