PUBERTY AND ESTRUS IN CONFINEMENT-REARED GILTS 1. U.S. Department of Agriculture 3, Clay Center, NE 68933

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1 PUBERTY AND ESTRUS IN CONFINEMENT-REARED GILTS 1 R. K. Christenson and J. J. Ford 2 U.S. Department of Agriculture 3, Clay Center, NE Summary The physiological mechanisms responsible for puberty in mammals remain poorly understood despite many investigations. The incidence of gilts that did not show regular estrous cycles at 9 months of age ranges from 10 to 40% and breed of gilt is an important factor contributing to this percentage. The percentage of Landrace gilts that showed regular estrous cycles at 6 months of age was higher (P<.01) than that of Hampshire, Large White, Yorkshire and Duroc gilts. Estrous activity for these five breeds of gilts at 8.5 months of age had reached a plateau and a higher percentage of Landrace, Large White, Hampshire and Duroc gilts showed regular estrous cycles than did Yorkshire gilts (78, 86, 71, 71 vs 56%, respectively). Of the noncyclic gilts slaughtered at 9 months of age, 55% were gilts with immature reproductive tracts (delayed puberty) and 45% were gilts with fully developed uteri and ovaries that had corpora lutea at different stages of development or preovulatory follicles and corpora albicans (behavioral anestrus). Behavioral anestrus can be reversed by methallibure treatment. Season of year and social environment, in addition to breed, influence puberty and regularity of estrous cycles in confinement-reared gilts, but the l Invitational paper presented at the 11th Meeting of the Midwestern Section ASAS, Southern Illinois Univ. Carbondale, January 5, Original research supported in part by USDA Cooperative State Research Service grant , USDA, Science and Education Administration-Agricultural Research, Roman L. Hruska U.S. Meat Animal Research Center, Clay Center, NE; and Department of Animal Science, Nebraska Agr. Exp. Sta., Lincoln, NE. 2The authors express their appreciation to the swine operation personnel of the Roman L. Hruska U.S. Meat Animal Research Center for their assistance. 3Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable. relative influence of each remains to be determined. (Key Words: Swine, Confinement, Estrous Activity, Puberty, Behavioral Anestrus.) I ntrod uction A successful swine breeding unit is dependent upon the reproductive efficiency of the breeding herd. Reproductive efficiency depends on the proportion of gilts reaching puberty, continuing regular estrous cycles and conceiving at first breeding. Rearing of gilts in total confinement units places the gilts in an environment different from their natural habitat. This environmental change, along with genetic change of swine as a result of selection for leanness, may have altered physiological and behavioral patterns of gilts. Few studies have been conducted to evaluate confinement rearing on puberty and continuation of regular estrous cycles of gilts. Therefore, the question remains whether total confinement adversely affects sexual maturation or whether management in confinement has increased the awareness of reproductive problems in swine. Although the understanding of sexual maturation remains obscure, gilts reared in confinement have delayed puberty and "quiet estrus" (Jensen et al., 1970) or delayed puberty and "behavioral anestrus" (Einarsson and Linde, 1974; Bane et al., 1976; Christenson and Ford, 1977). Our objective was to discuss factors associated with puberty in gilts reared in total confinement. Major factors to be discussed are physiology of puberty, breed of swine, season of sexual maturation and social environment as created by total confinement. Other factors known to influence puberty in gilts are nutrition and exposure to a boar. The influence of nutrition on age at puberty in gilts has been studied by several investigators (Robertson et al., 1951a; Self et al., 1955; Gossett and Sorensen, 1959; Zimmerman et al., 1960; Lodgeand McPherson, 1961; O'Bannon et al., 1966), but the results were inconsistent (Melampy and An- 743 JOURNAL OF ANIMAL SCIENCE, Vol. 49, No. 3 (1979)

2 744 CHRISTENSON AND FORD derson, 1972) and this conclusion is supported by subsequent reports (Cunningham et al, 1974; Friend, 1976, 1977). Exposure of gilts nearing puberty to a boar hastens the age at puberty in nonconfinement-reared gilts (Zimmerman et al., 1969; Brooks and Cole, 1970; Zimmerman et al, 1974) and confinementreared gilts (Thompson and Savage, 1978). Physiology of Puberty. Despite many investigations, physiological mechanisms responsible for sexual maturation are not clearly understood. Although several suggestions (Kallas, 1929; Pfeiffer, 1936; Byrnes and Meyer, 1951; Ramirez and McCann, 1965; Odell and Swerdloft, 1976) have been made as to the mechanisms by which puberty is attained, the most widely accepted hypothesis is a change in hypothalamic sensitivity to the negative feedback of gonadal steroids. As an animal matures, the sensitivity of the hypothalamus to negative feedback diminishes and greater quantities of gonadal steroids are required to inhibit gonadotropin secretion (Ramirez, 1974). In neonatal rats and sheep, negative feedback of gonadal hormones is functional in males because luteinizing hormone (LH)increases in serum after castration, but in females of these two species serum LH concentrations do not increase significantly after castration (Goldman et al., 1971; Foster et al., 1972). In neonatal Rhesus monkeys of both sexes and in neonatal male pigs, negative feedback is nonfunctional because LH does not increase after castration (Dierschke et al., 1974; Ford and Schanbacher, 1977). In postpuberal castrated rats and sheep of both sexes, serum LH is initially decreased in response to estrogen; but if additional estrogen is administered, serum LH is increased in only castrate females (Taleisnik et al, 1971; Neill, 1972; Short, 1974; Karsch and Foster, 1975). This observation demonstrates that negative feedback of estrogen is functional in both castrate sexes but positive feedback of estrogen is functional in only castrate female rats and sheep. Postpubertal monkeys and pigs differ from rats and sheep because estrogen exerts a biphasic effect (both negative and positive feedback) on LH release in both castrate sexes (Lantz and Zimmerman, 1972; Karsch et al., 1973; Ford and Schanbacher, 1977). The development of normal sexual behavior is also dependent on hypothalamic maturation. In mature castrate rats, injections of estrogens and progesterone in combination readily induce estrous behavior in females hut rarely induce estrous-like behavior in males (Grady et al., 1965; Pfaff and Zigmond, 1971). In contrast, in mature castrate pigs exogenous estrogen treatment induces estrous behavior in both males and females (Diehl et al., 1972; Ford and Schanbacher, 1977). Collectively, these data point out important species differences relating to physiological mechanisms associated with onset of puberty. The anterior pituitary of the 77-day-old gilt releases LH in response to synthetic gonadotropin stimulation, prepuberal gilts a'. 110 days of age show behavior estrus and ovulation in et al., 1973). In response to exogenous gonadotropin stimulation, prepuberal gilts at 100 days of age show behavioral estrus and ovulation in advance of the normal occurrence of puberty, but estrus and ovulation do not recur (Casida, 1935; Dzuik and Gehlbach, 1966). Schilling and Cerne (1972) have shown that recurring estrus and ovulation are established in gilts treated with gonadotropin at 150 days. Thus, for follicles in prepuberal gilt ovaries to develop and ovulate, the ovaries must mature near 110 days of age. Influence of Breed The influence of breed and mating systems on age at puberty in swine reared under nonconfinement conditions has been recognized (Robertson et al., 1951b; Foote et al., 1956; Zimmerman et al., 1960; Clark et al., 1970). Foote et al. (1956) reported differences in age at puberty among mating types within the purebred and crossbred systems of mating as well as the effect of heterosis on reducing age at puberty in swine. A recent study at the Roman L. Hruska US Meat Animal Research Center (MARC) determined age at puberty and continuation of regular estrous cycles in gilts of five breeds reared in total confinement. A total of 434 gilts of the Hampshire (89), Duroc (44), Yorkshire (119), Large White (114) and "Swedish" Landrace (68) breeds born in March and May were weaned at 28 days of age and then reared until 145 days of age in groups of 18 to 20 gilts in a building with partly slatted floors. Gilts were relocated to a building with open flushing gutters and divided into groups of nine with 1 m 2 of pen space per gilt. From 145 to 270 days of age, gilts were checked daily with a mature boar for estrus. Puberty was designated as the day of first estrus, and gilts showing estrus every 18 to 23 days were classified

3 PUBERTY OF CONFINEMENT-REARED GILTS 745 as having regular estrous cycles. Gilts not showing estrus were classified as noncyclic. The percentage of gilts showing regular estrous cycles in relation to chronological age (months) is presented in figure 1. Landrace gilts showed the highest estrous activity (number of gilts showing regular estrous cycles/number of gilts x 100) between 5 and 7 months of age, and the percentage of Landrace gilts showing regular estrous cycles at 6 months of age was higher (P<.01) than that of Hampshire, Large White, Yorkshire and Duroc gilts (69 vs 11, 4, 3, 0%, respectively). Estrous activity for the five breeds of gilts had reached a plateau at 8.5 months of age, and the percentage of gilts showing regular estrous cycles was greater for Landrace, Large White, Hampshire and Duroc gilts than for Yorkshire gilts (78, 86, 71, 71 vs 56%, respectively). At puberty, Landrace gilts were younger (P<.01), Hampshire and Large White were intermediate, and Yorkshire and Duroc gilts were older (P<.01) than gilts of other breeds (table 1). An improvement of the percentage of gilts showing regular estrous cycles at 6, 7 and 8 months of age is desirable in gilts reared in total confinement. Maintenance of gilts in total confinement from market weight and age to breeding age is costly, especially if a larger number of gilts must be retained to insure that the necessary number of gilts required for replacement are showing regular estrous cycles at breeding age. On the basis of other estrous data (tables 2 and 3), we anticipate that the percentage of gilts showing regular estrous activity would not increase significantly if gilts were checked for,y_, b o e: w g 6( 2C... V.~"~-.~ ~ _...--o----,, -~--~--~ ~.~:_-- "-~ L~I, NORACE(6r)~//I~ LARGE W~TF(II~ -f J,~" ~OC (44) BGE (MONTHS} Figure 1. Estrous activity in relation to age of gilts reared in confinement. Total number of gilts in parentheses. estrus for 3 more months (9 to 12 months of age). Maintaining noncyclic gilts beyond 8.5 to 9 months of age for replacement cannot be recommended from an economic standpoint. At 9 months of age noncyclic gilts were slaughtered and reproductive tracts were examined (table 1). The distribution of immature reproductive tracts (55%, gilts with delayed puberty) and fully developed reproductive tracts (45%, behaviorally anestrous gilts) was nearly equal. Predominately Duroc and Large White gilts had delayed puberty and Hampshire and Landrace gilts were behavioral anestrus. Fifty-seven percent of the noncyclic Yorkshire gilts had delayed puberty and 43% were behavioral anestrus. Behavioral Anestrus. Two questions were asked in hopes of further understanding behavioral anestrus. The first was: Do corpora lutea of the behaviorally anestrous gilts persist for longer than 16 days and consequently inhibit the expression of estrus and regular estrous cycles? To answer this question, 10 behaviorally anestrous gilts were laparotomized and corpora lutea marked with silk suture. The gilts were checked daily for estrus for 23 days and none showed estrus. Gilts were relaparotomized and marked corpora lutea had regressed in all gilts. This result indicated that the lifespan of corpora lutea was not prolonged. The second question was: Are corpora lutea of behaviorally anestrous gilts producing normal amounts of progesterone? Jugular blood was collected from 10 behaviorally anestrous gilts and the gilts were laparotomized after the first blood sample. In two gilts that had recently ovulated, serum progesterone concentrations during the next 20 days (figure 2) were similar to those of cyclic gilts (Henricks et al., 1972). This conclusion was supported by the serum progesterone concentrations determined for the other eight gilts. Despite ovarian cyclicity and a normally developed reproductive tract, behaviorally anestrous gilts rarely show estrous behavior. Detection of estrus in several groups of gilts from 5 months old to 9 or 12 months old suggests that gilts classified as behavioral anestrus fit several patterns. Some gilts will show an occasional estrous period during the 4- to 7-month estrous detection period, whereas others will show a single estrus early or late during this period. Still others will not show estrus throughout the estrous detection period (R. K. Christenson, unpublisbed data). Since the initiation of repro-

4 746 CHRISTENSON AND FORD TABLE 1. REPRODUCTIVE STATUS AND AGE AT PUBERTY FOR GILTS REARED IN CONFINEMENT Large Item Landrace White Hampshire Duroc Yorkshire No. of gilts No. of gilts with cystic ovaries a or ovotestis b I a 1 b I a 0 3 b Reproductive statusc: No. of gilts with delayed puberty No. of behaviorally anestrous gilts Avg age at puberty, days d e f f 224 3g 221 3g a,b,cgilt s slaughtered at 9 months of age. dmeans SE for those gilts that showed estrus. e'f'gmeans without a common superscript differ (P*(.01). ductive studies with the swine herd housed in confinement at MARC, behavioral anestrus has been observed in several groups of 8- to 12- month-old gilts studied and the magnitude has ranged from 6 to 28% (table 4). To further describe behavioral anestrus, the influence of methallibure on the estrous response induced by estradiol benzoate was studied in ovariectomized gilts (Christenson and Ford, 1979). Treatment with estradiol benzoate induced estrus in a higher percentage (P<.01) of behaviorally anestrous methallibure-treated and cyclic, control gilts than in behaviorally anestrous gilts (table 5). Methallibure treatment effectively reversed the inhibitory factors that prevented behaviorally anestrous gilts from showing estrus in response to estradiol benzoate. Several possibilities may explain the mechanism by which methallibure reverses behavioral anestrus. The possibility that altered adrenal activity may induce behavioral anestrus has been ex- Age TABLE 2. PERCENTAGE OF GILTS SHOWING REGULAR ESTROUS CYCLE AT 6 TO 12 MONTHS OF AGE AND AVERAGE AGE AT PUBERTY a Gilt showing regular estrous cycles, %b 8 gilts/pen 24 gilts/pen (31) c (48) 6 months months months months months months months Avgageat puberty, days ahampshire (Yorkshire-Duroc reciprocal crosses) crossbred gilts reared in confinement (1.2 m 2/gilt) and daily estrous detection initiated at 165 days of age (October). bnumber of gilts in parentheses. COne gilt removed from experiment. Age TABLE 3. PERCENTAGE OF GILTS SHOWING REGULAR ESTROUS CYCLES AT 6 TO 12 MONTHS OF AGE AND AVERAGE AGE AT PUBERTY a Gilt showing regular estrous cycles, %b 8 gilts/pen 24 gilts/pen (32) (46) c 6 months months months months months months months ~vg age at puberty, days ahampshire, Yorkshire and Duroc gilts reared in confinement (1.2 m 2/gilt) and daffy estrous detection initiated at 165 days of age (April). bnumber of gilts in parentheses. CTwo gilts died.

5 PUBERTY OF CONFINEMENT-REARED GILTS 747 A 40 r. v uj 30 z o LLI 03 2C Lfl L9 t (3_ IC DAYS I-3 kx \\ k~ k~ k\ \\ \\ DAYS 2-4 \'~ \\ DAYS AFTER FIRST LAPAROTOMY Figure 2. Peripheral progesterone concentrations in two behavioral anestrous gilts in relation to the ovarian cycle observed at laparotomy. plored. Normal cyclic gilts were ovariectomized on Day 5 of a 14-day treatment of either adrenocorticotrophic hormone (ACTH; 60 IU twice daily) or 6a,9c~-difluoro-11/3,17o~,21-trihydroxy - 16c~ -methylpregna - 1,4 - diene- 3,20 - dione (flumethasone; 1.5 mg twice daily). Estradiol benzoate (10 #g/kg body weight) was given on Day 7, and gilts were checked twice daily for estrus. ACTH and flumethasone treatment did not alter the estrous response to estradiol benzoate. Jensen et al. (1970) reported heavier adrenal weights in tethered than in nontethered gilts in some of their trials while Schilling and Rechenberg (1972) were able to block ovulation in cyclic gilts treated with exogenous ACTH (80 IU/day) and synthetic glucocorticold (flumethasone, 2.5 mg/day and dexamethasone, 20 rag/day). The effects of ACTH and glucocorticoids on ovulation were attributed to an inhibition of the release of LH. Serum progesterone concentrations for behaviorally anestrous and cyclic, ovariectomized gilts on the methallibure study were low (<.5 ng/ml) and not different at the time of estrogen treatment. However, the adrenal glands may be producing other hormones that are involved in behavioral anestrus. Because methallibure inhibits gonadotropin secretion (Paget et al., 1961) and its effects are mediated through the hypothalamus (Malven, 1971; Malven et al., 1971), we determined the ability of estradioi benzoate (10 ~tg/kg body weight) to induce estrus in ovariectomized gilts being fed methallibure. Three gilts were ovariectomized and fed methallibure (100 rag/day) for 10 days andon the fourth day of methallibure feeding were given a single injection of estradiol benzoate (10 #g/kg body weight). Approximately 3 days after the estradiol benzoate injection, the three gilts showed normal estrous behavior. Such behavior suggested that the "hypothalamic behavioral center" was not directly suppressed by the methallibure treatment, a conclusion that agrees with reports by Polge (1965) and Polge and Day (1969). Further evidence that methallibure treatment effectively reverses behavioral anestrus was shown when pregnancy was initiated in be- TABLE 4. MAGNITUDE OF BEHAVIORAL ANESTRUS AND DELAYED PUBERTY IN GILTS REARED IN CONFINEMENT a Month Gilts with estrous Age at Behaviorally delayed No. of checking slaughter, anestrous puberty, Breed b gilts initiated c month gilts, % % D, H, Y 105 July D, H, Y, LW, L 288 August D, H, Y, LW, L 140 October D, H, Y 78 April D, H, Y 59 December Y X (H X Y--D) d 18 June H X Y-D d 79 October L--LW d 27 December atrials conducted from 1976 through bd--duroc, H-Hampshire, Y-Yorkshire, L--Landrace, LW-Large White. CDaily estrous detection initiated at 145 to 165 days of age. dyorkshire and Duroc (Y--D) and Landrace and Large White (L-LW) reciprocal crosses.

6 748 CHRISTENSON AND FORD TABLE 5. ESTRADIOL BENZOATE (EB)-INDUCED ESTROUS RESPONSE IN ANESTROUS GILTS TREATED WITH OR WITHOUT METHALLIBURE BEFORE OVARIECTOMY Estrous response to EB, %a Day of EB Anestrous, Cyclic, injection after methallibure Anestrous controls ovariectomy (Group I) (Group II) (Group III) (11) (12) (15) Day b 8.3 c 93.3 b Day b 58.3 d 93.3 b Day b 41.7 d 93.3 b anumber of gilts in parentheses. b'c'dmeans in the same column or row without a common superscript differ (P<.01). haviorally anestrous gilts. Ten anestrous gilts were fed methallibure (100 mg/day) for 20 days followed by a subcutaneous injection (1,000 IU) of pregnant mare serum gonadotropin the day after the last feeding of methallibure. Estrus was detected in all 10 gilts and all gilts were artificially inseminated with 100 ml of fresh diluted semen at 12 and 24 hr after onset of estrus. Gilts were slaughtered 70 days after breeding and 6 of 10 were pregnant. The mechanism by which methallibure affected behaviorally anestrous gilts is unsolved. Influence of Season In the literature, "season of birth" has been used as a term related to age at puberty; however, swine management has changed to farrowing throughout the year and the term "season of birth" is restrictive. We choose to emphasize the influence of time of year when the gilt is approaching sexual maturation (puberty) as a season effect. Climatic conditions (season), whether modified by confinement management or not, are factors that might be directly influencing the processes of sexual maturation in gilts. An influence of season on the process of sexual maturation is often alluded to, but data to support this concept are not overwhelming. Mavrogenis and Robison (1976) reported that gihs born in the fall (reaching puberty during winter and early spring) reached puberty at a younger age than gilts born in the spring (reaching puberty during summer and early fall). In France Carrez et al. (1977) reported that gilts born from May through October reached puberty at a younger age than those born from November through April. However, Gossett and Sorensen (1959) and Sorensen et al. (1961)observed no effect of time of year on age at puberty. Two reports of Robertson et al. (1951a,b) are often cited as support for time of year effects on puberty. However, these studies were conducted with gilts born in spring only; and although a significant negative correlation between date of birth and age at puberty was observed, these results should not be extrapolated to support seasonal effects. In other studies with gilts born in spring, the correlation between date of birth and age at puberty is not always significant (Self et al., 1955), is significant in 1 year but not the next (Warnick et al., 1951), or is significant in one of two breeds (Zimmerman et al., 1960). Wiggins et al. (1950) and Scanlon and Krishnamurthy (1974) have studied reproductive tract development in slaughter-weight gilts throughout the year (figure 3). A total of 1,998 gilts (nonparous) ranging in weight from 81.5 to kg were examined by Wiggins et al. (1950) and 6,727 gilts ranging in weight from 88.6 to 90.7 kg were examined by Scanlon and Krisnamurthy (1974). The data from these two reports plus recent data (Bane et al., 1976) indicate that gilts at market weight going to slaughter in the late summer show less sexual development than at any other time of the year as evidenced by the higher percentage of gilts with immature reproductive tracts. These data suggest that gilts maturing during the summer are I00 - o 6o ~ 40 ~ 20; JAN APR, JUL. MONTHS Figure 3. Sexual development in slaughter-weight gilts in relation to time of year (adapted from Wiggins et al., 1950 ~; Scanlon and Krishnamurthy, 1974 u. OC~

7 PUBERTY OF CONFINEMENT-REARED GILTS 749 exposed to conditions that delay their rate of sexual maturation. Influence of Social Environment The social environment in which mammals are reared can also affect sexual maturation. Such effects have been extensively studied and reported in mice (Whitten, 1956; Bruce, 1959; Vandenbergh, 1974; Drickamer, 1975). But social factors appear to play a different role in the female pig (Mavrogenis and Robison, 1976) than they do in the mouse. The shift from small nonconfined swine production units to larger total confinement units either has increased reproductive problems in gilts or has increased the awareness of such problems. The physical and social environments created by the density of animals (space allowance per pig) and number of animals per pen have been suggested as possible factors that influence puberty and regular estrous cycles in gilts reared in confinement. Ford and Teague (1978) studied age at puberty in gilts (eight or 12 gilts/pen) reared from 3 to 8 months of age in pens with partly slatted floors; controls had an initial floor space of.37 m 2/pig and crowded gilts were restricted to one-half the space allowed to controls. Pen size was increased by.09 m 2/pig when the average weight of each pen reached 34.1 kg/pig and at every 13.6-kg increase thereafter. The number of gilts reaching puberty and average age at puberty were not affected by crowding. Because of the wide spectrum of housing for swine, the type of confinement (individual pens, tethering and gestation type stalls and group pens for small or large numbers) for gilts nearing puberty needs study. Individual isolation has been reported to alter the expression of puberty in swine. Jensen et al. (1970) reported that individual tethering increased age at puberty by only 4 days; but the expression of estrus was less discernible in tethered gilts and the incidence of immature reproductive tracts at 10 to 12 months of age was greater in tethered than in nontethered gilts (13/87 vs 0/87). Mavrogenis and Robison (1976) reported that puberty was 14 days later in individually penned gilts kept outside than in gilts kept in groups of 30/pen. Thus, the effect of individual isolation in gilts is not limited to confinement and we can conclude from these data that housing of gilts individually before and during the breeding season is detrimental. We are left with this question. Should gilts be housed in total confinement buildings in small or large groups of gilts per pen? Little is understood about the influence of social status (peck order, social dominance) on the expression of estrus by closely confined gilts. Two experiments were conducted to determine the influence of number of gilts per pen (8 vs 24 gilts/pen) with a constant area per gilt (1.2 m 2) on age at puberty and continuation of regular estrous cycles. In the first experiment, confinement-reared crossbred gilts (5.5 months old) were put on experiment in October and checked daily for estrus through April. In a second experiment, confinement-reared purebred gilts (Hampshire, Yorkshire and Duroc breeds, 5.5 months old) were put on experiment in April and checked daily for estrus through October. The results of these experiments (tables 2 and 3) are conflicting. Maintaining crossbred gilts in groups of eight gilts per pen appeared to be more beneficial than maintaining gilts in groups of 24 gilts per pen; whereas, with purebred gilts no real difference was observed. Because season of the year and breed of gilts are confounded in these two experiments (tables 2 and 3), a difference may not have been observed for the purebred gilts because expression of estrus by these gilts was low throughout the study. Conclusions It is apparent that understanding of biological events associated with puberty in gilts is limited and an explanation for the differences that have been reported in numerous studies is not obvious. In confinement-reared gilts we observed that age at puberty, percentage of gilts reaching puberty by 9 months of age, and characteristics of the reproductive tracts of those not showing estrus (delayed puberty and behavioral anestrus) were influenced by breed. Significant interactions between breed and environment appear to exist and future studies need to address the magnitude of these interactions. Literature Cited Bane, A., S. Einarsson and K. Larsson. 1976, Effect of season on age at puberty in crossbred gilts in Sweden. VIII Int. