Increased calf production in cattle selected for twin ovulations

The effects of increasing fetal numbers and their distribution between the left and right uterine horns on calf survival, calf BW at birth and weaning, gestation length, dystocia, and calf sex ratio were evaluated for single (n = 1,587), twin (n = 2,440), and triplet calves (n = 147) born to primiparous and multiparous females in the Twinner population at the US Meat Animal Research Center between 1994 and 2004. Cattle were distributed equally between the spring and fall breeding seasons. Fetal number and distribution in utero were determined by real-time ultrasonography at 40 to 70 d postbreeding. For cows and heifers combined, number of calves per parturition increased from 1.34 in 1994 to 1.56 in 2004. Gestation length was 6.8 d shorter (P < 0.01) for twins compared with singles (277.5 +/- 0.2 vs. 284.3 +/- 0.2 d) and 12.7 d shorter for triplets (271.6 +/- 0.8 d). Survival and BW of individual calves at birth decreased (P < 0.01) but total calf BW per dam increased (P < 0.01) as fetal number increased from single to triplet births. Twins resulting from bilateral twin ovulations had increased (P < 0.01) survival and BW at birth, a longer (P < 0.01) gestation length, and less (P < 0.01) dystocia than twins resulting from unilateral twin ovulations. Calf survival and BW at birth were 97.2 +/- 0.3% and 48.0 +/- 0.1 kg for singles, 92.0 +/- 0.4% and 39.0 +/- 0.2 kg for bilateral twins, 83.2 +/- 0.4% and 36.7 +/- 0.2 kg for unilateral twins, 73.8 +/- 1.4% and 30.6 +/- 0.7 kg for bilateral triplets, and 51.9 +/- 3.2% and 31.7 +/- 1.6 kg for unilateral triplets. Birth weight of single calves increased by 0.51 kg/d for each additional day of gestation length vs. 0.38 kg/d for individual twins. Calf BW at birth increased (P < 0.01) with age of dam from 2 to 4 yr. Twin and triplet births had a greater (P < 0.01) incidence of dystocia than single births. The ratio of male:female calves (0.52:0.48) at birth was not affected by type of birth. Postnatal calf survival was similar for all 3 types of birth. Total progeny BW at weaning for single, twin, and triplet births was 217.7 +/- 2.5, 328.3 +/- 3.2, and 378.4 +/- 15.0 kg, respectively (P < 0.01). Although most bovine females have the uterine capacity to gestate twin calves, decreased survival and BW of unilateral twins and of all triplets indicate that their growth and development may have been compromised by uterine crowding.     

Possible role of IGF2 receptors in regulating selection of 2 dominant follicles in cattle selected for twin ovulations and births

Abundance of IGF-2 receptor (IGF2R), FSH receptor (FSHR), and LH receptor (LHCGR) mRNA in granulosa cells (GCs) or theca cells (TCs) or both cells as well as estradiol (E₂), progesterone (P₄), and androstenedione concentrations in follicular fluid were compared in cows genetically selected (Twinner) or not selected (control) for multiple ovulations and twin births. Cows were slaughtered at day 3 to 4 (day 3) and day 5 to 6 (day 5) of an estrous cycle, and ovaries, follicular fluid, GCs, and TCs were collected. The two largest (F1 and F2) E₂-active (EA) and E₂-inactive (EI) follicles were selected according to their E₂-to-P₄ ratio and diameter. Androstenedione levels in EA F1 and F2 follicles were 5-fold greater (P < 0.05) in Twinner cows than in control cows on day 3 but did not differ on day 5. Twinner cows also had greater (P < 0.05) E₂ and P₄ concentrations, whereas steroid levels in EI follicles did not differ (P > 0.10) between genotypes. In EA F2 follicles, IGF2R levels in GCs were greater (P < 0.05) in control cows than in Twinner cows on day 3 and day 5, whereas IGF2R mRNA in TCs did not differ (P > 0.10). On day 3, FSHR mRNA levels were greater (P < 0.05) in GCs of EA F1 and EI F2 follicles of control cows than of Twinner cows. LH receptor mRNA expression was less in GCs and greater in TCs of EA F2 follicles in control cows than in Twinner cows (P < 0.05). We hypothesize that reduced GC IGF2R expression in F2 follicles of Twinner cows may play a role in the development of 2 or more dominant follicles.     

