Physiology and Endocrinology Symposium: Harnessing basic knowledge of factors controlling puberty to improve synchronization of estrus and fertility in heifers

Implementing Fixed-Time Artificial Insemination Programs in Beef Herds

The article discusses the importance of reproductive biotechnologies, including artificial insemination and fixed-time artificial insemination (TAI), in beef cow-calf operations. The use of TAI improves cow-calf productivity and profitability by shortening the breeding season and increasing the number of calves born earlier, resulting in heavier calves at weaning. However, adoption of TAI by beef producers in the United States has been slow compared with the dairy industry and internationally, such as Brazil. Current TAI protocols are effective in synchronizing ovulation and yield consistent pregnancy results. Factors affecting the success of TAI include cow/heifer factors, sire, nutritional status, and cattle temperament.

Understanding the underlying genetic mechanisms for age at first calving, inter-calving period and scrotal circumference in Bonsmara cattle

BACKGROUND

Reproduction is a key feature of the sustainability of a species and thus represents an important component in livestock genetic improvement programs. Most reproductive traits are lowly heritable. In order to gain a better understanding of the underlying genetic basis of these traits, a genome-wide association was conducted for age at first calving (AFC), first inter-calving period (ICP) and scrotal circumference (SC) within the South African Bonsmara breed. Phenotypes and genotypes (120,692 single nucleotide polymorphisms (SNPs) post editing) were available on 7,128 South African Bonsmara cattle; the association analyses were undertaken using linear mixed models.

RESULTS

Genomic restricted maximum likelihood analysis of the 7,128 SA Bonsmara cattle yielded genomic heritability's of 0.183 (SE = 0.021) for AFC, 0.207 (SE = 0.022) for ICP and 0.209 (SE = 0.019) for SC. A total of 16, 23 and 51 suggestive (P ≤ 4 × 10-6) SNPs were associated with AFC, ICP and SC, while 11, 11 and 44 significant (P ≤ 4 × 10-7) SNPs were associated with AFC, ICP and SC respectively. A total of 11 quantitative trait loci (QTL) and 11 candidate genes were co-located with these associated SNPs for AFC, with 10 QTL harbouring 11 candidate genes for ICP and 41 QTL containing 40 candidate genes for SC. The QTL identified were close to genes previously associated with carcass, fertility, growth and milk-related traits. The biological pathways influenced by these genes include carbohydrate catabolic processes, cellular development, iron homeostasis, lipid metabolism and storage, immune response, ovarian follicle development and the regulation of DNA transcription and RNA translation.

CONCLUSIONS

This was the first attempt to study the underlying polymorphisms associated with reproduction in South African beef cattle. Genes previously reported in cattle breeds for numerous traits bar AFC, ICP or SC were detected in this study. Over 20 different genes have not been previously reported in beef cattle populations and may have been associated due to the unique genetic composite background of the SA Bonsmara breed.

Effect of growth and carcass traits on likelihood of early pregnancy in Nellore heifers raised at intensive nutritional plan

This study evaluated the influence of the quantitative traits measured by real-time ultrasound (adjusted longissimus muscle area [AdjLMA], adjusted rump fat thickness [AdjRFT], and adjusted marbling [AdjMAR]) as well as age at first breeding [AFB] and adjusted weight [AdjWeight], on the probability of occurrence of early pregnancy (EP) in 55 Nellore heifers, and also performed an economic analysis. All calves received supplementation in creep-feeding (ad libitum), and at weaning (average age= 210 ± 30 days; average weight= 241 ± 33 kg) until first breeding by artificial insemination (May to November) all heifers were managed in the same group (two paddocks of 25 ha each evenly covered with Urochloa Marandu Grass) and received protein-energy supplementation (1% of average BW per animal/day). The quantitative variables were collected immediately after timed artificial insemination (average age= 392 ± 27 days; average weight= 431.90 kg), and the pregnancy diagnosis was completed at 30 days following insemination. For economic analysis, two systems were compared (age at first breeding at 13 and 24 months). The greater adjusted weight on the first breeding increased the probability of occurrence of early pregnancy, while the greater adjusted longissimus muscle area reduced. In addition, intensive meat production systems provide greater economic return throughout cow-calf operation.

