Search

The feral pig populations of many countries continue to increase. Scientific studies on population size are scarce, while the numbers of reported observations on presence of and damage caused by feral pigs are increasing. Feral pigs can carry and spread several diseases (including zoonotic), but African Swine Fever (ASF) is of most concern. It is a highly transmissible viral disease associated with an extremely high mortality rate. Since 2009 ASF has appeared in several European countries, with cases being identified first among local feral pigs and consequently in domestic pig production units, indicating a clear linkage with the movement of the feral pig population and the spread of the disease across national boundaries. Control of feral pig populations is currently under discussion. Because massive culling raises questions of animal welfare and ethics, fertility control could represent an important and effective means to control feral pig populations. Contraceptive vaccines have been used with some degree of success in many wild species because they are able to provide a long-term effect without any consequent health problems. However, extensive and efficacious use of vaccines to control feral pig populations is not simple. The aim of this article was to review the progress in immunocontraception use in feral pigs, providing an account of the current status and future perspectives., (© The Author(s) 2019. Published by Oxford University Press on behalf of the American Society of Animal Science. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Pigs immunized against gonadotropin-releasing factor (GnRF) have increased carcass fatness compared to entire males; however, the timing of this increase in fatness after the second immunization against GnRF has not been determined. An experiment was conducted to identify and compare the growth performance, body composition, and physiological changes in immunocastrated males (IC males) at different BW and feeding levels. A total of 64 pigs were used in a 2 × 2 × 2 factorial experiment with the treatments being 1) sex (entire males or IC males), 2) initial BW (45.9 kg [light] or 78.3 kg [heavy]), and 3) feeding regime (2.5 times maintenance [restricted] or ad libitum). The pigs were individually housed, and the diets were fed for 4 wk after the second immunization against GnRF until slaughter at either 68.4 kg BW (light) or 105.8 kg BW (heavy). Immunocastrated males on a restricted feed intake had a lower ADG compared to entire males from d 15 to 28 and d 0 to 28 ( 0.011 and 0.011, respectively). Fat deposition was not affected by sex from d 0 to 14, but from d 15 to 28 IC males deposited 45 g/d more fat than entire males ( = 0.025). Immunocastrated male pigs fed ad libitum deposited 87 g/d more fat from d 15 to 28 than entire males fed ad libitum ( = 0.036). However, there was no difference in fat deposition between IC males and entire males when feed intake was restricted from d 15 to 28. Plasma urea nitrogen levels were greater in IC males compared to entire males from d 7 after the second immunization against GnRF ( 0.05 for d 7, 10, 14, 21, and 28). Plasma concentrations of IGF-1 were lower for IC males compared to entire males on d 3, 7, 10, and 28 ( 0.05 for all days). The following conclusions were made: 1) when pigs are immunized at a light BW (50 kg) and/or are on a restricted feed intake, they have a reduced propensity to deposit fat; however, the restriction in feed intake adversely affects growth rate. 2) The majority of fat deposition for males immunized at heavy BW (80 kg) occurs from d 15 to 28 after the second immunization against GnRF.

