Evaluation of Three Estrous Synchronization Protocols in Beef Heifers

Objectives of this study were to evaluate synchronization, conception, and pregnancy rates of heifers synchronized with melengestrol acetate (MGA)-prostaglandin F_2α (PGF_2α,), Select Synch, or Select Synch preceded by MGA (MGA-Select Synch). Heifers in the MGA-PGF_2α group (n = 209; BW = 378 kg) received MGA (0.5 mg/ d per heifer) for 14 d and PGF_2α (25 mg) 19 d later. Select Synch heifers (n = 213; BW = 374 kg) received gonadotropin-releasing hormone (GnRH; 100 μg) followed by PGF_2α (25 mg) 7 d later. The MGA-Select Synch heifers (n = 210; BW = 373 kg) were fed MGA (0.5 mg/d per heifer) for 7 d, GnRH (100 μg) the day following the last MGA feeding, and PGF_2α (25 mg) 7 d after GnRH. More (P<0.01) heifers were in estrus 1 to 4 d before PGF2a administration in both the Select Synch (20%) and MGA-Select Synch (24%) groups than in the MGA-PGF_2α (4%) group. Pregnancy rates for heifers in estrus early (d 1 to 4 before PGF_2α) were greater (P<0.05) for both Select Synch (55%) and MGA-Select Synch (63%) compared with MGA-PGF_2α heifers (18%). Synchronization rate (detected after PGF_2α) was greater (P<0.01) for MGA-PGF_2α heifers (86%) compared with Select Synch (66%) and MGA-Select Synch (68%) heifers; however, conception rate did not differ (P=0.13) and averaged 72, 63, and 62% for MGA-PGF_2α, Select Synch, and MGA-Select Synch heifers, respectively. Select Synch (52%), MGA-Select Synch (58%), and MGA-PGF_2α protocols (61%) provided similar (P=0.18) overall AI pregnancy rates; however, more heifers were in estrus before PGF_2α administration in protocols using GnRH.     

Influence of Temperament Score and Handling Facility on Stress,Reproductive Hormone Concentrations,and Fixed Time AI Pregnancy Rates in Beef Heifers

The objectives were (i) to evaluate the effect of temperament, determined by modified 2‐point chute exit and gait score, on artificial insemination (AI) pregnancy rates in beef heifers following fixed time AI and (ii) to determine the effect of temperament on cortisol, substance‐P, prolactin and progesterone at initiation of synchronization and at the time of AI. Angus beef heifers (n = 967) at eight locations were included in this study. At the initiation of synchronization (Day 0 = initiation of synchronization), all heifers received a body condition score (BCS), and temperament score (0 = calm; slow exit and walk or 1 = excitable; fast exit or jump or trot or run). Blood samples were collected from a sub‐population of heifers (n = 86) at both synchronization initiation and the time of AI to determine the differences in serum progesterone, cortisol, prolactin and substance‐P concentrations between temperament groups. Heifers were synchronized with 5‐day CO‐Synch+ controlled internal drug release (CIDR) protocol and were inseminated at 56 h after CIDR removal. Heifers were examined for pregnancy by ultrasound 70 days after AI to determine AI pregnancy. Controlling for synchronization treatment (p = 0.03), facility design (p = 0.05), and cattle handling facility design by temperament score interaction (p = 0.02), the AI pregnancy differed between heifers with excitable and calm temperament (51.9% vs 60.3%; p = 0.01). The alley‐way with acute bends and turns, and long straight alley‐way had lower AI pregnancy rate than did the semicircular alley‐way (53.5%, 56.3% and 67.0% respectively; p = 0.05). The serum hormone concentrations differed significantly between different types of cattle handling facility (p < 0.05). The cattle handling facility design by temperament group interactions significantly influenced progesterone (p = 0.01), cortisol (p = 0.01), prolactin (p = 0.02) and substance‐P (p = 0.04) both at the initiation of synchronization and at the time of AI. Inter‐ and intra‐rater agreement for temperament scoring were moderate and good (Kappa = 0.596 ± 0.07 and 0.797 ± 0.11) respectively. The predictive value for calm and pregnant to AI was 0.87, and excited and non‐pregnant to AI was 0.76. In conclusion, the modified 2‐point temperament scoring method can be used to identify heifers with excitable temperament. Heifers with excitable temperament had lower AI pregnancy. Further, cattle handling facility design influenced the temperament and AI pregnancy.     