The effects of protein level on cytokines and chemokines in the uterine environment of beef heifers during development

The development of replacement heifers is crucial for breeding success and herd efficiency. Nutritional management can affect not only reproductive development but also the inflammatory status of the uterine environment, which may impact reproductive functions such as pregnancy establishment and development. The study herein evaluated the concentration of cytokines and chemokines in the uterus of heifers supplemented with different levels of protein. Angus heifers (n = 60) were blocked by body weight (BW) and randomly assigned to 1 of 3 treatments based on protein supplementation level: control of 10% crude protein (CON), 20% crude protein (P20), or 40% crude protein (P40). BW, body condition score, and blood samples were taken every 2 wk for 140 d to monitor development. Uterine flushes were performed monthly and concentrations of cytokines (IL-1α, IL-1β, TNF-α, IFN-γ, IL-10, VEGF-α, IL-17A, and IL-36RA) and chemokines (IL-8, MCP-1, MIP-1α, and MIP-1β) were quantified via ELISA multiplex. To test if there were mean differences in cytokines between the treatment groups or over time, PROC GLIMMIX (SAS v 9.4) was utilized. Concentrations of all cytokines and chemokines, except IL-1α, changed throughout heifer development (P < 0.05). Heifers in the P40 treatment group displayed reduced concentrations of MCP-1 (P = 0.007) and tended to have decreased concentrations of IFN-γ (P = 0.06). Cytokine IL-36RA tended (P = 0.06) to be affected by protein level, with the lowest concentrations observed in CON heifers. Most cytokines and chemokines increased following the initial month of supplementation (P < 0.05). The increase in concentrations after 1 mo may indicate an adaptive response in the uterus to diet change. Cytokines and chemokines fluctuated due to physiological changes occurring during development. Further research is needed to determine the influence of nutrition on uterine inflammation and long-term impacts on reproductive function.

Attainment and maintenance of pubertal cyclicity may predict reproductive longevity in beef heifers†

We hypothesized the manner that heifers achieve puberty may indicate their future reproductive longevity. Heifers with discontinued or delayed cyclicity during puberty attainment may have irregular reproductive cycles, anovulation, and infertility in their first breeding season contributing to a shorter reproductive lifespan. Therefore, plasma progesterone (P4) was measured from weaning to breeding on 611 heifers born 2012-2017 and four pubertal classifications were identified: (1) Early; P4 ≥ 1 ng/ml < March 12 with continued cyclicity, (2) Typical; P4 ≥ 1 ng/ml ≥ March 12 with continued cyclicity, (3) Start-Stop; P4 ≥ 1 ng/ml but discontinued cyclicity, and (4) Non-Cycling; no P4 ≥ 1 ng/ml. Historical herd records indicated that 25% of heifers achieved puberty prior to March 12th in the 10 years prior to the study. Start-Stop and Non-Cycling yearling heifers were lighter indicating reduced growth and reproductive maturity traits compared with Early/Typical heifers. In addition, Non-Cycling/Start-Stop heifers were less responsive to prostaglandin F2 alpha (PGF2α) to initiate estrous behavior and ovulation to be artificially inseminated. Non-Cycling heifers had fewer reproductive tract score-5 and reduced numbers of calves born in the first 21-days-of-calving during their first breeding season. Within the Start-Stop classification, 50% of heifers reinitiated cyclicity with growth traits and reproductive parameters that were similar to heifers in the Early/Typical classification while those that remained non-cyclic were more similar to heifers in the Non-Cycling group. Thus, heifers with discontinued cyclicity or no cyclicity during puberty attainment had delayed reproductive maturity resulting in subfertility and potentially a shorter reproductive lifespan.

Effect of progesterone supplementation in a resynchronization protocol on follicular dynamics and pregnancy success

The objective of this study was to evaluate the necessity of a controlled internal drug releasing device (CIDR) in a fixed-time AI resynchronization protocol as well as to compare a commercially available blood pregnancy test with transrectal ultrasonography for Day 28 pregnancy detection. Over a two-year period, beef cows and heifers from twelve herds were inseminated using the 7-day CO-Synch + CIDR protocol. On Day 21 following the first insemination, the protocol was repeated, with animals receiving either a CIDR or no CIDR. Pregnancy status (AI1) was determined on Day 28 by both transrectal ultrasonography and the IDEXX Rapid Visual Pregnancy Test. Non-pregnant animals by both methods (CIDR: n = 190 cows, n = 228 heifers; no CIDR: n = 185 cows, n = 223 heifers) received an injection of Prostaglandin F2alpha and were inseminated at the appropriate time or bred following detection of estrus. Corpora lutea (CL) number and largest follicle diameter were recorded on a subset of non-pregnant animals (CIDR: n = 66 cows, n = 46 heifers; no CIDR: n = 76 cows, n = 41 heifers) at time of pregnancy diagnosis on Day 28. Final pregnancy status was determined a minimum of 31 days following the second AI (AI2). The GLIMMIX procedure of SAS was utilized for estrus and pregnancy data; while the MIXED procedure was utilized for analyses of CL number and largest follicle diameter. There was no effect (P ≥ 0.55) of treatment on AI1 pregnancy, AI2 pregnancy, or overall pregnancy rates. The presence of a CIDR during the resynchronization increased (P < 0.001) estrus expression prior to AI2. There was an effect of treatment by age on AI2 pregnancy (P < 0.01); heifers that received a CIDR had greater AI2 pregnancy rates than heifers that did not receive a CIDR (P = 0.04), but there was no difference between cows with and without a CIDR. Treatment had no effect (P > 0.10) on embryonic loss (between the first and second pregnancy diagnosis), CL number, or follicle diameter. Although, there was a tendency for the interaction of treatment by age on follicle size (P = 0.07), with cows having larger follicles than heifers in the no CIDR group but not the CIDR group. In conclusion, use of a CIDR in this resynchronization protocol increased estrus expression, increased AI2 pregnancy for heifers, but did not improve pregnancies in cows, and did not influence overall pregnancy or embryonic loss.

Decreasing the dose of equine chorionic gonadotropin does not affect ovarian or pregnancy responses of purebred taurine and crossbred beef heifers

In this study there was evaluation of effects of different doses of equine chorionic gonadotropin (eCG: 200, 300, or 400 IU) administrated at progesterone (P4) plus estradiol-based timed AI (TAI). A total of 1080 heifers were included in the study. There was insertion of the intravaginal P4-device plus administration of 2 mg of estradiol benzoate IM. On D7, 12.5 mg of dinoprost tromethamine IM was administered and on D9, the P4 insert was removed and 0.5 mg of estradiol cypionate IM was administered. Heifers were categorized according to Reproductive Tract Status (RTS; 1-5) and were assigned to one of three treatments: 200 IU (n = 387), 300 IU (n = 357), or 400 IU (n = 336) of eCG. Estrous occurrence was evaluated at TAI 48 h later (D11). A subset of heifers (n = 213) had the largest follicle (LF) evaluated on D9 and on D11, and the formation of a new CL evaluated on D18.There was no effect of eCG treatment on LF on D11 (P = 0.79), occurrence of estrus (P = 0.92), and pregnancy at 30 days after AI (P/AI; 52.2%, 49.8%, and 51.5% for 200 IU, 300 IU, and 400 IU, respectively; P = 0.46). Regardless of the treatment, there was a greater P/AI when heifers had a functional CL, at initiation of the estrous synchronization treatment regimen. It, therefore, is efficacious to reduce the dose of eCG to 300 or 200 IU in purebred taurine and crossbred beef heifers without negative effects on ovarian, estrous or pregnancy responses.

Comparison of follicular development, timing of ovulation and serum progesterone, estradiol and luteinizing hormone concentrations in dairy heifers treated with 4- or 5-day CoSynch + CIDR protocols

