Heifers In Exp Research Articles

Effects of feeding duration, implant dose, and terminal window duration on growth performance and carcass characteristics of feedyard steers and heifers

Our objective was to evaluate effects of feeding duration (days on feed, DOF), terminal window (TW), and implant program dose on growth performance and carcass characteristics of beef steers and heifers. Heifers in Exp. 1 (n = 3,360; 351 ± 31 kg) and Exp. 2 (n = 3,778; 310 ± 28 kg) were used in randomized complete block designs with 3 × 2 factorial treatment arrangements. In Exp. 1, factor 1 was DOF (139, 162, and 183) and factor 2 was TW (60 or 100). In Exp. 2, factor 1 was implant program {(1) IH/H—Component TE-IH with Tylan on d 0 followed by a Component TE-H with Tylan [140 mg of trenbolone acetate (TBA):14 mg of estradiol; Elanco], (2) IH/200—Component TE-IH with Tylan on d 0 followed by a Component TE-200 with Tylan, and (3) 200/200—Component TE-200 with Tylan on d 0 and at reimplant} and factor 2 was TW (60 or 100). In Exp. 3, feedlot data from an industry database (Benchmark, Elanco Knowledge Solutions, Kansas City, KS) was used in an observational study and organized by sex and implant program [steers = IS/200 (total of 280 mg of TBA and 36 mg of estradiol), XS (200 mg of TBA and 40 mg of estradiol), and 200/200 (a total dose of 400 mg of TBA and 40 to 56 mg of estradiol or estradiol benzoate); heifers = IH/200 (total of 280 mg of TBA and 28 mg of estradiol) and 200/200]. Repeated-measure and nonlinear broken-slope models were used to evaluate TW on live performance. In Exp. 1, ADG decreased ( P = 0.03) and final BW, marbling score, hot carcass weight (HCW), backfat thickness, and calculated empty body fat (EBF) increased ( P < 0.03) as DOF increased. In Exp. 2, greater implant program dose tended ( P ≤ 0.10) to increase ADG and DMI and increase ( P ≤ 0.02) G:F, HCW, DP, and LM area. Furthermore, overall dose reduced ( P ≤ 0.03) marbling score and EBF. In Exp. 3, interactions between implant program, live performance, and TW were detected ( P < 0.001). Steers given 200/200 exhibited the greatest ( P < 0.05) ADG, followed by XS and IS/200 from TW 50 to 100. Inversely, steers given XS had the greatest ( P < 0.05) F:G, followed by IS/200 and 200/200 from TW 50 to 90. Heifers administered 200/200 exhibited greater ( P < 0.05) ADG between TW 50 to 70 and reduced ( P < 0.05) F:G through TW 110 compared with IH/200. Increased DOF resulted in reduced ADG and increased HCW and EBF. Increased implant dose improved live performance and HCW. There is a finite number of TW days before growth suffers from inadequate stimuli. Furthermore, cattle type, implant program dose, DOF, and TW all require consideration when designing implant programs.

The effect of frequency of supplementing rumen-protected unsaturated fatty acids on blood serum fatty acid profiles in beef heifers and lactating cows.