Follicle Turnover and Pregnancy Rates Following Oestrus Synchronization Protocols in Mediterranean Italian Buffaloes (Bubalus bubalis)

An ultrasound assessment of follicle turnover following two different protocols for synchronization of oestrus and ovulation, as well as an assessment of achieved synchronization between ovulation and AI and conception rates in nulliparous and pluriparous buffaloes were carried out during months of increasing day length. Nulliparous buffaloes (n = 30) were subjected only to Ovsynch protocol whereas pluriparous buffaloes (n = 31) were assigned to Ovsynch (n = 14) or to PRID-pregnant mare serum gonadotrophin (PMSG) (n = 17) protocol according to the presence of functional CL confirming cyclic and acyclic conditions. Ultrasound examination of ovarian follicular dynamics at critical days in the course of synchronization treatments was employed to monitor the fate of the largest available follicles at the beginning of treatments. Such available dominant follicle would persist throughout the protocol as ovulating follicle (no-follicle shift) or would regress giving way to a new follicle to become dominant and ovulate (follicle shift). Furthermore, ultrasound monitoring would determine the degree of synchronization of ovulation and final outcome represented by pregnancy rates. Pregnancy rate following Ovsynch protocol was 40% (12/30) and 42.8% (6/14) in nulliparous and pluriparous buffaloes respectively (p = 0.8575). Most ovulations were synchronized and recorded at AI and the following day in nulliparous (24/30; 80%) and pluriparous (12/14; 85.7%) buffaloes respectively (p = 1.000). A follicle shift was recorded in 14 of 30 (46.6%) and 11 of 14 (78.5%) in nulliparous and pluriparous buffaloes respectively (p = 0.0466). Among established pregnancies: eight derived from follicle shift (66.6%) and four from no-follicle shift (33.3%) in nulliparous buffaloes, p = 0.0729 whereas in pluriparous buffaloes five (83.3%) derived from follicle shift and one from no-follicle shift (16.6%), p = 0.6154. Collectively, from 18 pregnancies in nulliparous and pluriparous buffaloes following Ovsynch protocol, 13 derived from follicle shift (72.2%) and five from no-follicle shift (27.7%), p = 0.0860. Pregnancy rate in pluriparous buffaloes following PRID-PMSG protocol was 70.5% (12/17). The majority of ovulations were synchronized and recorded at first, second AI and following day (13/17; 76.4%). A follicle shift occurred in 15/17 buffaloes (88.2%) and among the 12 recorded pregnancies, 11 derived from follicle shift (91.6%), p = 0.5147. In conclusion, pregnancy rates following Ovsynch protocol were similar in nulliparous and pluriparous cyclic buffaloes. A progestagen treatment on acyclic buffaloes but still displaying some ovarian follicular dynamics, resulted in significantly higher pregnancy rate compared with Ovsynch (p = 0.0376). According to the time of scheduled AI, a high degree of synchronized ovulations were recorded following the implementation of both protocols.     

Pregnancy Rates in Repeat-breeder Heifers Following Multiple Artificial Inseminations during Spontaneous Oestrus

Hormonal asynchronies during oestrus, related to the presence of suprabasal plasma-progesterone (P4) concentrations and a delayed ovulation, interfere with the fertility of repeat-breeder heifers (RBH). Since tubal dysfunction can occur in connection with hormonal asynchronies and constrained availability of fertile spermatozoa at the time of ovulation, the present study tested the hypothesis that frequent sperm deposition from onset of oestrus to ovulation may improve pregnancy rates in RBH. Five RBH and five virgin heifers (VH; controls) were repeatedly artificially inseminated (AI) at 6 h intervals from onset of oestrus to spontaneous ovulation. Hormone analyses revealed suprabasal P₄ concentrations and a delay in the occurrence of the luteinising hormone (LH) surge, but a normal cortisol profile in RBH. Compared with controls, RBH presented longer interval from onset of oestrus to ovulation, and therefore, received more AIs. Pregnancy rates in RBH reached control levels (60%; NS), indicating that the hypothesis might be correct. Pregnancy rates in VH were below the expected range, presumably attributed to a deleterious influence of the frequent handling. The study suggests that pregnancy rates can be improved in RBH by frequent AI in relation to spontaneous ovulation. However, this practice of repeated manipulations, while seeming not to show adverse effects, lacks practicality for routine use.     

Extending Interval from Seventeen to Nineteen Days in the Melengestrol Acetate - Prostaglandin Estrous Synchronization Program for Heifers

Two experiments were conducted to determine whether extending the interval between removal of melengestrol acetate (MGA) from feed and injection of prostaglandin F_2α (PGF) from 17 to 19 d would affect synchronization of estrus, conception, and pregnancy rates of beef heifers. In both experiments, heifers were fed MGA for 14 d, and PGF was given at either 17 or 19 d after cessation of MGA feeding. Heifers were observed for estrus and artificially inseminated for 5 d after PGF injection. In Exp. 1, 240 yearling heifers were randomly assigned to either a 17- or a 19-d treatment group according to estrous status and day of the estrous cycle. In Exp. 2, 1409 yearling heifers on a cooperating ranch were randomly assigned to the same two treatment groups without knowledge of estrous status. The PGF injection at 19 d (Exp. 1) caused a higher (P<0.05) percentage of heifers to exhibit estrus by 72 h after the injection compared with heifers receiving the injection at 17 d. A greater percentage (P<0.01) of heifers in the 19-d group were in the late luteal phase of the estrous cycle at the time of PGF injection compared with the heifers in the 17-d group, and pregnancy rates were higher for the heifers in the late luteal phase. In Exp. 2, heifers injected with PGF at 19 d after MGA had a greater (P<0.05) percentage in estrus (10%) during the 5-d breeding period, and had higher (P<0.05) pregnancy rates in 5 d (7.6%) and 50 d of breeding (5.5%), compared with heifers injected with PGF 17 d after withdrawal of MGA. These results indicate that the PGF injection given at 19 d after removal of MGA from the diet increases synchronized estrous response and results in higher pregnancy rates in heifers compared with the 17-d injection treatment.     

Use of Vaginal Electrical Resistance to Diagnose Oestrus,Dioestrus and Early Pregnancy in Synchronized Tropically Adapted Beef Heifers

The primary objective of this study was to determine whether a single measurement of intravaginal electrical resistance (VER), using the commercially available Ovatec^® probe, can discriminate between dioestrus and oestrus in Bos indicus females, which had been treated to synchronize oestrus. Santa Gertrudis heifers (n = 226) received one of three oestrous synchronization treatments: double PGF_2α 10 days apart, 8‐day controlled internal drug release (CIDR) treatment or CIDR pre‐synchronization + PGF_2α 10 days after CIDR removal. The heifers were inseminated within 12 h following observed oestrus, or, if not observed, at a fixed time approximately 80 h, following the last synchronization treatment. They were palpated per rectum for signs of pregnancy 9 weeks after artificial insemination (AI). Vaginal electrical resistance measurements were taken at the completion of synchronization treatments (presumed dioestrus), immediately prior to AI (oestrus), and then at 3 and 9 weeks post‐AI. Mean VER differed between presumed dioestrus and oestrus (113.7 vs 87.4, p < 0.001). The area under the receiver operating characteristics (ROC) curve was 0.925, indicating that VER was highly discriminatory between dioestrus and oestrus. Vaginal electrical resistance at time of AI was negatively associated with odds of conception when all inseminations were included in the analyses [odds ratio (OR) = 0.97; 95% CI 0.95–1.00; p = 0.018], but not when fixed time AIs were excluded (OR = 1.00; 95% CI 0.97–1.03; p = 0.982). Mean VER readings differed between pregnant and non‐pregnant animals at both 3 weeks (120.5 vs 96.7, p < 0.001) and 9 weeks (124.0 vs 100.3, p < 0.001) post‐AI. However, 3‐ and 9‐week VER measurements were not highly discriminatory between pregnancy and non‐pregnancy (area under ROC curve = 0.791 and 0.736, respectively). Mean VER at time of AI for animals diagnosed in oestrus differed between each of the oestrous synchronization treatments (84.7, 73.6 and 78.9, groups 1–3 respectively, p < 0.001). These findings suggest that measurement of VER may improve accuracy of oestrus diagnoses when selecting cattle for AI following oestrous synchronization programmes involving tropically adapted cattle.     

Growth,Luteal Activity,and Pregnancy Rates of Three Breed Types of Dairy Heifers in a Forage-Based Development Program

Growth, luteal activity, and pregnancy rates were evaluated in 89 dairy heifers raised as contemporaries. Breed types were Holstein (H, n = 35), Jersey x H (JH, n = 30), and Brown Swiss x H (BSH, n = 24); they ranged in age from 129 to 205 d at the beginning of the trial. Heifers grazed pasture and received grain supplementation to ensure 0.9 kg of daily BW gain. Hip height, chest depth, and BW were obtained monthly; BCS was recorded at ca. 14 mo of age. At ca. 12 mo of age, two blood samples for each heifer were collected (10 d apart) by jugular vein puncture for progesterone assay. Heifers were considered cycling if progesterone concentrations were > 1 ng/ mL in either of the two serum samples. Heifers were bred artificially (AI) on a synchronized estrus starting at 14 mo of age, and pregnancy status was determined ultrasonically 60 d after breeding. The BSH and H had similar (P>0.05) weights and hip heights; whereas JH were lighter and shorter (P<0.05). No differences (P>0.05) occurred for depth of chest and mean BCS. Estrus occurrence by 12 mo of age was greater (P<0.05) for JH (90%) than for BSH (75%) and lowest (P<0.05) for H (47%). Pregnancy rates did not differ (BSH = 96%, JH = 87%, H = 77%). These data suggest that genetic effects of crossbreeding influence early growth and cyclicity at 12 mo of age for replacement dairy heifers. Forage-based development of dairy heifers may be a suitable option to concentrate feeding for dairy producers in Arkansas and the Southern Region.     

Growth,Luteal Activity,and Pregnancy Rates of Three Breed Types of Dairy Heifers in a Forage-Based Development Program

Growth, luteal activity, and pregnancy rates were evaluated in 89 dairy heifers raised as contemporaries. Breed types were Holstein (H, n = 35), Jersey × H (JH, n = 30), and Brown Swiss × H (BSH, n = 24); they ranged in age from 129 to 205 d at the beginning of the trial. Heifers grazed pasture and received grain supplementation to ensure 0.9 kg of daily BW gain. Hip height, chest depth, and BW were obtained monthly; BCS was recorded at ca. 14 mo of age. At ca. 12 mo of age, two blood samples for each heifer were collected (10 d apart) by jugular vein puncture for progesterone assay. Heifers were considered cycling if progesterone concentrations were ≥1 ng/mL in either of the two serum samples. Heifers were bred artificially (AI) on a synchronized estrus starting at 14 mo of age, and pregnancy status was determined ultrasonically 60 d after breeding. The BSH and H had similar (P>0.05) weights and hip heights; whereas JH were lighter and shorter (P<0.05). No differences (P>0.05) occurred for depth of chest and mean BCS. Estrus occurrence by 12 mo of age was greater (P<0.05) for JH (90%) than for BSH (75%) and lowest (P<0.05) for H (47%). Pregnancy rates did not differ (BSH = 96%, JH = 87%, H = 77%). These data suggest that genetic effects of crossbreeding influence early growth and cyclicity at 12 mo of age for replacement dairy heifers. Forage-based development of dairy heifers may be a suitable option to concentrate feeding for dairy producers in Arkansas and the Southern Region.     

母牛同期发情新技术

上个世纪60年代,同期发情技术开始得到人们的认识和研究,包括牛、羊、马、猪等,其中以牛的同期发情研究较多。最初牛的同期发情技术主要是应用孕激素及其类似物,经口服、皮下注射、埋植、阴道栓等方法,停药后药物抑制卵泡发育的作用消失,新的卵泡发育实现同期发情。70年代,前列腺素开始     

青年母牛在发情周期中卵泡发育波变化规律的研究

为揭示青年母牛卵泡生长发育的动态模式,作者利用B-型超声波诊断仪,对青年母牛在发情周期中卵泡生长发育的过程进行了连续观察。结果表明,卵巢上小卵泡(1~4 mm)、中卵泡(5~7 mm)和大卵泡(≥8 mm)的数量和直径均呈动态变化。大、中、小卵泡的数量、卵泡的发育要经历征集期、选择期、优势化、成熟期和闭锁退化的过程。青年母牛在整个发情周期中,优势卵泡的平均直径为13.6 mm,成熟卵泡的平均直径为13.80 mm,一个发情周期有3~4个卵泡发育波,其中有3个卵泡发育波的为多数(71.43%),有4个的为28.57%。     