The use of 4‐day CoSynch + Controlled internal drug release (CIDR) + timed artificial insemination (TAI) in dairy heifers has resulted in adequate pregnancy rates compared with the 5‐day CoSynch + CIDR + TAI protocol. The objective of this study was to compare follicular growth, timing of ovulation and serum progesterone (P₄), estradiol (E₂) and luteinizing hormone (LH) concentrations in dairy heifers treated with modified 4‐ or 5‐day CoSynch + CIDR protocols (CIDR for 4 or 5 days, PGF₂α at CIDR removal and GnRH + TAI 72 h later). Twelve cycling Holstein heifers were randomly assigned to either the 4‐ or 5‐day Co‐Synch+CIDR (n = 6/treatment) to receive an intravaginal insert CIDR^® containing 1.38 g of P₄ for 4 or 5 days, respectively. At CIDR removal, 25 mg of PGF₂α was injected IM; 72 h after CIDR removal, heifers received 100 μg of GnRH IM and timed artificial insemination (TAI). Follicular growth and timing of ovulation were assessed using transrectal ultrasonography. Blood samples were collected at the time of CIDR insertion and at frequent time points after CIDR removal for determination of P₄ (at TAI), E₂ (every 12 h) and LH (every 6 h during the first and second day and every 2 h on the third day). Heifers in the 4‐day group had smaller follicles from CIDR insert removal to ovulation compared with heifers in the 5‐day treatment. Five of six heifers (83.3%) in the 4‐day treatment ovulated at 90–96 h post CIDR insert removal, whereas most heifers in the 5‐day treatment (4/6; 66.6%) ovulated at 84–90 h post CIDR insert withdrawal. Heifers in the 5‐day treatment reached greater peak LH concentration between 48 and 72 h after CIDR insert removal and lesser E₂ concentration at TAI than heifers in the 4‐day treatment. In conclusion, heifers in the 4‐day treatment had smaller follicular diameter at 0, 30, 36, 42 and 48 h after CIDR insert removal, longer interval from CIDR insert removal to ovulation, greater E₂ concentrations at TAI, and lesser peak LH concentration than heifers in the 5‐day treatment. These results represent a baseline for further studies to determine if prolonging the interval to TAI by 6 h in the 4‐day CoSynch+CIDR would improve pregnancy risk.

Genomic association for sexual precocity in beef heifers using pre-selection of genes and haplotype reconstruction

Reproductive traits are of the utmost importance for any livestock farming, but are difficult to measure and to interpret since they are influenced by various factors. The objective of this study was to detect associations between known polymorphisms in candidate genes related to sexual precocity in Nellore heifers, which could be used in breeding programs. Records of 1,689 precocious and non-precocious heifers from farms participating in the Conexão Delta G breeding program were analyzed. A subset of single nucleotide polymorphisms (SNP) located in the region of the candidate genes at a distance of up to 5 kb from the boundaries of each gene, were selected from the panel of 777,000 SNPs of the High-Density Bovine SNP BeadChip. Linear mixed models were used for statistical analysis of early heifer pregnancy, relating the trait with isolated SNPs or with haplotype groups. The model included the contemporary group (year and month of birth) as fixed effect and parent of the animal (sire effect) as random effect. The fastPHASE® and GenomeStudio® were used for reconstruction of the haplotypes and for analysis of linkage disequilibrium based on r² statistics. A total of 125 candidate genes and 2,024 SNPs forming haplotypes were analyzed. Statistical analysis after Bonferroni correction showed that nine haplotypes exerted a significant effect (p<0.05) on sexual precocity. Four of these haplotypes were located in the Pregnancy-associated plasma protein-A2 gene (PAPP-A2), two in the Estrogen-related receptor gamma gene (ESRRG), and one each in the Pregnancy-associated plasma protein-A gene (PAPP-A), Kell blood group complex subunit-related family (XKR4) and mannose-binding lectin genes (MBL-1) genes. Although the present results indicate that the PAPP-A2, PAPP-A, XKR4, MBL-1 and ESRRG genes influence sexual precocity in Nellore heifers, further studies are needed to evaluate their possible use in breeding programs.

Feeding distillers dried grains in replacement of forage in limit-fed dairy heifer rations: Effects on metabolic profile and onset of puberty

The objective of this study was to determine the effect of increasing the inclusion rate of distillers dried grains (DDGS) in replacement of forage in limit-fed diets on the metabolic profile and onset of puberty in dairy heifers. A 16-wk randomized complete block design study was conducted using 48 Holstein heifers (199 ± 2 d of age) with 3 treatments. Treatments were (1) 30% DDGS (30DG), (2) 40% DDGS (40DG), and (3) 50% DDGS (50DG), with the remainder of the diet consisting of grass hay and 1.5% mineral mix. Heifers were housed in groups of 6 and individually limit-fed using Calan gates at 2.65, 2.50, and 2.35% of body weight (BW) on a dry matter (DM) basis for 30DG, 40DG, and 50DG, respectively. Jugular blood samples were collected during wk 0, 4, 8, 12, and 16 for metabolite and metabolic hormone analysis. Additional samples were taken during wk 16 for plasma fatty acid analysis. When heifers weighed 200 kg, coccygeal vein blood samples were taken twice per week for progesterone analysis to estimate onset of puberty. Blood samples continued until cycling was confirmed via ultrasound for the presence of a corpus luteum. A quadratic response and a linear tendency in the proportion of total fatty acids as linoleic acid were observed. Linear and quadratic responses for plasma concentrations of linoleic acid and arachidonic acid occurred. Overall results for fatty acid analysis demonstrated that total fatty acid and polyunsaturated fatty acids concentration in the blood were linearly increased, with a quadratic response for polyunsaturated fatty acids with 30DG and 50DG having the greatest concentrations. No interactions of treatment by week were observed for any of the metabolites and metabolic hormones measured. Glucose, insulin, insulin-like growth factor-1, leptin, and triglycerides were similar across treatments. A linear response of plasma urea nitrogen and a quadratic response tendency for cholesterol concentration were observed. Age and BW at puberty were similar across treatments. Limit-feeding heifers with greater inclusion rates of DDGS maintained energy status without the accumulation of excess adipose tissue as indicated by leptin. Treatments had no detrimental effects on age or BW at puberty.