The objective of this study was to determine if supplementation frequency of rumen-protected fat (RPF) influences circulating serum concentrations of fatty acids (FA), NEFA, and urea nitrogen in beef heifers and lactating cows. In Exp. 1, 12 early gestation beef heifers were supplemented 0.5 kg of corn gluten feed (CGF) daily during a 2-wk adaptation period. During the last 3 d of adaptation, blood samples were collected immediately before supplementation and then 8 and 16 h postsupplementation daily. Each heifer was then randomly assigned to 1 of 3 supplementation frequency treatments of RPF (3, 5, or 7 d/wk) for 3 wk in a 3 × 3 Latin square design with 3 periods (4 heifers per treatment per period), with each treatment receiving the same amount of RPF and CGF per wk (1.0 and 2.7 kg as fed, respectively). Blood samples were collected during the final 3 d of each supplementation period as described in the adaptation period. In Exp. 2, 18 Angus crossbred cows in early lactation were supplemented with 4.54 kg (as fed) of CGF weekly either at 3, 5, or 7 d/wk during a 2-wk adaptation period. Blood samples were collected during the last 3 d of adaptation as in Exp. 1. For the subsequent 3 wk, RPF (530, 318, and 227 g/d when supplemented) was added to the CGF supplement so that each supplementation frequency received 1.59 kg as fed/wk of RPF. Blood samples were collected during the last 3 d of supplementation as in Exp. 1. Serum FA profiles on a random subsample of 9 heifers in Exp. 1 and all animals in Exp. 2 were determined via gas chromatograph (GC), and values were analyzed using the MIXED procedure of SAS. In Exp. 1, there were no differences ( ≥ 0.53) in serum FA profile across supplementation frequencies. There was a decrease in serum 18:2, 18:1 -9 and total FA during the sampling time (time effect, < 0.02). In Exp. 2, there was treatment × time effect ( ≤ 0.001) for both 18:2 and total FA measured. The 7 d/wk frequency had a greater concentration of C18:2 and total FA at 8 h of the sampling period when compared to the other 2 frequencies. These results demonstrate that supplementation of RPF at 3, 5 or 7 d/wk resulted in no changes in serum FA profiles in growing heifers. In early lactating beef cows, there were minor time point changes in serum FA due to supplementation frequency; however, the vast majority of time points were not different, indicating no substantial changes in serum FA profiles.

Evaluation of 2 sugarcane molasses feeding strategies on measures of growth and reproductive performance of replacement beef heifers

ABSTRACT Our objectives were to compare the effects of 2 sugarcane molasses feeding strategies on growth (Exp. 1 and 2), reproductive performance (Exp. 1), and forage intake (Exp. 2) of replacement beef heifers. In Exp. 1 and 2, treatments consisted of cottonseed meal manually mixed with molasses in a slurry form (SLU), or cottonseed meal and molasses provided in separate bunks (SEP). In Exp. 1, 48 Brangus-crossbred heifers (BW = 236 ± 4 kg; age = 335 ± 5 d) were randomly assigned to 1 of 12 bahiagrass pastures (4 heifers per pasture; 6 pastures per treatment). Each pasture received 31.8 kg of molasses and 6.4 kg of cottonseed meal (as fed) twice weekly from d 0 to 70. In Exp. 2, Braford heifers were randomly assigned into 1 of 16 drylot pens to evaluate the daily hay DMI of heifers offered SEP or SLU supplementation twice weekly from d 0 to 21. Effects of treatment and treatment × day were not detected for BW and ADG of heifers in Exp. 1 and 2 ( P ≥ 0.27). In Exp. 1, overall pregnancy rates were similar ( P = 0.55), but puberty achievement tended to be greater for SLU versus SEP heifers ( P = 0.10). In Exp. 2, G:F and hay DMI did not differ between treatments ( P ≥ 0.21). Therefore, cottonseed meal and sugarcane molasses can be offered separately rather than in a slurry form without affecting growth, forage intake, and reproductive performance of grazing replacement beef heifers during breeding season.

Metabolic status, gonadotropin secretion, and ovarian function during acute nutrient restriction of beef heifers1,2