An experiment was conducted to determine the standardized ileal digestible (SID) Lys requirement of entire male and male pigs immunized against gonadotrophin releasing factor (GnRF; immunocastrates). A total of 420 entire male and immunocastrated (IC) male pigs weighing 60.1 kg BW (SEM 0.49) were used in a 2 × 5 factorial experiment with the main effects being gender (entire males or IC males) and 5 concentrations of SID Lys:DE ratio (0.32, 0.43, 0.54, 0.64, or 0.75 g SID Lys/MJ DE). The diets were fed for 6 wk until slaughter at 107.5 kg BW (SEM 5.72). Over the entire period, IC males had a greater ADG ( < 0.001), greater ADFI ( < 0.001), and lower G:F ( < 0.001) compared with entire males. Immunocastrated males had increased plasma urea nitrogen (PUN) concentrations compared with entire males from d 10 to 42 ( < 0.001 for all days). Plasma urea nitrogen concentration also increased as Lys concentrations increased from d 3 to 42 ( < 0.001 for all days). Using the linear-plateau model, the optimal ADG for entire males was achieved at SID Lys concentrations of 0.68, 0.62, 0.54, and 0.58 g/MJ DE whereas optimal G:F was achieved at SID Lys concentrations of 0.72, 0.60, 0.54, and 0.51 g/MJ DE for the time periods d 0 to 14, d 15 to 28, d 29 to 42, and d 0 to 42, respectively. For IC males, optimal ADG was achieved at SID Lys concentrations of 0.64, 0.43, 0.38, and 0.40 g/MJ DE whereas optimal G:F was achieved at SID Lys concentrations of 0.64, 0.43, 0.36, and 0.42 g/MJ DE for the same respective time periods. Using the quadratic polynomial model, maximum ADG for entire males was achieved at SID Lys concentrations of 0.62 and 0.58 g/MJ DE whereas maximum G:F was achieved at SID Lys concentrations of 0.59 and 0.68 g/MJ DE for d 29 to 42 and d 0 to 42, respectively. For IC pigs, maximum ADG was achieved at SID Lys concentrations of 0.69, 0.54, and 0.64 g/MJ DE whereas maximum G:F was achieved at SID Lys concentrations of 0.81, 0.54, and 0.64 g/MJ DE for d 0 to 14, d 29 to 42, and d 0 to 42, respectively. A solution could not be found using the quadratic polynomial model for entire males for d 0 to 14 for both ADG and G:F and for both entire males and IC males for d 15 to 28 within the range of Lys values tested. When both the growth performance and PUN values are considered, the results suggest that IC males show a response to dietary SID Lys similar to that of entire males for 2 wk after the second immunization against GnRF. After this, IC males have a lower requirement for SID Lys than entire males.

The objective of this study was to determine the effects of increasing balanced protein, with and without ractopamine hydrochloride (RAC), in the finisher diets of immunocastrates on their carcass cutting yields and respective muscle, fat, and bone proportions. Carcass traits and yields of 120 individually housed entire male pigs and immunocastrates were evaluated following a 2 × 2 × 3 factorial design. Vaccination occurred at 16 and 20 wk of age and slaughtering at 24 wk. Up until 20 wk of age, all pigs received a commercial grower diet. Low-, medium-, and high-protein (7.50, 9.79, and 12.07 g standardized ileal digestible [SID] lysine/kg, respectively) diets were then fed for the last 28 d with RAC supplemented at 0 or 10 mg/kg. Immunocastration and RAC supplementation increased ( = 0.003 and = 0.017, respectively) the live weight at slaughter. The interaction between sex and protein ( = 0.039) for caliper backfat depths showed greater backfat depths in immunocastrates fed medium protein whereas RAC decreased ( = 0.027) the caliper backfat depth. Feeding RAC increased the trotters ( = 0.040), tenderloins (fillets; < 0.001), shoulder ( = 0.002), hindquarter ( = 0.026), loin ( = 0.028), and belly ( = 0.044) percentages. The shoulder ( < 0.001), hindquarter ( < 0.001), and loin ( = 0.005) muscle percentages were increased and the hindquarter fat percentages were decreased ( = 0.032) with RAC supplementation. Immunocastration increased the belly cut ( = 0.006), loin fat ( < 0.001), and belly fat ( < 0.001) percentages. Therefore, carcass cutting and lean yields can be improved by RAC supplementation while decreasing the backfat depth, and the increase in backfat due to immunocastration could be avoided with the correct dietary protein level.

Growth performance and carcass characteristics of physically castrated (PC) and immunologically castrated (IC) pigs fed ractopamine hydrochloride (RAC; 5 mg/kg) were evaluated in 64 pens of 22 pigs each. Male pigs were randomly assigned to castration method at birth. Pigs in the PC group were physically castrated at 5 d of age while IC pigs were administered Improvest at 11 and 18 wk of age. Pigs entered the grow-finish barn at approximately 9 wk of age (d 0). Dietary treatments (control or RAC) were initiated on d 87. Final treatment arrangement was a 2 × 2 factorial of castration method and diet. Data were analyzed using a mixed model with fixed effects of castration method, diet, market group, and all 2- and 3-way interactions. Pen was the experimental unit. From d 0 to 65, IC pigs had 11.2% greater (P < 0.01) G:F and 11.6% less (P < 0.01) ADFI than PC pigs, but ADG was increased 1.0% in PC pigs compared with IC pigs (P < 0.01). From d 65 to 87, IC pigs had 7.9% greater (P < 0.01) ADG and 12.1% greater (P < 0.01) G:F than PC pigs while having similar (P = 0.16) ADFI. At the initiation of diet (RAC) treatments, BW of all treatments were similar (P ≥ 0.32). From d 87 to 120 (RAC feeding period), IC pigs had 10.0% greater (P < 0.01) ADG and 10.5% greater (P < 0.01) ADFI than PC pigs while having similar (P = 0.64) G:F. Feeding RAC increased (P < 0.01) ADG by 16.9% and G:F by 17.9% while having no effect (P = 0.42) on ADFI from d 87 to 120. There were no significant interactions between castration method and diet on growth performance from d 87 to 120. For the entire study (d 0-120), IC pigs had 2.6% greater (P < 0.01) ADG, 4.6% less (P < 0.01) ADFI, and 7.3% greater (P < 0.01) G:F than PC pigs. Averaged over market groups, IC pigs were 2.5 kg heavier (P < 0.01) and had similar (P = 0.10) carcass weights and 1.8 percentage units less (P < 0.01) dressing yields than PC pigs. Additionally, IC pigs had 1.3 mm less (P < 0.01) fat and 1.7 mm less (P < 0.01) loin depth than PC pigs. Pigs fed RAC were 2.9 kg heavier (P < 0.01) and had 2.3 kg heavier (P < 0.01) carcasses and 2.2 mm deeper (P < 0.01) loins but similar (P = 0.21) dressing yields and tended (P < 0.10) to have 0.4 mm less fat than control-fed pigs when averaged over market groups. Group 3 pigs were the heaviest (P < 0.01) at slaughter and had the heaviest (P < 0.01) carcasses, greatest (P < 0.01) dressing yields, and the most (P < 0.01) carcass fat of all market groups. Overall, immunological castration and RAC were additive in terms of improving growth performance and carcass characteristics.

The effects of immunocastration and time after second Improvest dose on proximate composition and fatty acid profile of adipose tissue from jowl and belly of finishing barrows were determined. Physically castrated (PC) and immunologically castrated (IC) barrows were assigned to treatments at birth. Within 5 d of age, PC barrows were physically castrated and IC barrows were administered Improvest (gonadotropin-releasing factor analog; Zoetis, Kalamazoo, MI) at 16 and 20 wk of age. Diets were formulated with corn and soybean meal and did not contain ethanol coproducts. Subsequently, PC (n = 23) and IC (n = 24) barrows were slaughtered biweekly from 22 to 28 wk of age, 2 to 8 wk following second Improvest dose. Adipose tissue samples were collected from the jowl and belly. Main effects of castration method and time after second Improvest dose and their interaction were analyzed with the MIXED procedure of SAS. Overall, IC barrows were heavier at slaughter (136.1 vs. 100.6 kg; P < 0.01), had similar HCW (P = 0.13), and were leaner (50.9 vs. 48.9%; P < 0.01) than PC barrows. In jowl adipose tissue, IC barrows had less lipid content (64.7 vs. 80.2%; P < 0.01) than PC barrows at 2 