Academic literature on the topic 'Peri-pubertal and pubertal heifers'

Abstract The objective was to evaluate the relationship of heifer puberty status at 10 months of age with body weight (BW), hair shedding score (HSS), and dam age. Puberty status was assessed at 10 months of age on 177 fall-born, Simmental × Angus heifers (birth date 9/26/2020 ± 18.7 d) using fabric heat patches (Estrotect; Hermitage, Tennessee) for 22 d. Patches were scored as 0, 50, or 100% activated with heifers scored 50 or 100% considered pubertal. Heifers with missing patches (n = 10) were excluded from the analysis. Heifers grazed endophyte-infected tall fescue pastures from 6 to 12 months of age. Heifers were assigned HSS at 10, 11, and 12 months of age. A HSS of 1 indicates a slick summer coat and a 5 is a full winter coat. The MIXED procedure of SAS 9.4 was used to analyze data. Heifers classified as pubertal at 10 months of age had greater (P = 0.01) birth BW than prepubertal heifers (33.1 kg vs. 30.8 kg, respectively). Heifers that were pubertal at 10 months of age also had greater (P ≤ 0.05) BW at 6, 8, 10, and 12 months of age than prepubertal heifers. Additionally, pubertal heifers had greater (P = 0.01) ADG from 6 to 12 months of age while grazing. Pubertal heifers tended (P = 0.10) to have a lesser, more desirable HSS at 10 months of age and had a lesser (P < 0.01) HSS at 11 and 12 months of age compared with prepubertal heifers. Dam age was not different (P = 0.60) between heifer puberty status at 10 months of age. In conclusion, pubertal heifers were heavier at birth, heavier from 6 to 12 months, and had more desirable HSS compared with prepubertal heifers at 10 months of age.

Thirty-three cross-bred beef heifer calves were implanted with zeranol (36 mg) at birth, 100 and 300 d of age and 40 heifer calves remained as untreated controls. Heifers were examined rectally by diagnostic ultrasonography every 14 d from 10 to 14 mo of age and then, every 21 d until the end of test (100 d after exposure to bulls). The presence of a CL, diameter of the largest follicle, total number of follicles ≥ 3 mm, diameter of the right uterine horn and age at puberty were recorded. At 15 mo heifers were exposed to bulls for 63 d. Mean (± SEM) age at onset of puberty in zeranol-implanted heifers (420 ± 8.9 d) was greater than control heifers (362 ± 6.2 d; P < 0.05). The diameter of the largest follicle was smaller (P < 0.05) in pre-pubertal zeranol-implanted heiferes than in pre-pubertal control heifers between 10 and 14 mo of age and at puberty. Within the zeranol-implanted heifers, the diameter of the largest follicle was smaller in pre-pubertal (P < 0.05) than pubertal heifers until 14 mo of age. The uterine diameter of zeranol-implaned heifers was smaller (P < 0.05) than that of control heifers during all examinations prior to breeding. Pubertal heifers had a larger (P < 0.05) uterine diameter than pre-pubertal heifers. Pregnancy rate after the first 21-d period of the breeding season and at the end of test was higher (P < 0.001) for control than for zeranol-implanted heifers (82.5 and 100%, respectively and 24 and 42.5%, respectively). Pregnancy loss in zeranol-implanted heifers was 37.5% compared with 0% for control heifers (P < 0.001). Heifers that became pregnant during the first 21 d of the breeding season had reached puberty earlier (396.9 ± 12.5 and 358.7 ± 6.4 d of age for zeranol-implanted and control heifers, respectively) than heifers not conceiving in the first 21 d (472.8 ± 10.8 d and 380.0 ± 17.7 d for zeranol-implanted and control heifers, respectively; P < 0.05). Heifers that became pregnant during the first 21 d of the breeding season had a larger (P < 0.001) uterine diameter (18.17 ± 0.21 mm) than non-pregnant heifers (16.28 ± 0.36 mm). Within the zeranol-implanted group, pregnant heifers had a larger (P < 0.05) uterine diameter (17.75 ± 0.05 mm) than non-pregnant heifers (15.78 ± 0.38 mm). Heifers that were not pregnant at the end of test began cycling later and had a numerically smaller uterine diameter than pregnant heifers (430 ± 17.5 d vs. 374.7 ± 6.2 d; and 15.1 ± 0.6 mm vs. 17.3 ± 0.24 mm, respectively), however these differences were not significant (P > 0.05). It was concluded that zeranol implants from birth delay the onset of puberty and decrease uterine horn diameter. Futhermore, the lower pregnancy rate in zeranol-implanted heiferes 100 d after exposure to bulls was caused by failure to cycle early in life, and in those that were cycling, failure to conceive and abortions between 25 and 45 d of gestation. Key words: Zeranol, bovine, puberty, fertility, uterine diameter, ultrasonography