The effect of acute nutritional restriction on metabolic status, gonadotropin secretion, and ovarian function of heifers was determined in 2 experiments. In Exp. 1, 14-mo-old heifers were fed a diet supplying 1.2 × maintenance energy requirements (1.2M). After 10 d, heifers were fed 1.2M or were restricted to 0.4 × maintenance requirements (0.4M; d 0). Heifers received PGF2α (25 mg, intramuscularly) on d -10, 0, and 10 to synchronize ovulation. After 30 d, 1.2M and 0.4M heifers were realimented to 1.2 M for 100 d. Blood samples were collected every other day from d 0 to 14 then 3 times weekly thereafter. Heifers in Exp. 2 were managed as in Exp. 1 except that animals were fitted with an indwelling jugular catheter and blood samples were collected at 10-min intervals for 8 h on d 9, 10, and 11. Concentrations of progesterone in plasma were used to quantify ovarian luteal function. All 1.2M heifers ovulated, whereas only 30% of 0.4M heifers ovulated in Exp. 1. Concentrations of NEFA were greater and concentrations of thyroxine and IGF-I were less (P < 0.05) in plasma of 0.4M heifers compared with 1.2M heifers. The size of dominant follicles in Exp. 1 was reduced (P < 0.05) in 0.4M compared with 1.2M heifers. Concentrations of IGF-I were increased and anovulatory heifers resumed ovarian cycles an average of 35 d after realimentation. Concentrations of insulin were greater (P < 0.05) in plasma of 1.2M compared with 0.4M heifers in Exp. 2. The frequency of LH pulses was reduced (P < 0.05) in 0.4M heifers on d 9, and FSH in plasma on d 11 was not influenced by treatment. Reduced concentrations of IGF-I in plasma of nutrient-restricted heifers were associated with the reduced size of dominant follicles and indicated a local effect of growth factors on follicles. The decreased LH pulse frequency of 0.4M heifers before luteolysis indicates that restriction of nutrients decreased LH support of follicle growth. A preovulatory increase in estradiol in plasma and an ovulatory surge of LH were not detected in nutrient restricted heifers that did not ovulate. It is concluded that restricting beef heifers to 0.4 × maintenance energy requirements reduced the availability of metabolic fuel and decreased metabolic hormones, resulting in changes within the reproductive neuroendocrine-ovarian axis that compromised the ability of the dominant follicle to secrete sufficient concentrations of estrogen to stimulate an ovulatory surge of LH.

116 PREGNANCY RATES AFTER ADMINISTRATION OF EQUINE CHORIONIC GONADOTROPIN (eCG) AT PROGESTERONE INTRAVAGINAL DEVICE REMOVAL AND 14 DAYS AFTER FIXED-TIME AI IN BEEF CATTLE

An important proportion of embryonic deaths in cattle occur during the critical period of embryo development and maternal recognition of pregnancy. High serum progesterone concentrations during this period are associated to higher embryo survival, and the use of eCG could enhance luteal activity. Two experiments were carried out to evaluate the effect of administering eCG at the time of removal of a progesterone-releasing device and 14 days after fixed-time artificial insemination (FTAI) in beef cattle. Multiparous suckled cows between 60 and 90 days postpartum (n = 297; Exp. 1), and two-year old prepuberal heifers (n = 689; Exp. 2) with a body condition score (BCS) between 3.5 and 5 (1 to 8 scale) were used. At the beginning of the experiments, none of females used had a corpus luteum detected by ultrasonography. Females received an intravaginal device containing 0.5 g progesterone (DIB 0.5, Syntex, Buenos Aires, Argentina) and 2 mg oestradiol benzoate i.m. (Benzoato de Oestradiol, Syntex). Eight days later, 500 μg of cloprostenol (Ciclase DL, Syntex) and 0.5 mg oestradiol cypionate (Cipiosyn, Syntex) were given i.m. and devices were removed; FTAI was performed 52 to 56 h after DIB removal. At the time of device removal, the animals were randomly allocated to 4 groups, to receive 400 IU of eCG i.m. (Novormon, Syntex) or placebo at device removal and/or 14 days after FTAI, in a 2 × 2 factorial design. Control group did not receive eCG. In Exp. 1, nose plates were placed on calves on the day of DIB insertion and left for 10 days in order to prevent suckling. Pregnancy diagnosis was performed by ultrasonography at 30 and 60 days after FTAI. Data were analysed by logistic regression. Pregnancy rate at 30 days after FTAI was affected by treatment (P &lt; 0.05; Table 1). Late embryonic/fetal loss rates (non-pregnant females at 60 days after FTAI over those pregnant at 30 days) were 1.6% (3/182) for cows in Exp. 1 and 4.8% (14/292) for heifers in Exp. 2; no differences were detected among groups. In conclusion, administration of 400 IU of eCG at device removal increases pregnancy rate, both in anestrus cows and heifers. However, the administration of a second dose of eCG 14 days after FTAI suggests a possible effect on embryo survival within the first 30 days of pregnancy in cows but not in heifers. Further studies are required to confirm these results. Table 1.Pregnancy rates after administration of eCG at time of progesterone device removal and/or 14 days after insemination in beef cattle Supported by Syntex.