Fertility after two doses of PGF2α concurrently or at 6-hour interval on the day of CIDR removal in 5-day CO-Synch progesterone-based synchronization protocols in beef heifers

Timed artificial insemination protocols in beef cattle are designed to synchronize ovulation in a greater proportion of females while simultaneously achieving acceptable pregnancy rates and a concise calving season. Protocols achieving such goals reduce time and labor associated with estrus detection and make advanced reproductive technologies implementable for beef producers. The objective of the study was to determine the effect of three different PGF2α (PGF) dosage schemes on artificial insemination (AI) pregnancy rates in beef heifers. We hypothesized that two doses of PGF administered concurrently at the time of controlled internal drug release (CIDR) removal would attain similar pregnancy rates compared with two doses given 6-hours apart-one at CIDR removal and the next 6 hours later in the 5-day CO-Synch progesterone-based synchronization protocol. Angus heifers (n = 875) at six locations in Washington, Idaho, and Oregon states were included in this study. Heifers within locations were assigned a body condition score (BCS). All heifers received a CIDR (1.38 g of progesterone) and 100 μg IM of GnRH on Day 0. The CIDRs were removed on Day 5, heifers were randomly allocated to one of three protocol groups: 1PGF (n = 291), received 25 mg IM of dinoprost (PGF); 2CO-PGF (n = 291), received 50 mg IM of dinoprost at CIDR removal, 2PGF (n = 293), received 25 mg IM of dinoprost at CIDR removal, and an additional 25 mg IM of dinoprost 6 hours later. Each heifer was given GnRH (100 μg, IM) and artificially inseminated at 56 hours after CIDR removal. Heifers were examined for pregnancy status between 50 and 70 days after AI to determine time of conception. A mixed-model procedure (PROC GLIMMIX of SAS) was used to evaluate the effect of treatments (1PGF, 2CO-PGF, and 2PGF) on AI pregnancy rates. Models included were treatments, BCS categories (≤5 and >5), and treatment by BCS category interaction. Location (state), handling facilities, handlers, inseminators, and AI sires were included as a random effect in the model. The 2PGF group had greater AI pregnancy rate of 63.6% (185/291), compared with the 2CO-PGF group at 51.9% (151/291) and 1PGF group at 54.9% (161/293; P < 0.001). An AI pregnancy rate of 50% (104/208) was observed for heifers with BCS less than or equal to 5 versus 58.9% (393/667) for heifers with BCS greater than 5 (P < 0.05). Location did not influence the AI pregnancy rate (P > 0.1). In conclusion, beef heifers received two 25-mg doses of PGF at 6-hour interval on Day 5 at CIDR insert removal in a 5-day CO-Synch + CIDR synchronization protocols achieved greater pregnancy compared with heifers received 50 mg of PGF concurrently at CIDR removal.

Feeding fat from distillers dried grains with solubles to dairy heifers: II. Effects on metabolic profile

The objective of this study was to determine if increased dietary fat from dried distillers grains with solubles (DDGS) in diets of growing heifers affected metabolic profile, plasma fatty acid profile, and reproductive maturation. Thirty-three Holstein heifers (133±18 d of age) were used in a 24-wk randomized complete block design with 3 treatment diets. Treatment diets were (1) control (CON) containing ground corn (15.9% of DM) and soybean products (17.9%), (2) low-fat (LFDG) containing low-fat DDGS (21.9%) and ground corn (11.9%), or (3) high-fat (HFDG) with traditional DDGS (33.8%). Diets were isonitrogenous and isocaloric, but the HFDG diet was formulated to contain 4.8% fat compared with 2.8% in the CON and LFDG diets. All 3 diets were limit-fed to 2.45% of body weight on a dry matter basis, and resulted in a mean average daily gain of 0.96kg/d across treatments. Every 4wk, jugular blood was collected for analysis of metabolites and metabolic hormones. During wk20 of the feeding period, blood samples were collected for analysis of plasma fatty acid profiles. When heifers weighed between 200 and 300kg of body weight, coccygeal blood samples were taken twice weekly for analysis of progesterone to determine if puberty had been reached. Plasma concentrations of nonesterified fatty acids were similar among treatments and consistent over the duration of the study. Plasma concentrations of β-hydroxybutyrate, insulin, insulin-like growth factor-1, and leptin were similar among heifers fed each treatment diet, but increased over the duration of the feeding period. Serum concentrations of glucose tended to be less in heifers fed HFDG compared with heifers fed the CON diet. Glucose concentrations fluctuated throughout the feeding period, but no treatment by time interactions were noted. Plasma urea N concentrations were less in heifers fed LFDG compared with heifers fed HFDG and CON diets. The concentrations of plasma urea N increased over the duration of the feeding period, with no treatment by week interaction. Total plasma cholesterol was greater in heifers fed HFDG compared with the CON and LFDG diets, and a significant week effect and a week by treatment interaction were observed. Fatty acid profiles also differed among treatments based on the supply of fatty acids from the diet. Progesterone analysis indicated that heifers fed HFDG tended to be pubertal at a younger age than heifers on CON. These results demonstrate that dietary fat from DDGS can be used in high-plane of nutrition rations for growing heifers and maintain metabolic energy status compared with starch from corn, but alters the concentrations of different blood lipids.

Optimising reproductive performance of beef cows and replacement heifers

A reproductively efficient beef cow herd is fundamental to meeting the protein and specifically, red meat demand of an ever increasing global population. However, attaining a high level of reproductive efficiency is underpinned by producers being cognisant of and achieving many key targets throughout the production cycle and requires significant technical competency. The lifetime productivity of the beef bred female commences from the onset of puberty and will be dictated by subsequent critical events including age at first calving, duration of the postpartum interval for each successive calving, conception and pregnancy rate and ultimately manifested as length of intercalving intervals and number of calves weaned over her lifetime. Puberty in heifers is a consequence of the interactive effects of genetics and both pre- and post-weaning nutrition. Early onset of puberty is essential to achieving the first main reproductive target for beef cow herds; first calving at 2 years of age. In calved heifers and mature cows, the onset of ovarian activity, postpartum is a key event dictating the calving interval. Again, this will be the product mainly of prepartum nutrition, manifested through body condition and the strength of the maternal bond between cow and calf, though there is increasing evidence of a modest genetic influence on this trait. Following the initiation of postpartum ovarian cyclicity, conception and subsequent pregnancy rate is generally a function of bull fertility in natural service herds and heat detection and timing of insemination in herds bred through artificial insemination. Cows and heifers should be maintained on a steady plane of nutrition during the breeding season, but the contribution of significant excesses or deficiencies of nutrients including protein and trace elements is likely to be minor where adequate pasture is available. While, increased efforts are being made internationally to genetically identify and select for more reproductively efficient beef cows, this is a more long-term strategy and will not replace the need for a high level of technical efficiency and management practice at farm level.

Effect of beef heifer development system on average daily gain, reproduction, and adaptation to corn residue during first pregnancy