Abstract Protein supplementation to replacement heifers is utilized to reach targeted body weights, which may affect the animal’s gut and reproductive microbiomes and potentially impact breeding outcomes. The objective of the study was to evaluate the effects of protein supplementation on the ruminal and uterine bacterial communities of developing heifers. Thirty-nine pre-pubertal commercial Angus heifers were blocked by body weight (BW) and randomly assigned to one of three supplementation groups: 10% crude protein (CP), 20% CP, or 40% CP. Supplements were provided four times weekly for 140d. Blood and BW were taken every 14d to monitor development. Every 56d, uterine flushes and rumen content were collected for bacterial identification. Uterine flushes were collected via 20 mL sterile saline flushed through a Foley catheter, and rumen content by esophageal tubing. Bacterial DNA was extracted and amplified targeting the V1-V3 region of the 16S rRNA gene. Microbiome analyses were performed in R 4.1 with dada2 and phyloseq packages. Statistical analyses were performed in R and SAS 9.4. The GLIMMIX procedure was used including fixed effects of protein, month, and pubertal status. Random effects included BW, interaction of BW and protein, and heifer within the interaction, with repeated measures of month. Alpha diversity differed by month in the rumen, and by month and pubertal status in the uterus (P &lt; 0.05). Bray Curtis analyses indicated clustering by month and pubertal status (P &lt; 0.001) in the uterus, and by month in the rumen (P &lt; 0.001). Multiple uterine and ruminal bacteria abundances differed over time and by protein supplementation and pubertal status (P &lt; 0.05). The impact of pubertal status and month of development may indicate maturation of the uterine bacterial communities through development. Protein supplementation affecting the uterine bacterial communities may provide opportunity to manipulate the uterine microbiome, potentially affecting future reproductive success.

Abstract A 2-yr study evaluated the impacts of supplementation frequency and amount on growth and puberty attainment of Brangus heifers. On d 0 of each year, 64 Brangus heifers were stratified by BW and age (244 ± 4 kg; 311 ± 18 d) and assigned to 1 of 16 bahiagrass pastures (4 heifers/pasture). Treatments were randomly assigned to pasture (4 pastures/treatment) in a 2 × 2 factorial design and consisted of heifers offered a soybean hulls-based supplement DM at 1.25% or 1.75% of BW delivered either daily (7X) or 3 times weekly (3X; Monday, Wednesday, and Friday). On d 56, heifers were inserted with a CIDR device for 14 d, followed by administration of PGF2α and a timed-AI+GnRH protocol on d 89. Heifers were exposed to bulls from d 89 to 167. Data were analyzed using the SAS GLIMMIX procedure. No rate × frequency interactions were detected (P ≥ 0.20). Overall ADG was greater for 7X vs. 3X heifers (P = 0.007). Daily supplementation increased (P ≤ 0.03) the percentage of pubertal heifers prior to CIDR insertion and at AI compared to 3X supplementation; however, percentage of pregnant heifers did not differ (P = 0.70). Supplementation at 1.25% vs. 1.75% enhanced the overall ADG (P = 0.02) but did not impact (P = 0.18) the percentage of pubertal heifers before CIDR insertion. After the puberty induction protocol, heifers supplemented at 1.75% of BW achieved greater puberty attainment at time of AI (P = 0.05) and final pregnancy rates (P = 0.02) than heifers supplemented at 1.25% of BW. When a puberty induction protocol was included, the percentage of pregnant Brangus heifers were not affected by supplementation frequency (daily vs. 3 times weekly) but enhanced when the supplement dry matter amount was offered at 1.75 vs. 1.25% of body weight.