Comparison of long-term progestin-based estrus synchronization protocols in beef heifers1

Two experiments evaluated long-term progestin-based estrus-synchronization programs on the basis of potential for use in facilitating fixed-time AI in estrous cycling and prepubertal beef heifers. In Exp. 1, heifers were assigned to 1 of 2 treatments by age, BW, and estrous cyclicity status. Heifers assigned to the melengestrol acetate-PGF(2α) protocol (MGA-PG; n = 50) received MGA (0.5 mg·animal(-1)·d(-1)) in a 1.0-kg carrier from d 0 to 13 and were administered PGF(2α) (25 mg, intramuscularly) 19 d after MGA withdrawal (d 32). Heifers assigned to the Show-Me-Synch protocol (n = 49) received a controlled internal drug release (CIDR) insert (1.38 g of progesterone) from d 2 to 16 followed by PGF(2α) administration 16 d after CIDR removal (d 32). All heifers were fitted with HeatWatch estrus-detection transmitters at the time of progestin removal for continuous estrus detection through the synchronized period after PGF(2α). In Exp. 2, heifers (n = 396) were assigned to the same 2 treatments described in Exp. 1 by age, BW, and reproductive tract score. Heifers in Exp. 2, however, were fitted with HeatWatch estrus-detection transmitters at PGF(2α) to characterize estrus-distribution patterns during the synchronized period after PGF(2α). Heifers in both experiments were inseminated approximately 12 h after the onset of estrus. In Exp. 1, estrous response after PGF(2α) and mean interval to estrus after PGF(2α) did not differ between MGA-PG and Show-Me-Synch treatments (P = 0.97). The variance for interval to estrus after PGF(2α) tended (P = 0.06) to be reduced among MGA-PG-treated heifers compared with Show-Me-Synch-treated heifers. Conception to AI, AI pregnancy, and final pregnancy rates did not differ (P > 0.1) between treatments. In Exp. 2, estrous response after PGF(2α) was greater (P = 0.01) among Show-Me-Synch-treated heifers (92%) compared with MGA-PG-treated heifers (85%); however, mean interval to estrus after PGF(2α) did not differ (P = 0.74) between MGA-PG (57.4 ± 2.5 h) and Show-Me-Synch (56.2 ± 2.5 h) treatments. The variance for interval to estrus after PGF(2α) was reduced (P < 0.01) among Show-Me-Synch-treated vs. MGA-PG-treated heifers. Conception to AI, AI pregnancy, and final pregnancy rates did not differ (P > 0.1) between treatments. In summary, the Show-Me-Synch protocol compared favorably with the MGA-PG protocol on the basis of estrous response, synchrony of estrus, and resulting fertility after treatment administration.

Evaluating relationships among blood glucose, plasma urea nitrogen, performance, morbidity, and mortality in high-risk feedlot heifers

Two experiments were conducted using crossbred beef heifers (Exp. 1: n=332, 403 lb [183 kg] BW; Exp. 2: n=334, 456 lb [207 kg] BW) to evaluate relationships between blood glucose, plasma urea nitrogen, average daily gain (ADG), and morbidity and mortality rates. Whole blood glucose (Glc) was measured 24 hours after arrival using a handheld Glc monitor. Plasma was obtained from 179 heifers in Exp. 1 and from 334 heifers in Exp. 2 to determine concentrations of plasma urea nitrogen (PUN) and plasma Glc measured using a spectrophotometer. Heifers were placed into blood Glc categories of high (HGlc) and low (LGlc) Glc and high (HPUN) and low (LPUN) plasma urea nitrogen categories based on whether they had higher or lower levels than the mean of the group with which they arrived. In Exp. 1 ADG was lower for heifers categorized as HGlc and HPUN (P&lt;0.05), but in Exp. 2 ADG was lower (P&lt;0.05) for LPUN than HPUN heifers. Clinical signs (CS), first treatment, and retreatment rates were not different among groups in Exp. 1, but mortality rate was highest for HGlc (10.3 vs 6.0%; P&lt;0.03). In Exp. 2. CS. first treatment, and retreatment rates were lower for heifers categorized as LGlc vs HGlc heifers (P&lt;0.03). Days to first treatment were greater (P&lt;0.05) for heifers categorized as HPUN vs LPUN heifers in Exp. 1 and 2. Elevated PUN may be an indicator of existing catabolism and stress in feedlot calves.

Feedlot performance and carcass traits of serially slaughtered finishing heifers (2004)

Two experiments were conducted at the KSU Agricultural Research Southeast Agricultural Research Center Center, Hays, Kansas, to measure feedlot gain and carcass traits of serially slaughtered, yearling crossbred heifers. In Exp. 1, 159 heifers averaging 792 lbs were randomly assigned to one of four slaughter groups, and slaughtered at 21-day intervals beginning at 92 days on feed. In Exp. 2, 181 heifers averaging 759 lbs were randomly assigned to one of four slaughter groups, and slaughtered at intervals of 19, 23 and 21 day, respectively, starting at 127 days. In both experiments, final weight, gain, and carcass weight increased with days on feed. Heifers did not gain body weight between 134 and 155 days on feed in Exp. 1, but heifers continued to gain body weight through 190 days on feed in Exp. 2. Despite having a lighter starting weight, final body weights and hot carcass weights were greater for heifers in Exp. 2 than in Exp. 1 because they had more time on feed. Ribeye area increased with time, although the ratio of ribeye area to carcass weight decreased over time. Increases in backfat and kidney, pelvic, and heart fat suggest that carcass gain increases in fat content over time. Yield grade and marbling scores also increased with each successive slaughter group. Quality grade improved with more days on feed in Exp. 1. Carcass quality was, however, hampered by significantly increased carcass maturity in Exp. 2. Although it is not well defined, the greatest increase in carcass fat deposition seemed to occur between 92 and 113 days on feed in Exp. 1, whereas the increases in carcass fat seemed to increase continually between 127 and 188 days on feed in Exp. 2.

Effects of prenatal androgenization, melengestrol acetate, and Synovex-H on feedlot performance, carcass, and sensory traits of once-calved heifers.

Three studies were designed using a 2 x 2 factorial arrangement of treatments within each experiment to evaluate the effects of various management techniques on growth and carcass traits of once-calved heifers. In Exp. 1, 33 yearling (16 control [C] and 17 prenatally androgenized [PA]) and 39 once-calved (20 C and 19 PA) nonpregnant heifers were used to evaluate the effects of age and PA. Once-calved heifers were approximately 31 kg heavier (P < .03) on-test, initially consumed 11% more (P < .01) DM, and gained 11% faster (P < .08), but had similar (P = .44) gain to feed ratios (.138 vs. .131) as yearlings. Growth performance was not affected by PA. The yearling and once-calved heifers were then slaughtered at approximately 707 and 1,080 d of age, respectively. Despite maturity differences (P < .01), marbling scores (P = .26) and percentage of choice carcasses (P = .27) were similar. However, on the basis of newly accepted grading changes, percentage of once-calved heifer carcasses grading choice decreased from 57.3 to 34.4%. Organoleptic properties did not differ. Experiments 2 and 3 were then conducted to evaluate effects of commercially available products (melengestrol acetate [MGA] and Synovex-H) on feedlot characteristics of heifers following weaning of their first calf. In Exp. 2, 48 once-calved Simmental-based heifers were fed during the summer to a constant compositional fat thickness of .8 cm. Experiment 3 was conducted in the late fall and early winter, during which 52 moderately sized heifers were fed for 116 d postweaning and slaughtered. Performance of heifers in Exp. 2 was not affected by MGA. However, heifers fed MGA in Exp. 3 gained 9% faster (P < .05) than controls. In Exp. 2, Synovex-H tended to improve ADG (P < .09) and feed efficiency (P < .07), but did not affect performance in Exp. 3. Most carcass traits were not affected by either treatment. Nonpregnant, once-calved heifers seem to have a large capacity for efficient weight gain as compared with yearling heifers. However, hormonal compounds, such as testosterone propionate provided pre- or postnatally (Synovex-H), and MGA do not appear to provide the consistent, positive responses often seen with younger, more typical feedlot heifers.