The Management of Drug-induced Manipulation of the Estrous Cycle in Normal Cows and Heifers

Part I Prostaglandin-induced Synchronization of Estrus in Beef Cattle     

Novel Long-Acting Progesterone Protocols Used to Successfully Synchronize Donor and Recipient Mares With Satisfactory Pregnancy and Pregnancy Loss Rates

The present study aimed to evaluate pregnancy and pregnancy loss rates of recipients treated with alternative long-acting progesterone protocols, designed to synchronize acyclic and cyclic mares, regardless of their cycle phase. A total of 150 Campolina breed mares were used as recipients. Recipient mares were assigned to six different groups with 25 animals each. Groups 1 to 5 were treated with progesterone at some point. Group 1 (acyclic recipients); group 2 (cyclic estrous recipients with one ≥35 mm follicle); group 3 (cyclic estrous recipients with an anovulatory follicle); group 4 (early estrous cyclic recipients); group 5 (diestrous cyclic recipients), and group 6 (cyclic recipients—control). Embryos (day 8) were transferred 4 days after ovulation or 4 days after progesterone injection. Pregnant diagnosis was performed by transrectal ultrasonography 1 week after embryo transfer. Pregnant recipients were evaluated for possible losses and mares treated every 14 days with 3 g (intramuscular) of long-acting progesterone, until 120 days of pregnancy. Pregnancy at 15 days and pregnancy loss rates were recorded and statistically evaluated through multivariate regression (P < .05). Pregnancy and pregnancy loss rates were similar within groups (G1: 76%–10.5%; G2: 76%–5.9%; G3: 56%–0%; G4: 80%–10%; G5: 60.9%–0%; and G6: 60%–13.3%). In conclusion, the novel long-acting progesterone protocols proposed in this study allowed successfully the utilization of mares with asynchronous cyclic as embryo recipients, serving as an alternative specially when few recipients are available and usual synchronization is not possible.     

氯前列烯醇、孕酮阴道栓单独或结合其它激素处理本地母水牛后的发情和受胎情况

本试验研究对比了PGc、CIDR、PGC +PMSG和CIDR +PGc对 2 95头本地母水牛同期发情定时输精的发情率和受胎率。同时对处理后发情的母水牛分别在第一次输精时肌注hCG、LRHA并以不注射的作对照 ,探讨hCG和LRHA对发情母水牛排卵和受胎的效果。也分析了同一处理方法对处女水牛和经产水牛同期发情率和受胎率的差异。结果表明 :采用CIDR +PGc的同期发情率和受胎率最高 (85 .13%、4 6 .0 3% ) ,PGc +PMSG、CIDR和PGc组的发情率和受胎率分别是 73.0 1%和 4 3.4 8%、78.0 2 %和 4 3.6 6 %、6 4 .18%和 4 1.6 8%。CIDR +PG组的发情率极显著高于PGc组 (P <0 .0 1) ,其它各组间无显著差异 (P >0 .0 5 )。各组母牛的受胎率无显著差异 (P >0 .0 5 )。发情的母水牛在第一次输精时肌注hCG或者LRHA3,排卵率为 89.78%和 91.38% ,极显著高于不处理母水牛 (5 9.74 % ) (P <0 .0 1) ;但受胎率与对照组无显著差异 (P >0 .0 5 )。本试验 4种同期发情处理方法在发情率上经产牛 (84 .4 7% )显著高于处女牛 (5 5 .91% ) (P <0 .0 5 ) ;但受胎率 (45 .6 1% :38.4 6 % )无显著差异 (P >0 .0 5 )。     