Abstract This study aimed to determine if heifers induced to puberty had damage in reproductive performance compared with heifers that reach puberty naturally. Nellore heifers (n = 210) were weaned at 8 ± 0.7 mo of age were assigned to 42 feedlot pens according to BW (184 ± 1 kg). The diet was compounded by Tifton-85 haylage (18% CP; 55% TDN) ad libitum and supplement (4g/kg of BW; 25% CP; 60% TDN). Puberty and growth were assessed weekly. At 14 mo of age, 46 heifers reached puberty naturally (precocious heifers; PH) and 164 non-pubertal heifers were submitted to puberty induction with progesterone (P4) insert for 10 days and estradiol cypionate (0.5 mg) at P4 removal (puberty induction; PI). After fourteen days, all heifers were submitted to TAI. Heifers were considered pregnant at 20 days (P20) after AI when CL had blood flow &lt; 25% of area (Power-Doppler). Pregnancy rate was also determined at 30 (P30) and 60 (P60) days after AI by embryo detection. Early (P20 – P30) and late (P30 – P60) embryonic loss were determined. Reproductive data were analyzed by GLIMMIX, and BW was analyzed using repeated measures over time (MIXED; SAS 9.3). The PH was heavier than PI from 8 to 16 mo of age, which BW was 296 and 285 ± 3 kg at 14 mo of age, respectively. At beginning of TAI, 88% of PI heifers had a CL. Pregnancy rate was similar (P &gt; 0.05) between treatments, in which pregnancy rate at 20, 30 or 60 days after AI was 65, 47 and 43% for PH and 61, 53 and 48% for PI, respectively. Early (26 vs 14%) and late (9 vs 9%) embryonic loss were similar (P &gt; 0.05) in PH and PI, respectively. In summary, natural or induced pubertal heifers had similar reproductive performance.

High fertility and early puberty in Bos indicus heifers are desirable and genetically correlated traits in beef production. The hypothalamus–pituitary–ovarian (HPO) axis synthesizes steroid hormones, which contribute to the shift from the pre-pubertal state into the post-pubertal state and influence subsequent fertility. Understanding variations in abundance of proteins that govern steroid synthesis and ovarian signaling pathways remains crucial to understanding puberty and fertility. We used whole ovaries of six pre-pubertal and six post-pubertal Brahman heifers to conduct differential abundance analyses of protein profiles between the two physiological states. Extracted proteins were digested into peptides followed by identification and quantification with massspectrometry (MS) by sequential window acquisition of all instances of theoretical fragment ion mass spectrometry (SWATH-MS). MS and statistical analysis identified 566 significantly differentially abundant (DA) proteins (adjusted p < 0.05), which were then analyzed for gene ontology and pathway enrichment. Our data indicated an up-regulation of steroidogenic proteins contributing to progesterone synthesis at luteal phase post-puberty. Proteins related to progesterone signaling, TGF-β, retinoic acid, extracellular matrix, cytoskeleton, and pleiotrophin signaling were DA in this study. The DA proteins probably relate to the formation and function of the corpus luteum, which is only present after ovulation, post-puberty. Some DA proteins might also be related to granulosa cells signaling, which regulates oocyte maturation or arrest in ovaries prior to ovulation. Ten DA proteins were coded by genes previously associated with reproductive traits according to the animal quantitative trait loci (QTL) database. In conclusion, the DA proteins and their pathways were related to ovarian activity in Bos indicus cattle. The genes that code for these proteins may explain some known QTLs and could be targeted in future genetic studies.

Abstract While sequence variation can be informative to associate regions of the genome with specific traits and improve genetic selection, the epigenome may provide a more powerful tool to manage cattle. Identifying practices that are producer friendly and effectively control epigenetic function within animals is crucial to translating developmental programming to a production setting. Initial studies of developmental programming investigated how environmental or nutritional stresses during fetal and peri-natal development impacted performance of animals later in life. These studies demonstrated changes in methylation, alterations in transcript abundance, and negative impacts on physiology, but they also suggested that we may be able to beneficially impact the epigenome and developmentally program animals to excel in their niche in the production system. Maternal nutritional status during the third trimester influenced date of conception of female progeny in several studies but failed to do so in other studies. Transcriptomic analyses provided evidence that nutritional treatments alter mRNA abundance in brain, liver, muscle, and ovary, but does not conclusively demonstrate that this is due to functional changes in the epigenome. If developmental programming is to be applied in production systems, responses must be consistent and beneficial. Reducing nutrient intake in heifers during peri-pubertal development increased number of primordial follicles in the ovaries and reproductive longevity. While nutritional programming of the ovarian reserve in peri-pubertal heifers appears to occur consistently across locations and studies, it does not ensure that subsequent environmental stressors will not induce changes in the ovarian reserve that will negate beneficial effects. These studies demonstrate that it is possible to developmentally program the epigenome in cattle in ways that will improve production traits; however, there remains a need for studies to improve the consistency of response and to determine best practices that fit into production systems. USDA is an equal opportunity provider and employer.