Effect of daily replacement therapy with recombinant bovine somatotropin on somatotropin, insulin-like growth factor I, and onset of puberty in beef heifers immunized against growth hormone-releasing factor.

Two experiments examined whether replacement therapy with recombinantly derived bovine somatotropin (rbST) would induce puberty in heifers that had been actively immunized at 6 mo of age against growth hormone-releasing factor (GRF). Heifers received daily i.m. injections of 25 mg of rbST (Exp. 1, n = 6; Exp. 2, n = 4) or vehicle (VEH; Exp. 1, n = 6; Exp. 2, n = 4) for 56 d. Serum concentrations of somatotropin (ST, nanograms/milliter) were low in all heifers before first injection in Exp. 1 (1.56 +/- .04) and 2 (.95 +/- .03). During treatment, serum ST was greater (P < .01) in rbST than in VEH heifers (75.4 +/- 4.8 vs 2.8 +/- .1 ng/mL, respectively) in both experiments and remained increased through d 57 (32.2 +/- 6.4 vs .90 +/- .01 ng/mL). IN Exp. 1 and 2, concentrations of serum IGF-I were similar in rbST and VEH heifers before treatment, increased (P < .01) 12 h after first rbST, and remained increased (P < .01) through d 57 in rbST heifers. Concentrations of serum insulin (INS) and plasma glucose (GLU) were similar (P > .10) in rbST and VEH heifers before first injection (Exp. 1 and 2). Serum INS (micro-units/milliliter) was greater (P < .01) in rbST (61.7 +/- 3.7 and 36.0 +/- 2.4) than in VEH (12.4 +/- 1.6 and 8.1 +/- 1.0) heifers on d 1 or 2 only, in Exp. 1 and 2, respectively. In Exp. 1, GLU was increased (P < .05) by rbST on d 2 through 57, but only on d 1 in Exp. 2. Proportion of heifers pubertal by d 21 tended to be greater (P < .07) in rbST (3 of 6) than in VEH (0 of 5) heifers in Exp. 1, but not in Exp. 2 (1 of 4 vs 1 of 4, respectively). All heifers in Exp. 1 and 50% of the heifers in Exp. 2 attained puberty by d 56. Daily rbST increased ST, IGF-I, INS, and GLU but did not hasten onset of puberty in heifers immunized against GRF.