Effects of twinning on postpartum reproductive performance in cattle selected for twin births

The effects of twinning, dystocia, retained placenta, and body weight on postpartum reproduction were evaluated for 3,370 single and 1,014 twin births. Females were bred by AI for 40 d followed by 20 or 30 d of natural service with equal numbers bred and calved in spring and fall. Percentage of dams cyclic by the end of the AI period was lower (P<.05) for dams birthing and nursing a single calf (92.4%) than for dams birthing twins and nursing zero (98.7%) or two (94.7%) calves. Whereas the interval from parturition to first estrus was shorter (P<.01) for dams birthing and nursing a single (56.9 d) than for dams birthing twins and nursing one (68.5 d) or two (69.6 d) calves, length of the interval was further reduced by dystocia in nonlactating dams of either twins or singles (type of birth x dystocia, P<.05). Ensuing pregnancy rates were also affected by type of birth and dystocia. Without dystocia, dams birthing and nursing a single calf had a higher pregnancy rate (79.2%) than dams birthing twins and nursing one (61.7%) or two (66.3%) calves, whereas the lower ensuing pregnancy rates associated with dystocia in dams of singles (71.9%) resulted in similar rates among dams of singles and twins with dystocia (type of birth x dystocia; P<.01). Having a retained placenta resulted in a lower incidence of (93.5 vs. 96.4%, with vs. without; P<.05) and a longer interval to (64.7 vs. 59.2 d; P<.01) estrus while reducing subsequent pregnancy rates (X = 9.6%) in 3 of the 7 yr evaluated (retained placenta x year, P<.01). Because all parous females were bred during the same calendrical period, the shorter gestation length for twin calves (275.6 vs. 281.3 d) resulted in a longer interval from parturition to conception for twin births, whereas means for conception date differed by only 2 d between dams of twins and singles. Furthermore, a reduction (P<.01) in the interval to conception occurred with dystocia in dams of singles (89.3 vs. 85.0 d, without vs. with dystocia) and of twins nursed by zero (116.9 vs. 83.5 d), one (100.2 vs. 92.8 d), or two (96.1 vs. 97.2 d) calves. Another detriment to fertility was the higher incidence of fetal mortality or abortions associated with twin vs. single pregnancies (12.4 vs. 3.5%; P<.01). However, despite the lower conception rates for dams of twins, the increased prolificacy provides an opportunity to increase total beef production with a twinning technology.     

Productivity and profitability of twin births in beef cattle

Data from 1,277 single and 85 twin calvings, occurring in both spring and fall from 1980 through 1987, were used to examine the productivity and profitability associated with twin births in beef cattle. Pregnancies in pure and crossbred cattle resulted from both AI and embryo transfer. Cows and calves were confinement-housed. Cows were individually fed to specification. Calves were given ad libitum access to creep feed and those born in 1986 and 1987 were fed to slaughter. The influence of birth number of gestation length, total calf birth and weaning weights, lactation yield, and cow fed intake during both the dry and lactating periods were examined. Twin-bearing cows had their gestation length shortened by 6.4 d (2%); yielded 25.5 (59) and 186.0 kg (73%) more weight of calf at birth and weaning, respectively; had lactation yield and lactation feed intake increased by 25 and 20%, respectively; and had precalving (dry) period feed intake no different from their single-bearing counterparts (P = .12). Postweaning growth was not different for single and twin calves (P = .50); twin gain, relative to initial size, was higher. Feedlot feed intake of twins was 85% of that for singletons (P = .20). Twins were 90% of singleton live weight at slaughter and yielded 93% of singleton hot carcass weight (P = .12). Twins were slightly older and significantly leaner at slaughter. Returns less feed costs showed twin births to be associated with increased profit for cow-calf programs. Returns less feed and overhead costs were higher for twin calves than for singles in the feedlot.     

Fetal development in cattle with multiple ovulations.