Abstract Scientists are starting to realize that some effects from the animal environment may affect their progeny’s phenotype. These epidemiological evidences of early life programming are very well described in humans and rodents but less documented in bovines, especially in relation to the peri-conceptual period. In fact, as other species the bovine gametes may carry non-genetic information that will impact the survival, the fitness and the phenotype of the embryo-fetus-offspring. Using a peri-pubertal age model we have compared the semen of bulls that we obtained at different ages just after puberty for whole genome DNA methylation, miRNA content and open DNA access regions to identify mechanistic relation to gene expression and DNA methylation in embryos created with the same bulls compared to themselves and mated by IVF to the same heifer several times. A similar experiment was also done with pre-peri and post puber heifers using the same adult bull for repetitive IVF. The overall results indicate that the same pathways are affected in embryos from immature parents. The targeted processes are overwhelmingly associated with cellular metabolism involving mitochondria function and the mTOR pathways amongst others. A very similar conclusion was also observed with embryos obtained from adult mothers under negative energy balance and the resulting calves obtained by surrogate gestations using blood DNA methylation to assess the intergenerational legacy. These results support a metabolic programming across generation associated with an energy deficient status of the parent.

AbstractThe response of Hereford × Friesian heifers to large intakes of two types of high glucosinolate rapeseed meal were studied during two 5-month periods in consecutive years. In year 1, six heifers were given a compound concentrate food containing 250 g/kg extracted rapeseed meal and six control animals were given concentrate containing 210 g/kg soya-bean meal. In year 2, a further six heifers were given concentrate containing 320 g/kg expeller rapeseed meal and six control animals were given the same soya-bean meal compound concentrate. The crude protein (CP): metabolizable energy (ME) ratio of all diets was 14·3 g CP per MJ ME. Barley straw was the only roughage offered.There were no differences between groups in ovarian activity assessed from plasma progesterone concentrations, nor in behavioural activity around oestrus. The heifers were slaughtered 6 weeks after artificial insemination. Pregnancy rate was proportionally 0·58 (7/12) in the rape-fed and 0·67 (8/12) in the control heifers (P > 0·05). Foetuses from heifers given rapeseed meal were slightly smaller than those from control heifers; the difference was significant in year 2 (P < 0·05).Thyroid glands of heifers given rapeseed meal showed histological evidence of goitrogenicity but weights of the glands did not reflect this. Plasma thyroxine values were depressed in heifers given rapeseed meal (year 1, P < 0·001; year 2, P < 0·05). Plasma thiocyanate was elevated in rapeseed meal heifers in both years (P < 0·001) compared with values for control animals.