Postweaning heifer development systems were evaluated at 2 locations in a 4-yr study for their effect on performance and subsequent adaptation to grazing corn residue as a pregnant heifer. In Exp. 1, heifers were blocked by BW and randomly assigned to graze winter range (WR) or graze winter range and corn residue (CR). In Exp. 2, heifers were assigned to graze winter range and corn residue (CR) or graze winter range and placed in a drylot (DL). Artificial insemination and natural mating were used at breeding on the basis of location. In Exp. 1, heifers developed on corn residue tended (P = 0.11) to have reduced ADG compared with WR heifers. Subsequently, BW at the end of the 82-d corn residue grazing period tended (P = 0.09) to be lower for CR compared with WR heifers. However, the proportion of heifers attaining puberty before the breeding season and pregnancy rates were similar (P ≥ 0.29) for CR and WR heifers. Developing heifers on winter range tended (P = 0.09) to reduce heifer development costs $36/pregnant heifer compared with CR heifers. In Exp. 2, DL heifers had greater (P < 0.01) overall ADG during development compared with CR heifers, resulting in greater (P < 0.01) prebreeding BW for DL heifers compared with CR heifers (355 vs. 322 ± 9 kg). At pregnancy diagnosis BW remained greater (P = 0.02) for DL compared with CR heifers (423 vs. 406 ± 7 kg). Corn-residue-developed heifers had increased (P = 0.03) AI conception rates compared with DL heifers (78% vs. 67% ± 6%). However, there was no difference (P ≥ 0.21) in percent pubertal before the breeding season or final pregnancy rates for CR and DL heifers. Developing heifers on corn residue reduced (P = 0.02) heifer development costs $38/pregnant heifer compared with DL-developed heifers. A subset of pregnant heifers from both experiments grazed corn residue fields in late gestation. As pregnant heifers grazing corn residue, WR heifers (Exp. 1) tended to have reduced ADG compared with CR heifers (0.34 vs. 0.43 ± 0.08 kg/d, P = 0.07). Furthermore, in Exp. 2 CR heifers had greater (0.41 vs. 0.30 ± 0.22 kg/d) ADG grazing corn residue as pregnant heifers compared with DL-developed heifers. Calving date, dystocia score, and calf birth BW were similar (P ≥ 0.15) between development systems in both experiments. There appears to be a potential learned grazing behavior for heifers developed on corn residue allowing them to better adapt to grazing corn residue as pregnant heifers compared with WR and DL heifers.

Determination of anti-Müllerian hormone at estrus during a synchronized and a natural bovine estrous cycle

Anti-Müllerian hormone (AMH) has been correlated with phenotypic indicators of fertility. However, the effects of exogenous hormones used during estrus synchronization on AMH have not been evaluated. Therefore, the objective of this experiment was to determine whether concentrations of AMH at estrus are similar between a synchronized compared with a natural estrous cycle. Nulliparous dairy and beef heifers (n = 68) were synchronized with the Select Synch + controlled internal drug release (CIDR) protocol (GnRH + CIDR-7 d-CIDR removal + PG). Heifers were observed for expression of estrus every 6 h until 84 h after the injection of PG. Visual detection of the subsequent estrus, considered natural estrus, occurred every 6 h from day 16 to 24 after synchronized estrus. At the time of standing estrus, ovarian structures in heifers were evaluated by transrectal ultrasonography. Blood samples were collected at estrus for analysis of concentrations of AMH during the synchronized and natural estrous cycles. The GLM and CORR procedures of SAS were used to analyze data. Concentrations of AMH between natural and synchronized estrus were positively correlated (r = 0.67; P < 0.001). Mean concentration of AMH did not differ (P > 0.05) between the natural (0.0543 ± 0.0076 ng/mL) or synchronized (0.0428 ± 0.0076 ng/mL) estrous cycles. In conclusion, concentrations of AMH were similar between natural and synchronized estrous cycles. Concentrations of AMH in natural and synchronized estrous cycles were highly correlated within individual heifers and varied among heifers with beef heifers having increased (P < 0.05) concentrations of AMH compared with dairy heifers (0.0638 ± 0.01 and 0.0402 ± 0.01 ng/mL, respectively).

Nutritional development and the target weight debate

Postnatal nutrition has immediate and long-lasting effects on beef heifer reproductive efficiency, longevity, and productivity. This article reviews the effects of nutrients and nutritional management on reproduction in developing heifers. In addition, the current debate on the preferred target weight for heifers at breeding is discussed.

Luteal function, largest follicle, and fertility in postpartum dairy cows treated with 14dCIDR-PGF2α versus 2xPGF2α-Ovsynch for timed AI

A method for timed artificial insemination (AI) that is used for beef cows, beef heifers, and dairy heifers employs progesterone-releasing inserts, such as the controlled internal drug release (CIDR; Zoetis, New York, NY, USA) that are left in place for 14 days. The 14-day CIDR treatment is a method of presynchronization that ensures that cattle are in the late luteal phase of the estrous cycle when PGF2α is administered before timed AI. The objective of this study was to test the effectiveness of the 14dCIDR-PGF2α program in postpartum dairy cows by comparing it with the traditional "Presynch-Ovsynch" (2xPGF2α-Ovsynch) program. The 14dCIDR-PGF2α cows (n = 132) were treated with a CIDR insert on Day 0 for 14 days. At 19 days after CIDR removal (Day 33), the cows were treated with a luteolytic dose of PGF2α, 56 hours later were treated with an ovulatory dose of GnRH (Day 35), and 16 hours later were inseminated. The 2xPGF2α-Ovsynch cows were treated with a luteolytic dose of PGF2α on Day 0 and again on Day 14. At 12 days after the second PGF2α treatment (Day 26), the cows were treated with GnRH. At 7 days after GnRH, the cows were treated with PGF2α (Day 33), then 56 hours later treated with GnRH (Day 35), and then 16 hours later were inseminated. There was no effect of treatment or treatment by parity interaction on pregnancies per AI (P/AI) when pregnancy diagnosis was performed on Day 32 (115/263; 43.7%) or Days 60 to 90 (99/263; 37.6%) after insemination. There was an effect of parity (P < 0.05) on P/AI because primiparous cows had lesser P/AI (35/98; 35.7%) than multiparous cows (80/165; 48.5%) on Day 32. Cows observed in estrus after the presynchronization step (within 5 days after CIDR removal or within 5 days after the second PGF2α treatment) had greater P/AI than those not observed in estrus (55/103; 53.4% vs. 60/160; 37.5%; observed vs. not observed; P < 0.01; d 32 pregnancy diagnosis). When progesterone data were examined in a subset of cows (n = 208), 55.3% of cows had a "prototypical" response to treatment (i.e., the cow had an estrous cycle that was synchronized by the presynchronization treatment and then the cow responded appropriately to the subsequent PGF2α and GnRH treatments before timed AI). Collectively, cows with a prototypical response to either treatment had 52.2% P/AI that was greater (P < 0.001) than the P/AI for cows that had a nonprototypical response (19%) (P/AI determined at 60-90 days of pregnancy). In conclusion, we did not detect a difference in P/AI when postpartum dairy cows were treated with 14dCIDR-PGF2α or 2xPGF2α-Ovsynch before timed AI. The primary limitation to the success of either program was the failure of the cow to respond appropriately to the sequence of treatments.

Physiology and Endocrinology Symposium: How single nucleotide polymorphism chips will advance our knowledge of factors controlling puberty and aid in selecting replacement beef females

The promise of genomic selection is accurate prediction of the genetic potential of animals from their genotypes. Simple DNA tests might replace low-accuracy predictions for expensive or lowly heritable measures of puberty and fertility based on performance and pedigree. Knowing with some certainty which DNA variants (e.g., SNP) affect puberty and fertility is the best way to fulfill the promise. Several SNP from the BovineSNP50 assay have tentatively been associated with reproductive traits including age at puberty, antral follicle count, and pregnancy observed on different sets of heifers. However, sample sizes are too small and SNP density is too sparse to definitively determine genomic regions harboring causal variants affecting reproductive success. Additionally, associations between individual SNP and similar phenotypes are inconsistent across data sets, and genomic predictions do not appear to be globally applicable to cattle of different breeds. Discrepancies may be a result of different QTL segregating in the sampled populations, differences in linkage disequilibrium (LD) patterns such that the same SNP are not correlated with the same QTL, and spurious correlations with phenotype. Several approaches can be used independently or in combination to improve detection of genomic factors affecting heifer puberty and fertility. Larger samples and denser SNP will increase power to detect real associations with SNP having more consistent LD with underlying QTL. Meta-analysis combining results from different studies can also be used to effectively increase sample size. High-density genotyping with heifers pooled by pregnancy status or early and late puberty can be a cost-effective means to sample large numbers. Networks of genes, implicated by associations with multiple traits correlated with puberty and fertility, could provide insight into the complex nature of these traits, especially if corroborated by functional annotation, established gene interaction pathways, and transcript expression. Example analyses are provided to demonstrate how integrating information about gene function and regulation with statistical associations from whole-genome SNP genotyping assays might enhance knowledge of genomic mechanisms affecting puberty and fertility, enabling reliable DNA tests to guide heifer selection decisions.