wk. However, there were no differences (P ≥ 0.75) in lipid content between IC and PC barrows from 4 to 8 wk. Total MUFA content was reduced (42.5 vs. 44.8%; P = 0.04) and PUFA content was greater (16.9 vs. 14.8%; P < 0.01) in IC compared to PC barrows. Increasing time after second dose reduced (P = 0.04) PUFA content from 17.5% at 2 wk to 15.4% at 8 wk. In IC barrows, iodine value (IV) was increased by 6.9 and 3.3 g/100 g compared with PC barrows at 2 and 4 wk, but IV was not different between castration methods (P ≥ 0.95) at both 6 and 8 wk. Meanwhile, in belly adipose tissue, IC barrows had less lipid content than PC barrows (75.3 vs. 82.7%; P < 0.01). Lipid content was unchanged over time (P = 0.34) in either PC or IC barrows. Total SFA and MUFA content of the belly were similar for both IC and PC barrows. Belly adipose tissue from IC barrows had greater concentrations of PUFA than PC barrows (16.2 vs. 14.7%; P < 0.01). In IC barrows, IV was increased by 5.3 g/100 g compared with PC barrows at 2 wk, but IV was not different between castration methods (P ≥ 0.91) from 4 to 8 wk. Results indicate that as time after second Improvest dose progressed, IV of jowl and belly was reduced. This was accompanied by an increase in lipid deposition in jowl, while lipid content remained constant in belly.

Effects of feeding ractopamine (RAC; 5 mg/kg) to physically castrated (PC) and immunologically castrated (IC) pigs on carcass characteristics, cutting yields, and loin quality were evaluated using 285 carcasses. Male pigs were randomly assigned to sex treatments (PC and IC) at birth and fed the same nursery diets before allotment into 32 pens with 22 pigs per pen in a grow-finish barn. Pigs in the PC group were physically castrated at approximately 5 d of age, and pigs in the IC group were administered Improvest at 11 and 18 wk of age. Diet treatments (control or RAC) were initiated on study d 87. Pigs were marketed at 12 d (4.5 wk post-second Improvest dose), 19 d (5.5 wk post-second Improvest dose), and 33 d (7.5 wk post-second Improvest dose) following the start of final diet treatments. Three carcasses per pen were selected for evaluation of cutting yields and loin quality. Data were analyzed using PROC MIXED in SAS with fixed effects of sex, diet, market group, and their interaction; carcass (N = 285) was the experimental unit. Carcasses from RAC-fed pigs were heavier (P < 0.01) and had deeper (P = 0.02) loins than control-fed carcasses. Carcasses from IC pigs were similar (P = 0.22) in weight but had less (P < 0.01) fat and shallower (P = 0.02) loins when compared to PC carcasses. There were differences (P < 0.05) among market groups for carcass weights, fat depths, loin depths, and estimated carcass leanness. For cutting yields, RAC-fed carcasses had greater (P ≤ 0.03) bone-in lean and total carcass cutting yields than control-fed carcasses while there were no differences (P > 0.05) between RAC-fed and control-fed carcasses when evaluating LM color, marbling, firmness, pH, drip loss, and tenderness. Carcasses from IC pigs had greater (P < 0.05) boneless lean yields, bone-in lean yields, and total carcass cutting yields than PC carcasses. There were minimal differences (P < 0.05) in LM marbling, firmness, composition, and tenderness between PC and IC pigs. There was an interaction (P = 0.03) between sex and diet for LM composition. Control-fed PC loins had more (P < 0.01) lipid than all other treatment combinations. Market group had effects (P < 0.05) on carcass cutting yields, LM color, marbling and firmness scores, pH, purge loss, composition, and tenderness. The results from this study indicated RAC and immunological castration were additive in terms of improving carcass cutting yields while having minimal effects on pork quality.