Angus-sired heifers were allotted by age (mean=4 mo), BW (mean=135 kg), and sire (n=4) to either a control (n=10) or infected group (n=11; 600 metacercariae of Fasciola hepatica, intraruminally) to test our hypothesis that puberty is delayed by experimental fascioliasis. Blood samples were collected biweekly for analysis of steroid hormone concentrations. At 2-wk intervals, BW was recorded, and samples were collected for analysis of liver enzymes and serum proteins and fecal egg counts. A radiotelemetry system (HeatWatch??) was used to detect estrus and ovulation was confirmed by an elevation in serum progesterone (P4) after estrus. Heifers were artificially inseminated (AI) at the second observed estrus. Serum γ-glutamyl transpeptidase (GGT) and aspartate aminotransferase (AST) increased (p<0.0008) between day 0 and 112 in the infected group. Serum estradiol (E2) and P4 concentrations did not differ (p>0.1) between treatment groups. Mean age at puberty was 10 days later (p>0.1) in the infected group. Conception rate did not differ between control and infected heifers. The HeatWatch?? data were used to compare mounting activity during estrus in pubertal and gestating heifers. Mean duration of estrus was longer (p<0.01) for the second than for the pubertal estrus, though total mount duration and number of mounts did not differ. Number of mounts at second estrus was greater (p<0.05) for heifers that conceived (n=9). Mean duration of estrus and total mount duration at second estrus were not associated with pregnancy outcome. Estrus events were detected in all nine heifers during pregnancy (total=73). A majority (75%) of the interestrus intervals during gestation was <17 d. Number of mounts (p=0.035) and total duration of mounts (p=0.022) at second estrus were predictive of number of mounts during gestation. Experimental infection of Fasciola hepatica did not alter serum steroid hormone concentration or delay pubertal development in heifers. Estrus duration was longer for the second estrus compared to the pubertal estrus, and the number of mounts received during the second estrus was greater in heifers that did conceive to AI. Estrus events were detected in each heifer during pregnancy; however, a normal interestrus interval occurred in only 10% of the estrus events.

Titulo retirado da capa
Dissertação (mestrado) - Pontifícia Universidade Católica do Paraná, São José dos Pinhais, 2012
Bibliografia: f. 62-64
O presente estudo objetivou verificar a influência do protocolo hormonal com três manejos, utilizando a progesterona intravaginal de uso único, em novilhas pré-púberes e púberes, visando a inseminação artificial em tempo fixo (IATF). Foram utilizadas 447
The present study aim to verify the influence of using hormone in tree handle protocol which use progesterone intra vaginal inserts in pre pubertal and pubertal beef heifers, in purpose to achieve a pre timed A.I. 447 cross breed heifers ( Nelore X Red An

The objectives of this study were to determine the effects of handling peri-puberal heifers for 2 h each week on in-chute behavior, isolation behavior, and the time required for each heifer to leave the testing area; and to determine if the location of the facial hair whorl was associated with any of the behavior scores or social dominance order. Crossbred beef heifers (n = 146) were assigned to be walked through, sorted and moved through a chute for 2 hr each wk for 20 wk (HANDLED) or allowed to remain on pasture unless handling was required to treat an injury or disease (CONTROL). In-chute behavior, isolation behavior and exit times were observed and scored at the beginning (0 wk), middle (10 wk) and end of the experiment (20 wk). The facial hair whorl on each heifer was classified as being high (above the eyes), middle (between the eyes), or low (below the eyes). At the end of the experiment pairs of heifers in the HANDLED group competed for a feed source and a social dominance order was estimated. Weekly handling decreased in-chute behavior scores of heifers with facial hair whorl positions classified as medium or low, but not in heifers that exhibited a hair whorl high on their face. Cattle in the HANDLED treatment group which had an initial isolation score of 2 or 3 had the greatest improvement in temperament over the entire experiment when compared to CONTROL animals with the same initial isolation score. The calmest heifers were not negatively affected by the handling, while the most agitated animals in the HANDLED had a similar overall change in isolation score as those animals in the CONTROL group. This indicates that while weekly handling improved the temperament and behavior of heifers with intermediate temperament rating at the outset of the experiment, weekly handling seemed unnecessary for the calmest heifers and did not have a beneficial effect on the heifers rated as the most nervous and agitated at the beginning of the experiment. Social dominance rankings were positively correlated (P < 0.10) with final in-chute behavior scores, but not with the other behavior scores or heifer body weight. Cattle with the hair whorls in the middle of the forehead had higher mean social dominant rank than those with hair whorls higher or lower on the face (P < 0.03). Overall, the results of this experiment indicate that behavior testing can reveal differences in the temperament of heifers and that, other than the most nervous and agitated heifers; repeated handling could serve to improve the temperament of the animals.
Master of Science

Osteoporosis, a major public health problem, likely has its origins in childhood. During periods of rapid skeletal growth, diet may influence accrual of bone mineral density (BMD) and adult bone health. This study used novel approaches in bone imaging to further characterize optimal skeletal development and enhance our understanding of key dietary components that influence attainment of peak bone mass (PBM) and contribute to determinants of peak bone strength in peri-pubertal females. The use of a validated food-frequency questionnaire (FFQ) enabled the influence of usual dietary intake on bone parameters to be examined.This study examined the relationship of dietary intake of micronutrients and bone macro-architectural structure in peri-pubertal girls. This study suggested that vitamin C and zinc intake are associated with objective measures of bone status in 4th, but not 6th grade girls. This indicates potential differences in micronutrient and bone associations at various age-associated stages of bone maturation.The impact of dietary fat on peri-pubertal skeletal growth is not well characterized. This study examined relationships of select dietary fatty acid (FA) intakes and measures of bone status in peri-pubertal girls. This study suggested that MUFA, total PUFA, n-6 and linoleic acid (LA) are inversely associated with bone status prior to menarche, but composition of dietary fat may be more important during the early-pubertal years. Decreased intakes of n-6 PUFA may benefit bone health in young girls.The impact of a dietary protein on volumetric bone mineral density (vBMD), bone mineral content (BMC) and bone strength throughout maturation remains controversial. Given evidence of both anabolic and catabolic affects of protein on bone health, this study examined relations of dietary protein from different sources with bone parameters in peri-pubertal girls. This study showed that dietary protein intake is related to higher trabecular but not cortical vBMD, BMC and BSI, and accounts for 2-4% of their variability in peri-pubertal girls. The relationship seems to vary by the source of dietary protein and calcium intake. However, a negative impact of animal protein on bone health is not supported. Large scale observational and intervention studies are needed to establish causality.

Genetic improvement in cattle has focused in recent years on the large reproductive potential that resides in the ovaries of females at an early age. It is estimated that approximately 150,000 oocytes are present in primordial follicles in foetal ovaries at birth, and recruitment of follicles from the primordial pool has been initiated by the time of birth. Further, follicular growth can be superstimulated in heifer calves from around 4 weeks of age by treatment with gonadotrophins, and oocytes recovered and placed through in vitro maturation and fertilisation procedures to produce viable embryos.

The capacity to use embryos derived from heifer calves has the potential to reduce generation intervals and increase the rates of genetic gain in cattle. Studies on embryo production from heifer calves have reported inconsistent and unpredictable responses to superstimulation of follicle growth with FSH, similar to that observed in sexually mature heifers. Heifer calves that had a relatively large (>10mm) follicle on the ovary at the end of superstimulation, had a smaller number of total follicles compared with heifer calves that did not have a large follicle on the surface of the ovary. This observation led to the suggestion that follicular interrelationships may occur from an early age in heifers, and that a large follicle may suppress the development of other follicles. Nutrition appears to influence ovarian follicle status in peri-pubertal and pubertal heifers and possibly the response to superstimulation of follicular growth in older animals. There may be a role, therefore, for nutrition in ovarian follicle growth and responses to superstimulation in heifer calves.

In a number of studies oocytes obtained from heifer calves were reported to have a reduced developmental competency in vitro compared with oocytes obtained from ovaries of post-pubertal heifers.

In cattle, treatment with agonists of gonadotrophin hormone releasing hormone (GnRH) desensitises the anterior pituitary gland to GnRH which blocks pulsatile secretion of LH but allows continued basal LH secretion. Antagonists of GnRH prevent both pulsatile and basal secretion of LH. It is possible that treatment with GnRH agonists and antagonists might be used to regulate gonadotrophin secretion in heifer calves and prevent the development of large (functionally dominant) follicles. Subsequent initiation of superstimulation when a pool of small gonadotrophinresponsive follicles are present on the ovaries, and maturing these follicles in synchrony in vivo, may allow a pool of oocytes at similar stages of maturation to be collected for in vitro procedures.

The first two experiments in this thesis examined the requirement of LH for oocyte maturation by treating calves with gonadotrophin hormone releasing hormone (GnRH) agonist and antagonist before and during superstimulation with FSH. Simultaneous treatment with a GnRH agonist during superstimulation of ovarian follicle growth with FSH tended to increase the number of follicles stimulated to grow and significantly increased the number of Grade A and Grade B oocytes collected. In a second experiment, treatment with a GnRH antagonist tended to increase blastocyst development rate after in vitro fertilisation. It was concluded from these findings that exposure of oocytes to pulsatile secretion of LH, and/or a 'pre-ovulatory like' surge release of LH, is not an obligatory requirement for oocyte growth and development in heifer calves.

