Bos Indicus-influenced Beef Heifers Research Articles

143 Short- and long-term effects of heat stress in cow-calf pairs adapted to tropical and subtropical regions

Abstract Chronic exposure to high temperatures and humidity can limit beef cattle performance in tropical/subtropical environments, regardless of breed. Despite better heat adaptability compared with Bos taurus cattle, heat-stressed Bos indicus cattle also experience an increase in body temperature above the physiological limit. As body temperature increases, cascades of behavioral and physiological events are triggered to lessen heat load but often at the expense of productive outcomes. Contrary to other species and feedlot beef cattle, the effects of heat stress on physiology and performance of grazing cow-calf pairs have not been evaluated in detail, particularly for Bos indicus cattle. Preliminary data showed opposite outcomes to heat stress in Bos indicus vs. Bos taurus cattle, limiting the implications of the existing data generated from studies using beef and dairy Bos taurus cattle. This presentation will share some nutritional and management practices to mitigate heat stress in grazing beef cattle. Practices mentioned herein include altering the timing of body weight (BW) growth pattern (stair-step, SST) and immunomodulatory feed ingredient (IFI) supplementation for replacement beef heifers, and maternal access to shade to reduce heat load. For 100 d before the breeding season, Bos indicus-influenced beef heifers grazing warm-season grasses received a constant supplementation amount or SST strategy (50 d of low then 50 d of high supplementation), with or without IFI. Under severe heat stress conditions, IFI supplementation reduced heifer vaginal temperature but did not increase their growth and reproduction, whereas the SST strategy lowered internal body temperature, enhanced growth performance and pregnancy success compared with a conventional constant supplementation practice. Our group is also focused on the critical need to determine the specific effects of heat stress mitigation during pre- and postnatal phases on physiology and performance of Bos indicus-influenced cow-calf pairs. Study 1 evaluated the impacts of maternal access to artificial shade from 83 d prepartum until 50 d postpartum on body thermoregulation, performance, and physiology of beef heifers and their first offspring. Removing maternal access to shade led to greater maternal respiration rate, vaginal and body surface temperatures and reduced maternal body condition score and calf BW at birth. Nonetheless, offspring born from cows with no access to shade were remarkably heavier during a 60-d post-weaning period and had decreased plasma indicators of inflammation and positive effects to humoral immune response. Study 2 is a multi-year, USDA/NIFA-funded study designed to capitalize on the ability of cattle to reprogram physiological/immunological systems following exposure to stress during gestation. Study 2 combined gestational and postnatal heat abatement to enhance the overall growth/immune/reproductive performance of grazing Bos indicus-influenced cow-calf pairs. Partial data on long-term growth and reproductive performance, physiology, metabolome, and microbiome of both cows and their female offspring will be shared in this presentation.

134 A 12-year summary of effects of body weight at breeding and synchronization protocols on reproductive success of Bos indicus-influenced beef heifers grazing warm-season grasses

Abstract A retrospective analysis of 5 studies (2011 to 2022) evaluated the effects of body weight and synchronization protocols before the start of breeding season on reproduction of Bos indicus-influenced beef heifers. Data were collected from 594 Brangus crossbred heifers weaned on average at 280 ± 26 d of age, allocated to bahiagrass pastures (4 to 5 heifers and 0.8 to 1.0 ha/pasture), and supplemented with soybean hulls-based concentrate (0.5 to 1.8% of body weight, (BW); dry matter basis] from 2 weeks post-weaning until the end of the breeding season (189 ± 24 days of supplementation). Heifers were initially sorted into those that were supplement (S) and were not supplemented (NS) assigned to an estrous synchronization protocol before the start of the breeding season. Then, within each synchronization group, heifers were sorted into those that were below (BE) or above (AB) the average BW of all heifers (300 kg) at the start of the breeding season (n = 130, 126, 146, and 192 for BE+NS, BE+S, AB+NS, and AB+S heifers, respectively). Data were analyzed using GLIMMIX procedure of SAS using synchronization, BW at breeding, and resulting interaction as fixed effects, and heifer (synchronization × BW), study, and pasture(study) as random effects. Effects of synchronization × BW at breeding and synchronization were not detected for heifer BW and overall average daily gain (ADG). However, AB heifers were always (P < 0.0001) heavier from post-weaning until the end of the breeding season and had (P < 0.0001) greater overall ADG than BE heifers. Effects of synchronization × BW at breeding tended (P = 0.07) to detected for puberty attainment at the start of the breeding season, which was least (P < 0.01) for BE+NS, did not differ between BE+S vs. AB+NS (P = 0.37), and was greatest for AB+S heifers (31, 66, 60, and 81 ± 7.4%, respectively). However, final pregnancy and calving percentages differed (P < 0.01) only for BE vs. AB heifers (68 vs. 81 ± 4.4% and 63 vs. 75 ± 5.2%, respectively). Effects of synchronization × BW at breeding tended (P = 0.10) to detected for percent of heifers calving within the first 30 days of the calving season, which was least (P < 0.03) for BE+NS and did not differ (P = 0.20) among BE+S, AB+NS, and AB+S heifers (48, 69, 61, and 64 ± 5.9%, respectively). Hence, in Bos indicus-influenced beef heifers, BW above 300 kg at the start of the breeding season led to improved overall reproductive performance compared to BW < 300 kg, whereas estrous synchronization protocols had positive effects to puberty attainment (regardless of heifer BW) and improved early calving distribution only when heifers were below 300 kg at the start of the breeding season.