The study used a randomized complete block design (blocking factor was date of start on test) with 5 treatments: 1) physically castrated barrows (PC), 2) intact males (IM), 3) gilts (G), 4) immunologically castrated barrows (IC), and 5) immunologically castrated barrows fed ractopamine at 5 mg/kg (IC+RAC). The study used 192 pigs and was performed from the 16 wk of age (67.2 ± 2.52 kg BW) to a pen mean BW of 132.5 ± 3.60 kg. For IC+RAC, ractopamine was fed for the final 23 d of the study. Pigs were housed in groups of 4 (10 groups for PC, IM, G, and IC and 8 groups for IC+RAC) in a finishing building at a floor space of 1.18 m(2)/pig. Diets were formulated to meet requirements of IM except that the diet for the IC+RAC fed during the ractopamine feeding period was formulated to meet requirements of pigs on that treatment. Pigs had ad libitum access to feed and water throughout the study period and were individually weighed at the start, wk 2 and 4, and subsequently every week until the end of study. For the overall study period, IC had greater (P ≤ 0.05) ADG than the other genders (1,150, 1,024, 1,064, and 954 g/d for IC, PC, IM, and G, respectively; SEM = 25.8) and required fewer days to reach slaughter weight than the other genders (58.1, 61.6, 61.6, and 66.5 d for IC, PC, IM, and G, respectively; SEM = 1.26). Overall ADFI was less (P ≤ 0.05) for IM and G than IC and PC, which were similar (P > 0.05) in this respect (3.11, 3.06, 2.68, and 2.75 kg/d for IC, PC, IM, and G, respectively; SEM = 0.061). Overall G:F was greater (P ≤ 0.05) for IM than the other genders; IC had greater overall G:F than PC and G, which were similar in this respect (0.371, 0.335, 0.397, and 0.347 kg/kg for IC, PC, IM, and G, respectively; SEM = 0.0068). Immunologically castrated barrows had greater (P ≤ 0.05) ADG (30.7%) and ADFI (22.5%) than PC from the second week following the second Improvest dose to the end of the study. During the ractopamine feeding period, IC+RAC had greater (P ≤ 0.05) ADG (11.6%) and G:F (17.3%) than IC. The results of this study confirmed previously observed gender differences and effects of ractopamine on growth performance and that IC grew faster and had greater feed efficiency than PC during the study period.

The aim of this study was to evaluate safety (in terms of detecting possible adverse clinical effects attributable to vaccination), efficacy, and effects on growth performance of a gonadotropin releasing factor analog-diphtheria toxoid conjugate (commercially distributed as Improvac; Zoetis, Zaventem, Belgium) in male pigs raised in a commercial Greek farm. A total of 1,230 male pigs was enrolled in 16 weekly batches and allocated to 3 groups: barrows (castrated on the next day after birth [study Day 0]), pigs vaccinated with the above-mentioned product, and intact boars. Vaccinated pigs were injected subcutaneously with 2 mL of the anti-gonadotropin releasing factor (GnRF) vaccine at 9 to 11 wk of age (60-78 d) and 15 to 17 wk of age (102-120 d) and slaughtered at 22 to 25 wk of age (152-176 d). No clinical abnormalities or adverse events attributable to vaccination occurred. Mean BW of vaccinated pigs was 6% greater compared with barrows at slaughter (P < 0.0001). The vaccinated pigs had greater ADG than barrows from castration to slaughter (8%). In detail, a lower ADG from first to second vaccination (-12%; P < 0.0001) and a 27% greater ADG from second vaccination to slaughter (P < 0.0001) were observed. The ADG of vaccinated pigs and intact boars was not significantly different throughout the study, except from first to second vaccination (boars greater; P = 0.0059) and second vaccination to slaughter (vaccinates greater; P = 0.0390). Feed conversion ratio of barrows was 11 and 8% greater compared with vaccinated pigs (P = 0.0005) and boars (P = 0.0062) from first to second vaccination but was 23 to 26% lower compared with vaccinated pigs (P < 0.0001) and intact boars (P < 0.0001) from first vaccination to slaughter and 7 to 9.5% lower from the second vaccination to slaughter (P = 0.0029 and P = 0.0003 for vaccinates and intact boars, respectively). At slaughter, the belly fat androstenone concentration of all vaccinated pigs and 64% of intact boars was below 200 ng/g. Belly fat skatole concentration was below 20 ng/g in samples from all groups. In conclusion, vaccination against GnRF using the GnRF analog-diphtheria toxoid conjugate tested did not induce adverse clinical effects, proved effective, and compared with physical castration, resulted in greater BW at slaughter and improved ADG and feed conversion ratio from first vaccination to slaughter.