A third experiment examined the effects of nutrition and growth rate on maturation of the reproductive endocrine axis and the response of calves to superstimulation of ovarian follicle growth with FSH. Heifer calves were raised on two planes of nutrition (relatively low and high) and subsequently superstimulated with FSH. The nutritional treatments resulted in a significant difference in growth rate between the two groups of heifers. However, there were no apparent differences in ovarian follicular responses to stimulation with FSH, oocytes recovered, or in vitro developmental competency of oocytes, between the two groups of heifers.

In the fourth experiment, in vitro developmental competency was compared between oocytes obtained from heifer calves superstimulated with FSH, heifer calves that had not undergone superstimulation and post-pubertal heifers and cows that had not been stimulated with FSH. There were no differences in developmental competency between Grade A and Grade B oocytes derived from the three groups of animals. This finding demonstrated that oocytes obtained from pre -pubertal heifers do not have an intrinsic reduced capacity for in vitro development compared with oocytes obtained from post -pubertal heifers.

The ultimate test of viability of embryos derived from heifer calves is the transfer to recipients and the birth of calves. The aim of the fifth experiment therefore was to test the viability of embryos derived from 10 week-old heifer calves in which ovarian follicular growth was superstimulated with FSH. Transfer of blastocysts produced from oocytes obtained from heifer calves to recipient sexually mature heifers resulted in the birth of normal calves.

In summary, the competency of oocytes collected from heifer calves from an early age has been well established in the series of experiments undertaken in this thesis. The use of superstimulation protocols with heifer calves pre-treated with a GnRH agonist or antagonists increased the number of Grade A and Grade B oocytes, and tended to increase the development to blastocyst post-fertilisation. Grade A and Grade B oocytes collected from heifer calves form blastocysts at rates comparable to oocytes collected from mature cows, and establish pregnancies which result in the birth of calves.

INTRODUCTION: Osteoporosis has, in part, been attributed to physical inactivity. The direct health care costs of osteoporosis in Canada exceed 650 million dollars annually. Targeted physical activity may be an effective and feasible primary prevention strategy to lessen the burden of this debilitating disease. AIM: The primary aim was to determine the effects of a randomized, controlled, school-based, bone-loading exercise intervention on bone mineral accrual. METHODS AND RESULTS: Subjects: Children were a mixed ethnic group of 383 initially 9-11 year old boys and girls. Ethnic comparisons at baseline: For prepubertal girls (n=56), general physical activity, calcium intake, and total body (TB) and femoral neck (FN) bone mineral content (BMC) (adjusted for body size) were significantly (p<0.05) lower in Asians than Caucasians. For early pubertal girls (n=75), loaded physical activity and sport nights, calcium intake, TB, proximal femur (PF) and FN BMC and areal bone mineral density (aBMD) (adjusted for body size) were significantly lower (-10%) in Asians than Caucasians. 7-month change (girls): Fourteen schools were randomized to intervention (n=7) or control (n=7). I evaluated the effect of a 10-minute, 3x/week, exercise program on bone accrual at the TB, LS, PF, FN and trochanter (TR) in girls. Early pubertal (n=43), but not prepubertal (n=44) intervention girls gained significantly more (+~2%) adjusted BMC, aBMD, and vBMD (FN only) at the FN and LS than same-maturity controls (pre: n=26, early: n=63). 7-month change (boys): Prepubertal Asian and Caucasian boys in the exercise schools (n=61) gained significantly more adjusted TB BMC and PF aBMD (+1-2%) than prepubertal controls (n=60) over 7 months. Accrual was similar between ethnicities. 20-month change (girls): I investigated the continued bone health benefits from exercise over 20 months in girls. Intervention girls (N=32) gained significantly more adjusted FN and LS BMC (+4-5%) than controls (N = 43). CONCLUSIONS: Risk factors for osteoporosis were more prevalent in early pubertal Asian girls compared with Caucasians. A targeted, inexpensive and feasible school-based exercise program, implemented over 7 or 20 months, offered an effective strategy to enhance bone mineral accrual during growth. The skeletal response to loaded physical activity was sex-, site- and maturity-specific.