Removing maternal heat stress abatement during gestation modulated postnatal physiology and improved performance of Bos indicus-influenced beef offspring.

This study evaluated the growth and immune response of beef calves born from Bos indicus-influenced beef heifers provided pre- and postpartum heat abatement on pasture. On 83 ± 4 d prepartum (day 0), 64 Brangus crossbred beef heifers (~¼ B. indicus) were stratified by body weight (BW; 454 ± 37 kg) and body condition score (BCS; 6.3 ± 0.28; scale 1 to 9), and then allocated into 1 of 16 bahiagrass pastures (1 ha and 4 heifers per pasture). Treatments were randomly assigned to pastures (8 pastures per treatment) and consisted of heifers provided (SH) or not (NSH) access to artificial shade (4.5 m2 of shade area per heifer) from 83 d prepartum to 50 d postpartum (days 0 to 133). Heifers and calves were managed similarly from day 133 until the start of the breeding season (day 203). Calves were weaned on day 203 (at 119 ± 19 d of age), limit-fed the same drylot diet at 3.5% of BW (DM basis) days 209 to 268 (3 to 4 calves per pen; 8 pens per treatment) and vaccinated against respiratory disease pathogens on days 222 and 236. Heifer intravaginal temperatures from days 35 to 42 were lower (P ≤ 0.03) for NSH vs. SH heifers from 0000 to 0800 hours but greater (P ≤ 0.05) for NSH vs. SH heifers from 1100 to 1800 hours. Heifer intravaginal temperature from days 126 to 132 did not differ (P = 0.99) between NSH and SH heifers. Heifers assigned to NSH had greater respiration rates from days 20 to 96 (P ≤ 0.0007), greater plasma concentration of cortisol on days 35 (P = 0.07) and 55 (P = 0.02), less plasma concentration of insulin-like growth factor 1 (IGF-1) on days 35 (P = 0.10), 55, and 133 (P ≤ 0.05), and less BCS from days 55 to 203 (P ≤ 0.01) compared to SH heifers. Calves born from NSH heifers had less birth BW (P = 0.05), greater overall plasma haptoglobin concentrations (P = 0.05), greater seroconversion against bovine respiratory syncytial virus on day 222 (P = 0.02), tended to have greater ADG from days 209 to 268 (P = 0.07), and had greater BW on day 268 (P = 0.05) compared to SH offspring. Plasma concentrations of cortisol and serum titers against other respiratory disease pathogens did not differ (P ≥ 0.15) between NSH and SH offspring. Hence, removing maternal access to artificial shade: (1) increased prepartum intravaginal temperature and plasma concentrations of cortisol but reduced prepartum BCS and plasma concentrations of IGF-1 in grazing B. indicus-influenced beef heifers; and (2) increased post-weaning BW gain and had positive effects on humoral immune response of their offspring.

310 Effects of Stair Step Strategy and Immunomodulatory Feed Ingredient Supplementation on Thermoregulation, Growth, and Reproduction of Grazing Bos Indicus-Influenced Beef Heifers

Abstract This 2-yr study evaluated the combination of stair step supplementation strategy and OmniGen-AF inclusion on body thermoregulation, growth, and reproduction of Bos indicus-influenced beef heifers. Brangus heifers (n = 64/year) were stratified by BW and age (257 ± 20 kg; 271 ± 22 d) and allocated into 16 bahiagrass pastures. Treatments were randomly assigned to pastures in a 2×2 factorial arrangement. Treatments consisted of concentrate dry matter (DM) supplementation at 1.50% of BW from day 0 to 100 (CON) or concentrate DM supplementation at 1.05% of BW from day 0 to 49 and 1.95% of BW from day 50 to 100 (SST), which included or not OmniGen-AF from day 0 to 100 (OMN; 4 g/45 kg of BW). Heifers were submitted to an estrus synchronization protocol from day 100 to 114, inseminated from day 112 to 114, and exposed to bulls from day 120 to 210. Data were analyzed using SAS GLIMMIX procedure. Heifer BW did not differ (P ≥ 0.49) on days 0 and 56, but on days 100 and 210 it was greater (P ≤ 0.01) for SST vs. CON. Intravaginal temperatures from day 26 to 30, between 0830 and 1600h were least (P ≤ 0.03) for SST offered OMN and did not differ (P ≥ 0.17) among all remaining treatments. Body surface temperature on day 25 and plasma IGF-1 concentrations on day 75 were greater (P ≤ 0.04) for SST vs. CON. Percentage of pubertal heifers, estrus detection, and pregnancy to AI did not differ (P = 0.36) between SST and CON, but final pregnancy percentage was greater (P = 0.04) for SST. Thus, OMN decreased intravaginal temperature of SST heifers but failed to improve their growth and reproduction, whereas the stair step strategy improved body thermoregulation, growth, and final pregnancy percentage of Bos indicus-influenced beef heifers.