Immunization against GnRH reduces the occurrence of boar-taint-causing compounds in entire male pigs and is likely to alter growth performance, including whole-body protein deposition (PD; N retention × 6.25). Thirty-six male pigs were used to determine the effects of immunization against GnRH with GnRH analog on N retention and plasma urea nitrogen (PUN; a measure of AA catabolism). Four treatments were used: 1) conventional, early castrates (EC), 2) entire male pigs (EM), 3) entire male pigs immunized with GnRH analog (IM), and 4) entire male pigs surgically castrated after 6 wk of age (between 25 and 40 kg BW; late castrates; LC). The GnRH analog was injected at 30 and 70 kg BW. Within each of 9 litters, 4 males were randomly assigned to the 4 treatments. Pigs were fed corn- and soybean-meal-based diets that were not limiting in essential nutrients for high PD. Five consecutive N balances were conducted between d -9 and -4, 1 and 7, 9 and 16, 20 and 26, and 30 and 36, relative to administration of the booster dose of GnRH analog at d 0. Blood was sampled on d -4, -1, 2, 5, 8, 11, 14, 19, 28, and 37. There was an interactive effect of treatment and time on N retention (P < 0.001). Across periods, N retention for EC and LC were similar (32.7 vs. 33.6 g/d) and lower than EM (39.2 g/d, P < 0.001). The N retention in EM and IM were similar up to d 7 (37.8 vs. 38.5 g/d), tended to be greater for EM than IM between d 9 and 16 (38.4 vs. 34.9 g/d, P = 0.07) and was greater for EM than IM after d 20 (40.9 vs. 34.9 g/d, P < 0.05). Between d 9 and 36, N retention in IM was similar to EC and LC. The PUN concentrations were similar in EC and LC across sampling times (15.50 vs. 15.86 mg/dL) and greater than EM (9.33 mg/dL, P < 0.05). The PUN concentrations were similar in EM and IM up to d 5 (9.88 vs. 9.59 mg/dL), tended to be less in EM than IM on d 8 (9.08 vs. 11.85 mg/dL, P < 0.10), and were lower in EM than IM from d 11 to 37 (8.94 vs. 14.80 mg/dL, P < 0.05). After d 8, PUN concentrations were similar for IM, EC, and LC (14.31, 15.13, and 15.55 mg/dL, respectively). In conclusion, the results of the current study show that N retention and PUN patterns in EC and LC are very similar and lower than those in EM. Between d 7 and 16 after administration of the booster dose of GnRH analog, N retention and PUN in IM changed gradually from EM levels to approach levels in EC and LC, which should be considered when developing feeding programs for IM.