Effect of sequential implanting with Synovex on steer and heifer performance

Studies were conducted using 72 (Exp. 1) and 248 (Exp. 2) steer and heifer calves from 1 to 2 mo of age through slaughter to determine whether preweaning implants affect postweaning feed conversion of cattle that are rapidly grown and(or) finished in the feedlot. In Exp. 1, treatments were three Synovex (S) implants administered 0, 70, and 140 d postweaning (NSSS) or Synovex-C (C) administered preweaning and three implants administered postweaning (CSSS). In Exp. 2, treatments were no implants (NNNN), implants administered 0, 74, and 148 d postweaning to calves that had either received no preweaning implant (NSSS), or C (CSSS), and CSSS plus trenbolone acetate (TBA) administered with the last S implant (CSSS-TBA). Synovex-S or -H implants were administered postweaning to steers and heifers, respectively. Steer and heifer calves implanted with C in the preweaning phase were 9.0 and 13.0 kg (Exp. 1) and 7.5 and 15.0 kg (Exp. 2) heavier (P < .10), respectively, at weaning than nonimplanted steer and heifer calves. In Exp. 1, preweaning implant had no affect on postweaning performance or quality grade. In Exp. 2, preweaning implants (NSSS vs CSSS) decreased (P < .10) steer postweaning gains, whereas postweaning implants (NSSS vs NNNN) increased (P < .10) DM intake; however, feed:gain ratio was not affected by implant strategy. Heifers in Exp. 2 that received implants had greater postweaning (P < .10) DM intakes and daily gains than heifers not receiving implants (NNNN); however, feed:gain ratios among treatment groups were unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

Active immunization of heifers against prostaglandin F2 alpha: potential as a sterilization vaccine.

Four experiments were conducted with 210 heifers in an attempt to develop a sterilization vaccine by active immunization against prostaglandin F2 alpha (PGF2 alpha) and to evaluate feedlot performance following immunization against the combination of PGF2 alpha and estrogens. The objectives were: development of a PGF2 alpha-ovalbumin conjugate that would induce antibody formation when used with complete Freund's adjuvant (CFA); comparison of CFA with other adjuvants in relation to PGF2 alpha antibody binding and maintenance of the corpus luteum; examination of growth performance in immunized heifers against both PGF2 alpha and estradiol-17 beta and evaluation of sterilization in PGF2 alpha-immunized heifers maintained with bulls. A PGF2 alpha-ovalbumin conjugate was developed that resulted in antibody production against PGF2 alpha, although antibody binding was quite low. The antibody response in heifers was higher in Exp. I and II than in Exp. III and IV (P less than .05). Complete Freund's adjuvant was the best adjuvant in inducing antibody formation compared with all other adjuvants tested (P less than .01). Corpus luteum (CL) function was maintained for 2.5 mo and ovulation was apparently blocked in Exp. I. The results of Exp. II confirmed those of Exp. I. Fewer than half of the heifers in Exp. III and IV had prolonged estrous cycles. In Exp. IV, immunized heifers became sterile for at least 4 mo when kept with bulls, although the success rate was only 37%. The low levels of antibody titers to PGF2 alpha in Exp. III and IV may be the reason for the failure to maintain CL function in some heifers.(ABSTRACT TRUNCATED AT 250 WORDS)

Photoperiod influences age at puberty of heifers.

Two experiments were conducted to determine if exposure of prepubertal heifers to supplemental lighting hastens the onset of puberty. In Exp. 1, 16 heifers were paired according to birth date (April 21 to July 4) and assigned randomly to exposure to either 18 h light/d (L) or natural photoperiods (N) from 22 wk of age until puberty. Twenty-two heifers in Exp. 2, born between February 27 and March 31 and between May 3 and May 17, 1981, were exposed to L or N from 24 wk of age until March 23, 1982. In Exp. 2, animals were bred at all estrous periods until conception. Age at first ovulation and first estrus were less (P less than .01 for Exp. 1 and P less than .10 for Exp. 2) for L than N heifers. Average ages at first estrus were 318 (L) and 367 d (N) for Exp. 1 and 367 (L) and 394 d (N) for Exp. 2. Age at conception in Exp. 2 was similar for L (380 d) and N (396 d) groups. There were no significant differences between L and N heifers in changes in body weight for either experiment. There was a photoperiod X age interaction (P less than .06) for ovarian volume in Exp. 1 because the rate of ovarian growth was greater for L than N heifers. Concentrations of LH were not affected by photoperiod in Exp. 1 and not measured in Exp. 2. There were no significant changes in LH concentrations between 22 and 34 wk of age. When expressed relative to first ovulation, LH levels were highest at 7 and 2 wk before first ovulation. Concentrations of prolactin in Exp. 1 were not significantly affected by photoperiod. It was concluded that supplemental lighting after 22 or 24 wk of age reduced ages at first ovulation and first estrus in heifers born from February to July. These effects of photoperiod were accompanied by changes in ovarian development.

Evidence for Seasonal and Nutritional Modification of Ovarian and Pituitary function in Crossbred Heifers and Brahman Cows2,3

Alterations in endocrine response in the bovine female after consumption of monensin or exposure to changes in season were observed in two experiments. Thirty-three monensin-fed (M) or control (C) crossbred heifers in Exp. 1A were given a porcine follicle stimulating hormone (FSH-P) challenge on d 16 to 21 postestrus. Nine M heifers ovariectomized (OVX) on d 11 after the FSH-challenged estrus had a greater number of smaller corpora lutea (CL; P less than .005) than did nine d 11 OVX C heifers. Serum progesterone (P4) concentrations were greater in M on d 5 through 13 following the FSH-challenged estrus (P less than .001). Seven M and seven C Brahman cows in Exp. 1B given an identical FSH challenge had similar increases in CL number and size. In Exp. 2, blood samples were obtained from 14 M and 13 C Brahman cows during winter (WI), early spring (ESp) and late spring (LSp) to characterize the preovulatory LH surge. During each seasonal period, blood samples were taken hourly from estrus through 24 h postestrus for each cow. Only one of five WI-C cows had a preovulatory LH surge compared to five of five WI-M cows (P less than .01). No differences were found in the number of cows having an LH surge in ESp or LSp groups. Analysis of combined WI and ESp group values indicated a difference in timing of the LH surge by M or C treatment. Of those cows that had an LH surge, only three of 10 C had peak LH values later than the first sample taken, compared with 10 of 15 M (P less than .10). A heightened LH response was detected in M cows in all comparisons of LH surge profiles (P less than .005). Concurrent with changes in season from WI to ESp was an increase in number of C cows having an LH surge (one of five vs nine of 10; P less than .005), and elevated LH values in ESp-C and M groups compared with WI-C and M groups (P less than .05). Midluteal blood samples taken after every estrus indicated P4 to be greatest in February M cows (P less than .10) and P4 tended to be greater in February and lower in January for all groups. Conclusions are that monensin affects the FSH-P-induced ovulation rate of bovine females of all ages. Seasonal effects occurring between the shortest and longest days of the year exert their greatest influence on the preovulatory LH surge and P4 concentrations in Brahman cows between January and March. Seasonal effects appear to be partially modulated by nutritional factors.

Induction of twins in different breeds of cattle.

Two experiments were conducted examining bilateral transfer of Hereford X Angus crossbred embryos to Angus, Hereford and Hereford x Angus crossbred recipients. In Exp. 1, both virgin heifers and parous cows were used as recipients, in Exp. 2, only virgin heifers were used. No differences in production of twins due to breed of recipient was observed. In Exp. 1, heifers had as high a twinning rate and proportion of pregnancies carried to term as cows. About 9% of the heifers in Exp. 2 experienced late gestation abortion (based on recipients pregnant at 42 d), a problem which was not observed among heifers in Exp. 1. No association between body weight at transfer and abortion was observed. Twin pregnancy was associated with a higher incidence of retained placentas (Exp. 1, P less than .05; Exp. 2, P less than .10), lower birth weight/calf (P less than .01 in both experiments) and shorter gestation period (P less than .01 in both experiments). Calf mortality at birth was similar among twins and singles and, in Exp. 2, a lower (P less than .02) incidence of dystocia was observed among heifers giving birth to twins than among those with single calves. The results of these experiments indicate that both beef heifers and beef cows of three common breed types are capable of maintaining twin pregnancies and successfully producing twins.