PSIII-A-19 Thermotolerance and Residual Feed Intake in Bos-Indicus Influenced Beef Heifers

Abstract This study investigated the relationship between thermotolerance and residual feed intake (RFI) in Bos-indicus influenced replacement beef heifers. Multiple individual vaginal temperatures (VT) collected during the summer in Florida were used to select the most (n = 14; thermotolerant) and least (n = 13; non-thermotolerant) heat tolerant heifers to undergo a feed efficiency test in the fall. The maximum initial VT difference between groups was of 0.19 and 1.2 ± 0.065 °C (P < 0.01) for thermotolerant and non-thermotolerant heifers, respectively. Heifers (398 ± 11.3 kg; approximately 720 d of age) were randomly allocated to four concrete-floored pens equipped with two GrowSafe feed bunks each and received a common ad libitum total mixed ration (16.9% crude protein and 52% total digestible nutrients) for 56 d (14 d adaptation and 42 d evaluation period). Body weight (BW) was collected on d -1, 0, 14, 28, 41, and 42, while VT was recorded (10 min intervals for 4 consecutive d) from d 29 to 32, using temperature probes placed intravaginally with a hormone-free controlled internal drug release device. Final BW, average daily gain (ADG), dry matter intake (DMI), DMI as a percentage of BW, RFI, and VT were analyzed as a completely randomized design. Final BW, ADG, DMI, and VT collected during the feeding period were not affected by thermotolerance phenotype (P ≥ 0.42); however, heifers previously classified as thermotolerant had decreased RFI (P = 0.01; -1.32 vs. 1.27 ± 3.31 kg, respectively) and DMI as a percentage of BW (P = 0.01) when compared with non-thermotolerant heifers. In conclusion, Bos-indicus influenced replacement beef heifers previously classified as thermotolerant based on multiple vaginal temperature measurements collected throughout the summer had decreased residual feed intake when measured in the fall, indicating the potential of more heat tolerant heifers to be more feed efficient.

PSIV-10 Effects of stair-step vs. constant supplementation amount on growth, reproduction, and intravaginal temperature in Bos indicus-influenced beef heifers

Abstract This study investigated growth and reproductive performance of Bos indicus-influenced beef heifers provided a concentrate supplement at either a constant or stair-step supplementation. Sixty-four Brangus heifers were stratified by initial BW and age (255 ± 20 kg; 270 ± 22 d) and assigned to 1 of 16 bahiagrass pastures (4 heifers/pasture). Treatments were randomly assigned to pastures (8 pastures/treatment) and consisted of concentrate DM supplementation at: (1) 1.50% of BW from d 0 to 100 (CON) and (2) 1.05% of BW from d 0 to 49 and 1.95% of BW from d 50 to 100 (STP). Supplementation started on d 0 and continued until start of estrous synchronization on d 100. Intravaginal temperatures (16 heifers/treatment) were collected every 30 minutes from d 25 to 31 (September) and d 85 to 91 (November). All heifers were provided supplement at 1.50% of BW from d 101 to 211. Heifers were bred by AI from d 113 to 115 and Brangus bulls were placed with heifers from d 121 to 211. Data were analyzed using the MIXED procedure of SAS with treatment, time, and interaction as fixed effects. From d 0 to 49, ADG did not differ (P = 0.87); however, STP heifers had greater (P = 0.01; 0.73 vs. 0.56 kg/d) ADG from d 50 to 100 compared to CON heifers. Percentage of pubertal heifers, percentage of heifers in estrus, or reproductive tract scores did not differ between treatments (P ≥ 0.40). Intravaginal temperatures were greater (P < 0.0001) for CON vs. STP heifers from d 25 to 31 but did not differ (P = 0.74) from d 85 to 91. Using a stair-step vs. constant supplementation amount (1.05 to 1.95% vs. 1.50% of BW) before the breeding season reduced intravaginal temperatures and increased overall ADG in Bos indicus-influenced beef heifers.

PSII-30 Effects of Prenatal and Postnatal Nutrition on the Concentration of Neuropeptide Y in the Third Ventricle Cerebrospinal Fluid in Beef Heifers

Abstract Nutrition during gestation and early postnatal life can program developmental changes within the reproductive neuroendocrine system that persist until adult life. Here we tested the hypotheses that maternal nutrition during the second and third trimester of gestation and dietary energy intake during the juvenile period interact to affect concentrations of neuropeptide Y (NPY) in the third ventricle cerebrospinal fluid (CSF) after fasting (basal) and in response to peripheral leptin administration in heifer offspring. Bos indicus-influenced beef heifers were developed using a 3 × 2 factorial arrangement of pre- and postnatal dietary treatments. Beginning at the onset of the second trimester of gestation, pregnant Braford and Brangus cows were fed to reach body condition scores of 7.5–8 (H, obese), 5–5.5 (M, moderate), or 3–3.5 (L; very thin) during the third trimester. Heifer offspring were weaned at ~3.5 mo of age and fed to gain at either a relatively high (H; 1 kg/d) or low (L, 0.5 kg/d) rate between 4–8 mo of age, then fed a common diet subsequently. A subgroup of pubertal heifers (n = 6/group) representing 3 of the maternal × postnatal groups (HH, MH, and LL) were ovariectomized and received estradiol replacement after puberty. Cannulas were placed surgically in the third ventricle. After at least a 2-wk recovery period, heifers were fasted for 54 h and CSF was collected every 30 min for 5.5 h, including a 30-min basal and 5-h treatment period involving 3 hourly IV injections of recombinant oleptin (0.2 µg/kg BW). Basal and post-leptin NPY concentrations in the CSF did not differ (P > 0.10) among LL, MH, and HH heifers. Treatment with leptin had no effect (P > 0.10) on NPY concentration in any group. Therefore, neither the interaction of pre- and postnatal nutrition nor exogenous leptin affected third ventricle NPY tone in this experimental model.

Supplementation frequency and amount modulate postweaning growth and reproductive performance of Bos indicus-influenced beef heifers.

This 2-yr study evaluated the growth and puberty attainment of Bos indicus-influenced beef heifers offered 2 different postweaning concentrate supplementation amounts and delivery frequencies. On day 0 of each year, 64 Brangus crossbred heifers were stratified by initial body weight (BW) and age (mean = 244 ± 22 kg; 314 ± 17 d) and assigned into 1 of 16 bahiagrass pastures (4 heifers/pasture/yr). Treatments were randomly assigned to pastures in a 2 × 2 factorial design (4 pastures/treatment/yr) and consisted of concentrate dry matter (DM) supplementation at 1.25% or 1.75% of BW which were offered either daily (7×) or 3 times weekly (3×) for 168 d. On day 56 of each year, heifers were assigned to an estrus synchronization protocol consisting of intravaginal controlled internal drug release (CIDR) insertion on day 56, CIDR removal on day 70, i.m. injection of 25 mg of prostaglandin F2α (PGF2α) on day 86, and i.m. injection of 100 µg of gonadotropin-releasing hormone and timed-AI at 66 h after PGF2α injection (day 89). Heifers were exposed to Angus bulls from day 89 to 168 (1 bull/pasture). Pregnancy diagnosis was assessed on day 213 of each year. Supplementation amount × frequency effects were not detected (P ≥ 0.12) for any variable, except for plasma concentrations of glucose (P = 0.10) and urea nitrogen (PUN; P = 0.01). Herbage mass, herbage allowance, and nutritive value did not differ (P ≥ 0.12) among treatments. Increasing supplementation DM amount from 1.25% to 1.75% of BW increased (P ≤ 0.05) plasma concentrations of insulin-like growth factor 1 (IGF-1), overall average daily gain (ADG), final BW, percentage of pubertal heifers on day 89, pregnancy and calving percentages, and percentage of heifers calving within the first 21 d of the calving season. However, reducing the supplementation frequency from daily to 3× weekly, regardless of supplementation amount, did not impact overall pregnancy and calving percentages (P ≥ 0.42), but caused (P ≤ 0.05) fluctuations in plasma concentrations of insulin and IGF-1 and decreased (P ≤ 0.03) overall ADG, final BW, puberty attainment on days 56, 89, and 168, and percentage of heifers calving during the first 21 d of the calving season. Hence, increasing the supplement DM amount did not prevent the negative effects of reducing the frequency of supplementation (3× vs. 7× weekly) on growth and reproduction of replacement Bos indicus-influenced beef heifers.

Comparison of long-term progestin-based protocols to synchronize estrus prior to natural service or fixed-time artificial insemination in Bos indicus-influenced beef heifers

This experiment was designed to evaluate breeding strategies involving natural service or fixed-time artificial insemination (FTAI) in Bos indicus-influenced beef heifers (n = 1456) when there were field-type management conditions. Body weights and reproductive tract scores (RTS; Scale 1–5) were obtained for heifers before assignment to one of five treatments: 1) Non-synchronized control exposed for natural service (NS), n = 299; 2) melengestrol acetate + natural service (MGA + NS; 0.5 mg/heifer/d), n = 295; 3) 14-d controlled internal drug release insert + natural service (CIDR + NS), n = 289; 4) 14-d MGA-prostaglandin F2α (PG) + FTAI, n = 295; or 5) 14-d CIDR-PG + FTAI, n = 278. Fertile bulls were placed in pastures with heifers of the three NS treatment groups for a 65-day period which began 10 days after progestin treatments (MGA or CIDR) ended. Heifers in FTAI treatment groups were administered PG (25 mg, IM) 16 days after CIDR removal or 19 days following MGA withdrawal, respectively, and FTAI was performed at 66 (CIDR-PG) or 72 h (MGA-PG) after PG. Gonadotropin-releasing hormone (GnRH; 100 μg, i.m.) was administered at FTAI. Pregnancy status was determined at the end of a 65-day breeding period. Pregnancy rates on Days 21 and 65 of the breeding period differed among treatment groups based on pre-treatment pubertal status (P ≤ 0.02) and body weight (P ≤ 0.05) but did not differ by group. These data highlight the need for continued research efforts to improve reproductive management of Bos indicus-influenced females.

22 Puberty induction protocol, but not supplement amount, overcomes the negative impacts of reduced frequency of supplementation on reproduction of beef heifers

Abstract This study evaluated the performance of Bos indicus-influenced beef heifers submitted to a puberty induction protocol and offered two supplementation amounts and delivery frequencies. Sixty-four Brangus crossbred heifers were stratified by BW and age (244 ± 4 kg; 311 ± 18 d) and assigned into 1 of 16 bahiagrass pastures on d 0. Treatments were randomly assigned to pastures in a 2 × 2 factorial design (4 pastures/treatment) and consisted of soyhulls-based supplementation at 1.25% or 1.75% of BW (DM basis) offered daily or 3 times weekly for 168 d. The puberty induction protocol consisted of CIDR insert on d 56 and removal on d 70, 5-mL PGF2a injection on d 86, 2-mL GnRH injection and timed-AI on d 89. Heifers were exposed to Angus bulls from d 89 to 167 (1 bull/pasture). Pregnancy diagnosis was assessed on d 212. Frequency × rate effects were not detected (P ≥ 0.19) for any variable. Reducing the supplementation frequency from daily to 3X weekly tended (P = 0.07) to decrease overall ADG and puberty attainment on d 56 (53 vs. 41 ± 6%), but not puberty attainment on d 89 (P = 0.45; 84 vs. 78 ± 6%) and pregnancy percentage on d 212 (P = 0.97; 69 vs. 69 ± 7%). Increasing the supplement amount from 1.25% to 1.75% of BW increased (P = 0.04) overall ADG, tended (P = 0.09) to hasten puberty attainment on d 56 (40 vs. 54 ± 6%) and d 89 (74 vs. 88 ± 6%), and enhanced (P = 0.03) pregnancy percentage on d 212 (53 vs. 80 ± 7%). Therefore, the inclusion of a puberty induction protocol, but not increasing the supplement amount, enabled a reduced frequency of supplementation (3 vs. 7 times weekly) without detrimental impacts on percentage of pregnant yearling beef heifers.

103 Puberty induction protocol, but not supplement amount, overcomes the negative impacts of reduced frequency of supplementation on reproduction of beef heifers

Abstract This study evaluated the performance of Bos indicus-influenced beef heifers submitted to a puberty induction protocol and offered two supplementation amounts and delivery frequencies. Sixty-four Brangus crossbred heifers were stratified by BW and age (244 ± 4 kg; 311 ± 18 d) and assigned into 1 of 16 bahiagrass pastures on d 0. Treatments were randomly assigned to pastures in a 2 × 2 factorial design (4 pastures/treatment) and consisted of soyhulls-based supplementation at 1.25% or 1.75% of BW (DM basis) offered daily or 3 times weekly for 168 d. The puberty induction protocol consisted of CIDR insert on d 56 and removal on d 70, 5-mL PGF2a injection on d 86, 2-mL GnRH injection and timed-AI on d 89. Heifers were exposed to Angus bulls from d 89 to 167 (1 bull/pasture). Pregnancy diagnosis was assessed on d 212. Frequency × rate effects were not detected (P ≥ 0.19) for any variable. Reducing the supplementation frequency from daily to 3X weekly tended (P = 0.07) to decrease overall ADG and puberty attainment on d 56 (53 vs. 41 ± 6%), but not puberty attainment on d 89 (P = 0.45; 84 vs. 78 ± 6%) and pregnancy percentage on d 212 (P = 0.97; 69 vs. 69 ± 7%). Increasing the supplement amount from 1.25% to 1.75% of BW increased (P = 0.04) overall ADG, tended (P = 0.09) to hasten puberty attainment on d 56 (40 vs. 54 ± 6%) and d 89 (74 vs. 88 ± 6%), and enhanced (P = 0.03) pregnancy percentage on d 212 (53 vs. 80 ± 7%). Therefore, the inclusion of a puberty induction protocol, but not increasing the supplement amount, enabled a reduced frequency of supplementation (3 vs. 7 times weekly) without detrimental impacts on percentage of pregnant yearling beef heifers.

127 Comparison of long-term progestin-based protocols to synchronize estrus prior to fixed-time artificial insemination or natural service in Bos indicus-influenced beef heifers.

127 Comparison of long-term progestin-based protocols to synchronize estrus prior to fixed-time artificial insemination or natural service in Bos indicus-influenced beef heifers.

Preweaning injections of bovine ST enhanced reproductive performance of Bos indicus-influenced replacement beef heifers.

A 3 yr study evaluated the effects of three preweaning injections of bovine ST, administered 14 d apart, on growth and reproductive performance of Bos indicus-influenced beef heifers. On d 0 of each year, suckling Angus × Brangus heifers (n = 15 heifers/treatment/yr) were stratified by BW (147 ± 20 kg) and age (134 ± 11 d) and randomly assigned to receive an s.c. injection of saline (SAL; 5 mL; 0.9% NaCl) or 250 mg of sometribove zinc (BST; Posilac, Elanco, Greenfield, IN) on d 0, 14, and 28. Heifers and respective dams were managed as a single group on bahiagrass (Paspalum notatum) pastures from d 0 until weaning (d 127). From d 127 to 346, heifers were grouped by treatment, allocated to bahiagrass pastures (1 pasture/treatment/yr) and fed a molasses-based supplement (2.9 kg/heifer daily; DM basis) until d 346. Blood samples were collected on d 0, 14, 28, 42, and then every 9-10 d from d 179 to 346. In yr 3, liver biopsy samples were collected on d 0, 42, and 263. Heifers were exposed to mature Angus bulls from d 263 to 346. Growth performance and physiological parameters were analyzed using the MIXED procedure, whereas reproductive variables were analyzed using the GLIMMIX procedure of SAS. Effects of treatment × year and treatment × year × time were not detected for any variable measured in this study (P ≥ 0.14), except for calving percentage (P = 0.03). Heifers assigned to BST injections had greater overall plasma concentrations of IGF-1 and ADG from d 0 to 42 (P ≤ 0.05), less ADG from d 42 to 127 (P = 0.04), but had similar BW at weaning and postweaning ADG (P ≥ 0.25) compared to SAL heifers. Heifers assigned to BST tended to achieve puberty 26 d earlier (P = 0.10), had greater percentage of pubertal heifers on d 244, 263, 284, and 296 (P ≤ 0.04), tended to have greater overall pregnancy percentage (P = 0.10), and had greater (P ≤ 0.05) calving percentages in yr 1 and 2 (but not yr 3; P = 0.68) compared to SAL heifers. Liver mRNA expression of GHR-1B and IGF-1 on d 0 and 42 did not differ between treatments (P ≥ 0.15), but was greater for BST vs. SAL heifers on d 263 (P ≤ 0.02). Hence, administering three injections containing 250 mg of sometribove zinc at 14 d intervals before weaning (between 135 and 163 d of age) induced long-term impacts on liver gene expression and may be a feasible management practice to enhance puberty and pregnancy attainment in B. indicus-influenced replacement beef heifers.

Estrus synchronization and fixed-time artificial insemination alter calving distribution in Bos indicus influenced beef heifers

To determine the effects of estrus synchronization (ES) and fixed-time artificial insemination (TAI) on calving distribution in Bos indicus influenced heifers, 751 Bos taurus × Bos indicus beef heifers were enrolled in a completely randomized design at 2 locations from January to May of 2016. Within location, all heifers were randomly assigned to one of two treatments: 1) SYNCH (n = 371); heifers were exposed to the 5-day CO-Synch + CIDR protocol where they were treated with 100 μg of GnRH, 25 mg of PGF2α, and a controlled internal drug releasing insert (CIDR) on d 0; heifers received 50 mg of PGF2α at CIDR removal on d 5, and were treated with 100 μg of GnRH and TAI 66 ± 2 h later on d 8; or 2) CONTROL (n = 380); heifers were exposed to natural service without ES or TAI. On d 9, all heifers were exposed to bulls for the remainder of the breeding season at each location. Blood samples were collected on d −9 and on d 0 to determine pretreatment estrous cyclicity (progesterone ≥ 1.0 ng/mL). Pregnancy was diagnosed via transrectal ultrasonography 54 d after TAI by determining the presence of a viable fetus. Fetal age was estimated based on fetal size and structural features at the time of pregnancy diagnosis. Pregnancy rates on d 54 differed (P < 0.001) between locations, but did not differ (P = 0.777) between CONTROL and SYNCH treatments. Pregnancy rates on d 54 were greater (P < 0.001) in cycling compared with non-cycling heifers (63.9 vs 42.4%). A greater (P < 0.05) proportion of SYNCH heifers became pregnant in the first 19 d of the breeding season compared with CONTROL heifers (52.2 vs 46.4%). Overall breeding season pregnancy rates did not differ (P = 0.982) between treatments. In summary, ES and TAI increased the percentage of heifers that conceived in the first 19 d of the breeding season, and therefore, potentially altered the calving distribution by ensuring that more heifers calve early during the subsequent calving season.

510 Impact of estrus synchronization and fixed-time artificial insemination on calving distribution in Bos indicus influenced beef heifers

To determine the effects of estrus synchronization (ES) and fixed-time artificial insemination (TAI) on calving distribution in Bos indicus influenced heifers, 751 Bos taurus × Bos indicus beef heifers were enrolled in a complete randomized design at 2 locations from January to May of 2016. Within location, all heifers were randomly assigned to one of two treatments: 1) SYNCH (n = 371); heifers were exposed to the 5-day CO-Synch + CIDR protocol where they received a 100-µg injection of GnRH (Factrel; gonadorelin hydrochloride; Zoetis Animal Health), 25 mg of PGF2α (Lutalyse; dinoprost tromethamine; Zoetis Animal Health), and a controlled internal drug releasing (EAZI-BREED CIDR; 1.38 g of progesterone; Zoetis Animal Health) insert on d 0, heifers received 50 mg of PGF2α (Lutalyse; dinoprost tromethamine; Zoetis Animal Health) at CIDR removal on d 5, followed by a 100-µg injection of GnRH and TAI 66 ± 2 h later on d 8; or 2) CONTROL (n = 380); heifers were exposed to natural service without any ES or TAI. On d 9, all heifers were exposed to bulls for the remainder of the breeding season at each location. Blood samples were collected on d -9 and on d 0 to determine pretreatment estrous cyclicity (progesterone ≥ 1.0 ng/mL). Pregnancy was diagnosed via transrectal ultrasonography 54 d after TAI by determining the presence of a viable fetus. Fetal age was estimated based on fetal size and structural features at the time of pregnancy diagnosis. Pregnancy rates on d 54 differed (P < 0.001) between locations, but did not differ (P = 0.777) between CONTROL and SYNCH treatments (64.5 vs 65.7%, respectively). Pregnancy rates were greater (P < 0.001) in cycling compared with non-cycling heifers (63.9 vs 42.4%). A greater (P < 0.01) proportion of SYNCH heifers became pregnant in the first 21 d of the breeding season compared with CONTROL heifers (52.2 vs 46.4%). Overall breeding season pregnancy rates did not differ (P = 0.982) between treatments. In summary, ES and TAI increased the percentage of heifers that conceived in the first 21 d of the breeding season, and therefore, potentially altered the calving distribution by ensuring that more heifers calve early during the subsequent calving season.

497 Effects of post-weaning plane of nutrition and estrus synchronization on reproductive performance of Bos indicus-influenced beef heifers

A 3-year study evaluated the effectiveness of estrus synchronization of beef heifers undergoing different post-weaning growth rates. On day 0 of each yr, Angus × Brahman heifers (60 heifers/year; 310 days of age) were stratified by initial BW and age, and randomly assigned, in a 3 × 2 factorial design, to achieve a low (LOW; 0.45 kg/d), medium (MED; 0.73 kg/d), or high (HIG; 1.00 kg/d) growth rate from d 0 to 168 (5 heifers/pasture; 4 pastures/diet/yr). Supplements were formulated to allow LOW, MED, and HIG heifers to achieve 55.0, 62.5, and 70.0%, respectively, of mature weight at initiation of breeding season (d 84). On d 65, all heifers within each pasture (2 pastures/diet) were enrolled (SYNC) or not (NOSYNC) in an estrus-synchronization protocol, which consisted of intravaginal controlled internal drug release (1.38 g P4) insertion on d 65 and removal on d 75, followed by 100-µg i.m. injection of GnRH on d 77, and 25-mg i.m. injection of PGF2α on d 84. Heifers were stratified by diet (1 pasture/diet) and exposed to Angus × Brahman bulls from d 84 to 168 (1 yearling bull/pasture). Pasture was the experimental unit, whereas year, heifer (pasture), and pasture (diet × synchronization) were included as random effects in all analyses. Growth performance, and puberty and calving distribution were analyzed using MIXED and GLIMMIX procedures of SAS, respectively. Interactions among growth rate, estrus synchronization, and time were not detected for any variable (P ≥ 0.26). Overall heifer ADG achieved only 51 to 56% of targeted ADG, and mature BW on d 84 did not differ between MED and HIG heifers (57.6 vs. 58.6 ± 2.40%; P = 0.30), but both were heavier than LOW heifers (53.7 ± 2.40%; P < 0.0001). Percentage of pubertal heifers on day 84 was greater for MED and HIG vs. LOW heifers (44.4 and 45.4 vs. 26.5 ± 6.77% P ≤ 0.05), whereas estrus synchronization increased attainment of puberty on d 84 (23.5 vs. 48.7 ± 4.64% for NOSYNC and SYNC heifers; P < 0.01). Pregnancy and calving rates were not affected by diet or estrus synchronization (P ≥ 0.24), however heifers calved 9 d earlier with the use of estrus synchronization (P = 0.02). Estrus synchronization resulted in earlier attainment of puberty and pregnancy in Bos indicus-influenced beef heifers; however, at the low rates of BW gain achieved in the current study, estrus synchronization did not impact overall reproductive success.

501 Pre-weaning injections of bovine somatotropin altered liver gene expression, and enhanced puberty attainment and calving rates of Bos indicus-influenced beef heifers

A 3-yr study evaluated the effects of three 14-d apart injections of bovine somatotropin (bST) to suckling beef heifers on pre-weaning (yr 1, 2 and 3) and post-weaning growth and reproductive performance (yr 1 and 2 only). On d 0 of each yr, Angus × Brangus heifers (n = 15 heifers/treatment/yr; BW = 147 ± 20 kg; Age = 134 ± 11 d) were stratified by BW and age, and randomly assigned to receive s.c. injections of saline (SAL; 5 mL; 0.9% saline) or bST (250 mg of sometribove zinc; Posilac, Elanco, Greenfield, IN) on d 0, 14 and 28. Cow-calf pairs were allocated to 4 bahiagrass (Paspalum notatum) pastures (7-8 pairs/pasture/yr) from d 0 until weaning (d 127). Unshrunk BW and blood samples were collected on d 0, 14, 28 and 127. From d 127 to 346, heifers were pooled by treatment and allocated to bahiagrass pastures (1 pasture/treatment/yr) and fed soybean hulls-based concentrate at 1.1% BW (DM basis). Liver samples were collected via needle biopsy on d 0, 42, and at start of breeding season (d 262) of yr 3 only (6 heifers/treatment). Blood samples were collected every 9 to 10 d from d 127 to 346. Heifers were considered pubertal when 2 consecutive plasma samples had P4 ≥ 1.5 ng/mL. Resulting interactions among treatment, yr, and day were not detected for any variable measured in the study (P ≥ 0.12). During pre-weaning phase, bST heifers had greater ADG from d 0 to 42 (1.15 vs. 1.07 ± 0.026 kg/d; P = 0.03), but similar BW at weaning compared to SAL heifers (261 vs. 259 ± 1.3 kg, respectively; P = 0.25). Liver mRNA expression of GHR-1B and IGF-1 did not differ on d 0 and 42 (P ≥ 0.15), but was greater for bST vs. SAL heifers on d 262 (P ≤ 0.02). During post-weaning phase (d 127 to 347), bST heifers had similar ADG (0.17 vs. 0.12 ± 0.07 kg/d; P = 0.11), a greater (P ≤ 0.02) percentage of heifers attaining puberty at initiation of the breeding season (53 vs. 27 ± 7.9%), pregnancy (93 vs. 70 ± 6.6%) and calving rates (90 vs. 53 ± 7.3%) compared to SAL heifers. Calving distribution did not differ between treatments (P ≥ 0.15). Therefore, three 250 mg injections of bST administered at 14-d intervals, between 135 and 163 d of age, enhanced reproductive performance of Bos indicus-influenced beef heifers.

Effects of post-weaning growth rate and puberty induction protocol on reproductive performance of Bos indicus-influenced beef heifers.

A 3-yr study evaluated the effectiveness of puberty induction using exogenous reproductive hormones in beef heifers undergoing different post-weaning growth rates. On d 0 of each year, grazing Angus × Brahman heifers (60 heifers/yr) were stratified by initial BW and age (BW = 253 ± 30 kg; age = 310 ± 18 d), and randomly assigned, in a 3 × 2 factorial design, to achieve a low (LOW; 0.45 kg/d), medium (MED; 0.73 kg/d), or high (HIGH; 1.00 kg/d) growth rate from d 0 to 168 (5 heifers/pasture; 4 pastures/growth rate/yr). Dietary supplements were offered 3 times weekly (Mondays, Wednesdays, and Fridays) at 0800 h to provide 1.8, 2.9, and 4.4 kg of supplement DM per heifer daily from d 0 to 168. On d 0 of each year, 2 pastures/growth rate treatment were randomly assigned to receive (SYNC) or not (NOSYNC) a puberty induction protocol, which consisted of intravaginal controlled internal drug release (CIDR) insertion on d 65, CIDR removal on d 72, followed by 100-µg i.m. injection of GnRH on d 74, and 25-mg i.m. injection of PGF on d 84. Immediately after the puberty induction protocol, all heifers were grouped by growth rate treatment (1 group/growth rate/yr) into a single pasture and exposed to yearling Angus × Brahman bulls from d 84 to 168 (1 bull/group/yr). Interactions among growth rate and puberty induction protocol were not detected for any variable ( ≥ 0.26). Overall heifer ADG achieved only 51 to 56% of targeted ADG, and percentage of mature BW on d 84 did not differ between MED and HIGH heifers ( = 0.30), but both were heavier than LOW heifers ( < 0.01). Percentage of pubertal heifers on d 84 was greater for MED and HIGH vs. LOW heifers ( ≤ 0.05), whereas puberty induction protocol increased puberty attainment on d 84 ( < 0.01). Overall pregnancy and calving percentages after first breeding season were not affected by growth rate or puberty induction protocol ( ≥ 0.25). However, heifers calved 11 d earlier when enrolled in the puberty induction protocol ( = 0.02). In summary, puberty induction and HIGH and MED growth rates resulted in earlier attainment of puberty and pregnancy in -influenced beef heifers. However, neither growth rate or puberty induction impacted overall heifer pregnancy and calving rates on their first breeding season.

1274 Pre-weaning injections of bovine somatotropin enhanced puberty attainment of bos indicus-influenced beef heifers

1274 Pre-weaning injections of bovine somatotropin enhanced puberty attainment of bos indicus-influenced beef heifers