Three experiments were conducted to evaluate methods of immunization against GnRH on antibody titer, luteal activity, and pregnancy in beef heifers. Experiment 1 evaluated the efficacy of adjuvants with 30 heifers. Control heifers were immunized against human serum albumin (HSA) emulsified in Freund's complete adjuvant (FCA). The other 4 treatments contained GnRH conjugated to HSA (HSA-GnRH) emulsified in FCA, Freund's incomplete adjuvant (FIA), DEAE dextran (DD) + mineral oil (MO), or DD+FIA. Treatment was in the mammary gland for all experiments. Titers against GnRH for heifers immunized against HSA-GnRH with FCA, DD+MO, or DD+FIA were greater than titers for HSA-GnRH with FIA or control heifers (P < 0.01). Body weight was reduced (P < 0.05) in control and FCA heifers compared with FIA, DD+MO, and DD+FIA heifers. Heifers immunized with DD+MO and DD+FIA had fewer granulomas in mammary glands than heifers treated with FCA (P < 0.01). In Exp. 2, 36 heifers were used to determine the effect of the protein conjugated to GnRH on titers against GnRH. Heifers (6/treatment) received a primary immunization against GnRH conjugated to HSA (HSA-GnRH), ovalbumin (OA-GnRH), or keyhole limpet hemocyanin (KL-GnRH), or heifers were immunized against each carrier protein. Antigens were emulsified in DD+FIA. Immunization of heifers against OA-GnRH, KL-GnRH, or HSA-GnRH suppressed luteal activity (P < 0.01) for 23, 16, and 12 wk, respectively, and antibody titers against GnRH were greater (P < 0.01) for 19, 5, and 7 wk, respectively, compared with heifers immunized against the carrier proteins. In Exp. 3, 90 heifers were used to determine the effect of immunization against GnRH on ovarian activity and pregnancy rate. Heifers (30/treatment) received a primary and 2 or 3 booster immunizations against GnRH conjugated to OA, and controls received a primary and 2 booster immunizations against OA. All antigens were emulsified in DD+FIA. At 8 wk after primary immunization, heifers were exposed to fertile bulls for 24 wk. Pregnancy rate was less (P < 0.01) for 3-booster heifers (13%) compared with control (83%) and 2-booster (62%) heifers. We conclude that immunization against GnRH, conjugated to OA and emulsified in DD+FIA, does not influence ADG and produces sufficient titers against GnRH to prevent estrous cycles with few mammary granulomas. Immunization against GnRH with 3 booster immunizations prevented luteal activity and pregnancy in most beef heifers for more than 4 mo.

The objectives of this study were to evaluate the effects of immunization against recombinant GnRH fusion proteins and growth promotants on onset of puberty, feedlot performance, and carcass characteristics of beef heifers. Heifers were immunized against an ovalbumin fusion protein containing 7 GnRH peptides (oGnRH, n = 12), a thioredoxin fusion protein containing 7 GnRH peptides (tGnRH, n = 12), a combination of oGnRH plus tGnRH (otGnRH, n = 12), or a combination of ovalbumin and thioredoxin (control, n = 11). Each heifer received a primary immunization containing 1 mg of protein in 1 mL of adjuvant injected into the mammary gland at wk 0 (mean age = 38 wk) and booster immunizations at wk 6 and 12. Six heifers within each treatment received Synovex H implants at wk -2. Weekly blood samples were collected from wk -2 to 26 for determination of serum progesterone concentrations and GnRH antibody titers. In GnRH-immunized heifers, GnRH antibody titers increased after the first booster injection, peaked after the second booster injection, and remained elevated through the end of the study (P < 0.01). Heifers immunized against oGnRH achieved greater (P < 0.05) GnRH antibody titers than tGnRH heifers but did not differ (P = 0.20) from otGnRH heifers. During the 26-wk study, ovulation was prevented (P < 0.05) in 10 out of 12, 12 out of 12, 11 out of 12, and 0 out of 11 tGnRH, oGnRH, otGnRH, and control heifers, respectively. At slaughter, uterine weights were lighter (P < 0.01) for GnRH-immunized heifers than control heifers. Synovex H-implanted heifers had greater (P < 0.05) ADG from wk -2 to 26, greater LM area, and lesser percentages of KPH, yield grade, and quality grade than nonimplanted heifers, regardless of the immunization treatment. Immunization against GnRH fusion proteins resulted in production of antibodies against GnRH that prevented ovulation in 92% of the heifers without affecting feedlot or carcass performance. Implanting heifers with Synovex H improved ADG, LM area, and yield grade. Improvements in delivery of the oGnRH vaccine may provide a feasible alternative to surgical spaying of heifers.