胚胎移植受体牛同期发情处理效果分析

本文对不同月份，批次的胚胎移植受体牛同期发情处理效果进行了分析，找出发情率，黄体合格率的差异原因，从而克服一些不利因素对受体牛同期发情处理效果的影响。     

Effects of Feeding Supplemental Fat During Gestation to First-Calf Beef Heifers

Study 1, pregnant crossbred, first-calf heifers (n = 149; BW 493.8 ± 6.3) received gestation diets: control (CON), or added safflower seeds (SAFF), raw soybeans (SOY), or sunflower seeds (SUN). Diets were formulated isocaloric-isonitrogenous, contained 2.4, 4.7, 3.8, or 5.1% fat, and were fed for the last 65.3 ± 4.6 d precalving. Supplemental fat feeding was terminated at calving. Diet effects on dam BW or condition scores and calf birth BW, calving difficulty, and dam estrous cyclicity were generally nonsignificant (P>0.10). Fat-supplemented dams had greater pregnancy rates (P<0.05) and fall calf BW (P=0.08): CON, 79%, 182.4 kg; SAFF 94%, 194.9 kg; SOY, 90%, 197.7 kg; SUN, 91%, 196.8 kg. Study 2, pregnant crossbred, first-calf heifers (n = 83; BW 439.8 ± 7.3) received gestation diets: control (CON2) or added sunflower seeds (SUN2). Diets were formulated isocaloric-isonitrogenous, contained 2.2 and 6.5% fat, and were fed for the last 68.2 ± 5.5 d before calving. Supplemental fat feeding was terminated at calving. Blood samples were collected during the feeding period. Diet effects on dam BW, condition scores, estrous cyclicity, and pregnancy percentage were nonsignificant. Calf birth BW from SUN2 dams tended (P=0.06) to be greater. Diet effects on blood components were nonsignificant except for NEFA concentrations tending to be lower in SUN2 dams at the initial (P=0.08) and mid-gestation feeding (P=0.06) sampling. Major differences were found in forage availability between Studies 1 and 2. We conclude that dietary fat or fatty acids may be an important “reproductive fuel,” and effects of supplemental gestation fat may be masked when adequate nutrients are available in forages consumed postpartum.     

Feeder Cattle Health Management: Effects on Morbidity Rates,Feedlot Performance,Carcass Characteristics,and Beef Palatability

The effects of morbidity, defined as hospital visits per calf during the feeding period, on feedlot performance, carcass characteristics, and beef palatability traits were determined using 273 steer calves originating from one of two preconditioning programs or auction barns. Cattle treated more than once at the feedyard had a 12% lower ADG through reimplant (67 d; P<0.05), but ADG did not differ through harvest (185 d), compared with those cattle not treated at the feedyard. Cattle treated more than once also had lower hot carcass weights, lower marbling scores, and lower yield grades (P<0.05) than cattle not treated. Cattle that originated from the two preconditioning programs had a lower (P<0.05) average number of hospital visits compared with cattle that originated from the auction market. No differences (P>0.05) were noted in the effects of hospital visits or preconditioning treatments on beef tenderness and palatability measures. Overall, morbidity resulted in economic losses as a result of mortality and increased costs associated with hospital treatment. Morbidity also decreased (P<0.05) marbling deposition and USDA quality grade.     

Feeder Cattle Health Management: Effects on Morbidity Rates,Feedlot Performance,Carcass Characteristics,and Beef Palatability,

The effects of morbidity, defined as hospital visits per calf during the feeding period, on feedlot performance, carcass characteristics, and beef palatability traits were determined using 273 steer calves originating from one of two preconditioning programs or auction barns. Cattle treated more than once at the feedyard had a 12% lower ADG through reimplant (67 d; P<0.05), but ADG did not differ through harvest (185 d), compared with those cattle not treated at the feedyard. Cattle treated more than once also had lower hot carcass weights, lower marbling scores, and lower yield grades (P<0.05) than cattle not treated. Cattle that originated from the two preconditioning programs had a lower (P<0.05) average number of hospital visits compared with cattle that originated from the auction market. No differences (P>0.05) were noted in the effects of hospital visits or preconditioning treatments on beef tenderness and palatability measures. Overall, morbidity resulted in economic losses as a result of mortality and increased costs associated with hospital treatment. Morbidity also decreased (P<0.05) marbling deposition and USDA quality grade.     

蛋氨酸铜对西门塔尔牛发情周期生殖激素分泌的影响

选用16头18月龄,体况良好,体重380±26 kg的西门塔尔牛育成母牛,采用完全随机区组设计分为4组,以蛋氨酸铜为铜源,研究日粮加铜(0、8、16和24 mg/kg)对发情周期生殖激素分泌规律的影响。结果表明:8 mg/kg组和16 mg/kg组显著提高了发情周期LH、FSH、P4和E2水平(P<0.05);24 mg/kg组有所提高,但不显著(P>0.05)。说明添加蛋氨酸铜8~16 mg/kg对发情周期生殖激素分泌有显著促进作用。考虑基础日粮的含铜量,建议日粮铜水平为:13.87~21.87 mg/kg。