Treatment of lactating and nonlactating parous cows (n = 379) with 12 mg of FSH-P to evaluate development of multiple bovine fetuses resulted in ovulation rates ranging from 1 to 27 corpora lutea (CL). Fertilization rate (i.e., ova fertilized at 6 to 8 d postmating, 80.0%) was not affected by ovulation rate. The percentage of fetuses developing normally at 51 to 53 d postmating decreased (P less than .01) as ovulation rate increased; 1 CL, 100.0%; 2 CL, 100.0%; 3 CL, 66.7%; 4 CL, 45.8%; 5 CL, 33.3%; 6 to 10 CL, 13.6%; and greater than 10 CL, 8.9%. Of the 86 cows permitted to calve, 47 produced singles, 22 twins, 9 triplets, 7 quadruplets, and 1 quintuplets. Calf birth weight and gestational length decreased (P less than .01) as the number of calves born increased from one to two to three. Smaller decreases (P less than .05) in birth weight occurred among triplets, quadruplets, and quintuplets, whereas gestational length did not differ (P greater than .1) among these groups. Systemic progesterone concentrations in the dam were proportional (P less than .01) to the number of fetuses in utero between d 126 and 266 for dams gestating one, two, or three or more fetuses; estrone sulfate was lower (P less than .01) in dams with one than in those with two or more fetuses. Placental weight (i.e., cotyledons plus intercotyledonary membranes) per fetus at 52 +/- 1 d of gestation and at term decreased as the number of fetuses increased. The chorioallantoic membranes were often fused among multiple fetuses and contained either all viable or all dead fetuses, but not both, within the same anastomosed placental unit. These results suggest that ovulation rate is the first limiting factor to increasing cow productivity for beef cattle because some bovine females had the capacity to gestate up to three fetuses per uterine horn, or a total of five fetuses, above which pregnancy was terminated.     

Ovarian follicular growth, function and turnover in cattle: a review

Studies in cattle assessing changes in number and size of antral follicles, concentrations of estradiol, androgens and progesterone in serum and follicular fluid, and numbers of gonadotropin receptors per follicle during repetitive estrous cycles and postpartum anestrus are reviewed. The rate of growth of small follicles (1 to 3 mm) into larger follicles increases as the estrous cycle progresses from d 1 to 18 (d 0 = estrus). Size of the largest antral follicle present on the ovary also increases with advancement of the estrous cycle. Most large follicles (greater than 10 mm) persist on the ovarian surface for 5 d or more between d 3 and 13 of the bovine estrous cycle. After d 13, most of these large follicles are replaced more frequently by new growing follicles (turnover) with an increased probability for recruitment of the ovulatory follicle after d 18. More research is needed to determine the time required for growth of bovine follicles from small to large antral size and evoke recruitment of the ovulatory follicle. Factors that regulate selection of the ovulatory follicle are unknown but may involve increased frequency of LH pulses in blood, altered blood flow and(or) changes in intrafollicular steroids and proteins. Quantitative evaluation of ovarian follicles indicated occurrence of consistent short-term changes in fluid estradiol and numbers of luteinizing hormone receptors in cells of large follicles only during the pre-ovulatory period. Presumably, low concentrations of follicular estradiol found during most of the estrous cycle are not due to a lack of aromatizable precursor or follicle-stimulating hormone receptors. Follicular fluid concentrations of progesterone increase only near the time of ovulation. Little is known about changes in follicular growth, turnover and function during postpartum anestrus in cattle. However, preliminary data suggest that the steroidogenic capacity of large follicles changes markedly during the postpartum period.     

Increased abundance of aromatase and follicle stimulating hormone receptor mRNA and decreased insulin-like growth factor-2 receptor mRNA in small ovarian follicles of cattle selected for twin births

Cattle genetically selected for twin ovulations and births (Twinner) exhibit increased ovarian follicular development, increased ovulation rate, and greater blood and follicular fluid IGF-1 concentrations compared with contemporary cattle not selected for twins (Control). Experimental objectives were to 1) assess relationships among aromatase (CYP19A1), IGF-1 (IGF1), IGF-2 receptor (IGF2R), and FSH receptor (FSHR) mRNA expression in small (≤5 mm) antral follicles and 2) determine their association with increased numbers of developing follicles in ovaries of Twinner females. Ovaries were collected from mature, cyclic (d 3 to 6) Twinner (n = 11), and Control (n = 12) cows at slaughter and pieces of cortical tissue were fixed and embedded in paraffin. Expression of mRNA was evaluated by in situ hybridization using (35)S-UTP-labeled antisense and sense probes for CYP19A1, FSHR, IGF1, and IGF2R mRNA. Silver grain density was quantified within the granulosa and theca cells of individual follicles (2 to 7 follicles/cow) by Bioquant image analysis. Follicles of Twinners tended to be smaller in diameter than Controls (1.9 ± 0.1 vs. 2.3 ± 0.1 mm; P = 0.08), but thickness of granulosa layer did not differ (P > 0.1) by genotype. Relative abundance of CYP19A1 (P < 0.01) and FSHR (P < 0.05) mRNA was greater in granulosa cells of Twinners vs. Controls, respectively, whereas IGF2R mRNA expression was less in both granulosa (P < 0.01) and theca (P < 0.05) cells in follicles of Twinners vs. Controls, respectively. Abundance of CYP19A1 mRNA in granulosa cells was correlated negatively with IGF2R mRNA expression in both granulosa (r = -0.33; P < 0.01) and theca (r = -0.21; P = 0.05) cells. Expression of IGF1 mRNA was primarily in granulosa cells, including cumulus cells, and its expression did not differ between Twinners vs. Controls (P > 0.10). Detected increases in CYP19A1 and FSHR, but not IGF1, mRNA expression along with decreases in IGF2R mRNA expression in individual follicles of Twinners support the hypothesis that increased follicular development and steroidogenesis in Twinner females result from increased extra-ovarian IGF-1 production. Furthermore, a reduction in follicular IGF2R mRNA expression accompanied by a reduction in receptor numbers would increase availability of free IGF-2 and its stimulation of follicular development in Twinners.     

Ovarian follicular dynamics in lines of sheep (Finn, Merinos) selected on ovulation rate

Ewes from selected lines of sheep from each of two breeds, Finns (high ovulation rate, low ovulation rate and control lines with respective ovulation rates of 5.4, 2.7 and 3.3) and Merinos (T Merinos selected for increased ovulation rate and control Merinos with respective ovulation rates of 1.9 and 1.2) were used to examine how selection to alter ovulation rate had altered follicle development. Ovarian antral follicles were counted, measured, classified as nonatretic or atretic (more than five pyknotic bodies). The growth of ovulatory follicles in vivo, followed by repeated follicle ink marking, also was compared in the three lines of Finns. Regardless of breed, ewes selected for high ovulation rate had a similar number of antral follicles and a similar extent of atresia compared with their controls. Alterations induced by selection were located in the last stages of folliculogenesis. T Merinos exhibited a lower proportion of atretic follicles among follicles greater than 3 mm and a larger diameter of the largest healthy follicle when preovulatory follicles were excluded. High-line Finn ewes recruited more follicles, which produced smaller preovulatory follicles, each containing a smaller number of granulosa cells compared with either the low- or control-line ewes. Hence, physiological selection for high ovulation rate raised it by different methods in Merino than in Finn ewes.     

Dynamics of ovarian follicular development in cattle following hysterectomy and during early pregnancy

Three experiments were conducted to evaluate ovarian follicular dynamics and functional activity during pregnancy in cattle. In 11 pregnant Charolais cows of Experiment I, size of largest follicle, number of follicles and accumulated follicle size were reduced by day 27 of pregnancy on the ovary bearing the corpus luteum (CL) but not on the non-CL bearing ovary. In experiment II, local attenuation of ovarian follicular development on the CL bearing ovary of seven pregnant heifers was evident compared to the contralateral ovary without the CL. However, in four hysterectomized heifers, follicular development was sustained on both the CL- and non-CL bearing ovaries when CL maintenance was achieved without presence of the uterus or conceptus. In Experiment III, steroidogenic characteristics of the largest and second largest follicles at 17 d postestrus were evaluated for seven pregnant and six cyclic cattle. Follicle by physiological status interactions were detected for both aromatase activity of the follicle and follicular fluid concentrations of estradiol and progesterone. In cyclic cows, the largest follicle had appreciably more aromatase activity than did the second largest follicle; whereas, aromatase activity of the largest follicle from pregnant cows was less than that of cyclic cows. However, in pregnant cows the second largest follicle became the estrogen-active follicle, and this follicle occurred with a higher frequency on the ovary contralateral to the CL-bearing ovary. These changes in aromatase activity were reflected by parallel changes in estrogen concentrations of follicular fluid. The higher progesterone concentration in follicular fluid of the largest follicle in pregnant cows provided further confirmation of their atretic status. In conclusion, during early pregnancy the conceptus and/or uterus ipsilateral to the conceptus appear to secrete compounds which alter local follicular steroidogenic activity and attenuate subsequent follicular growth between 17 to 34 d of pregnancy on the CL-bearing ovary. This local mechanism acting within the ovary may contribute to the antiluteolytic effects of early pregnancy in cattle.     

Ovarian follicular dynamics in Caspian mares

The characteristics of the major follicular waves (primary and secondary) throughout estrous cycle were studied in 7 healthy Caspian mares (age, 4-15 years; weight, 198.6 ± 0.9 kg) during the breeding season. Ovarian follicular dynamics were monitored by using an ultrasound scanner equipped with a 5-MHz, B-mode, linear-array, rectal transducer throughout 2 complete estrous cycles. The diameters of antral follicles (5 mm) were measured, averaging the narrowest and widest dimensions. To detect follicular wave emergence, the diameter profile of the 3 largest follicles per ovary of each mare was determined without considering day-to-day identity of follicles but with maintenance of distinction between left and right ovaries. The primary waves originated on day 6.4 ± 0.81 (ovulation = day 0) when the mean diameter of ovarian follicles was 9.6 ± 1.05 mm. Divergence between the dominant preovulatory follicle and subordinate follicles occurred on day 13.4 ± 0.81, when the dominant follicle was 18.1 ± 2.67 mm in diameter. The intervals from emergence to divergence and from divergence to ovulation were 7 ± 0.68 and 8.7 ± 0.68 days, respectively. Secondary major follicular waves were not observed during this study. In conclusion, only 1 major follicular wave was detected in a Caspian mare, confirming the data previously described in other equine breeds. It is also indicated that the occurrence of 1 major follicular wave per cycle is a more common phenomena in equine species.     

Monitoring follicular development in cattle by real-time ultrasonography: a review.

The application of real-time ultrasonography to monitoring ovarian function in mammals has advanced the understanding of follicular dynamics and its regulation. Follicular development is a wave-like sequence of organised events. The waves consist of the synchronous growth of small (4 to 5 mm) antral follicles, followed by the selection and growth of one dominant follicle which achieves the largest diameter and suppresses the growth of the subordinate follicles. In the absence of luteal regression, the dominant follicle eventually regresses (becomes atretic) and a new follicular wave begins. The dominant follicle regulates the growth of the subordinate follicles, because the appearance of the next wave is accelerated if the dominant follicle is ablated, and delayed if the lifespan of the dominant follicle is prolonged. During bovine oestrous cycles, two or three successive waves emerge, on average, on the day of ovulation (day 0) and day 10 for two-wave cycles, and on days 0, 9 and 16 for three-wave cycles. During the oestrous cycle there are thus two or three successive dominant follicles, and the last of these ovulates. Ovarian folliculogenesis is a complex process involving interactions between pituitary gonadotrophins, ovarian steroids and non-steroidal factors. Subtle changes in the hormonal milieu regulate folliculogenesis and the emergence of a follicular wave is preceded by a small increase in the concentration of plasma follicle-stimulating hormone. The mechanisms that promote the selection of a dominant follicle have not been elucidated, but considerable progress has been made in understanding follicular development and its regulation. Most treatments designed to control the development of follicular waves have been based on the physical or hormonal removal of the suppressive effect of the dominant follicle, and the consequent controlled induction of the emergence of a new follicular wave. The studies reviewed here describe current methods for regulating the bovine ovarian cycle, interesting models for future studies, and information that may be used for improving reproductive efficiency.     

Proteolysis of insulin-like growth factor binding proteins during preovulatory follicular development in cattle

The present study was conducted to evaluate changes in follicular fluid (FF) insulin-like growth factor binding protein (IGFBP) proteolytic activity and levels of steroids and IGFBP during follicular development in cattle. Estrous cycles of cows were synchronized with two injections of prostaglandin F2alpha (PGF) 11 d apart and follicular growth monitored via daily rectal ultrasonography in order to identify the dominant follicle. All cows were ovariectomized 48 hr after the second injection of PGF. Follicular fluid was collected individually for all follicles > 5 mm and pooled for small (1 to 5 mm) follicles. Follicular fluid estradiol and androstenedione levels were greater (P < 0.05) and progesterone and IGFBP-3 levels not different (P > 0.10) in large dominant than in small (1 to 5 mm) or large (>5 mm) subordinate follicles, whereas IGFBP-2, -4 and -5 levels were less (P < 0.05) in large dominant than in small or large subordinate follicles. To evaluate proteolysis of IGFBPs, FF was incubated with recombinant human (125) I-labeled IGFBP-2, -3, -4, and -5 and proteins separated by 12% SDS-PAGE. Follicular fluid caused little or no proteolysis of (125)I-lableled IGFBP-2 or -3. However, cleavage of (125)I-labeled IGFBP-4 and -5 by FF from large dominant follicles was greater (P < 0.05) than by FF from small or large subordinate follicles indicating that a protease to IGFBP-4 and -5 exists in estrogen dominant follicles. We conclude that lower levels of IGFBP-2 in estrogen dominant follicles of cattle are not due to increased proteolysis, whereas decreases in IGFBP-4 and -5 levels are likely due, in part, to increased protease activity. Changes in IGFBP may alter levels of bioavailable IGFs that stimulate steroidogenesis and mitogenesis in developing bovine follicles.     

An economic assessment of twin births in British dairy herds.

The effect of twinning on the subsequent health, production and reproductive performance of dairy cattle was studied by analysing the data derived from 19,755 calvings which occurred during three years on 37 farms. The data formed part of the database of a veterinary practice operating the DAISY dairy cow recording scheme for its dairy farmer clients. The average twinning rate was 2.5 per cent. For first calf heifers the rate was 0.9 per cent, and the rate increased with increasing parity to over 5 per cent for cows calving for their sixth and subsequent lactations. Although they produced more milk than their contemporaries, twin-bearing cows suffered an increased incidence of retained placenta and vulval discharges and their calving to conception interval was extended by 33 days. Furthermore, 35 per cent of these cows were culled compared with 21 per cent of their contemporaries. The benefit of having more calves for sale was reduced owing to 15 per cent of them being born dead. It is calculated that producing twins resulted in an average loss of income of 74 pounds/cow, a deficit of 15 per cent compared with cows having single calves.     

Ovarian activity and oocyte development during follicular development in pigs at different reproductive phases estimated by the repeated endoscopic method

The aim of the present study was to assess follicular and oocyte development in the same gilts during three phases of their reproductive life [prepuberal gilts (PP; 6.0 months of age), puberal gilts (P; 9.5 months of age) and primiparous sows (S)]. Follicular development was stimulated by the injection of 1,000 IU of equine chorionic gonadotropin (eCG) followed by 500 IU of human chorionic gonadotropin (hCG) 72 h later. Cumulus-oocyte-complexes (COCs) were recovered by endoscopic ovum pick up/aspiration from preovulatory follicles of the left ovary, and the follicular fluid (FF) from the right ovary was collected 34 h after the hCG treatment by endoscopy. Altogether, 19 pigs were used in the PP and P trials and 12 in the S trial. From the left ovaries, 168, 190 and 82 follicles were aspirated and 106, 125 and 42 COCs, respectively, were recovered (recovery rate 61 +/- 27, 63 +/- 21 and 53 +/- 22%, respectively). The mean number of follicles was greater in the P phase than in the PP phase (19.7 +/- 6.8 vs. 15.7 +/- 6.8; p=0.06) and S phases (14.2 +/- 4.0; p<0.05). More uniform oocytes with an expanded cumulus were aspirated in the P and PP phases than in the S phase (90 and 78 vs. 46%; p<0.05). Furthermore, the meiotic configuration in oocytes (T I/M II stage) differed between the three phases (56 and 62 vs. 0%; p<0.05). Progesterone (P4) levels in FF decreased from 590.0 +/- 333.6 (PP) to 249.1 +/- 72.6 (P) and 161.4 +/- 75.2 ng/ml (S) (p<0.05). Estradiol-17beta (E2) levels differed between PP and P gilts and S sows (9.3 +/- 2.9, 21.9 +/- 10.6 and 94.0 +/- 15.9 pg/ml, respectively; p<0.05), and the P4/E2 ratio was 72, 15 and 5, respectively. These results indicate differences in follicular and oocyte development between the reproductive phases investigated. Puberal gilts should preferably be used in IVF and breeding programs. The lower reproductive potential of primiparous sows must be taken into consideration in breeding. Any prediction of lifetime performance based on individual ovarian reactions of prepuberal gilts is unreliable.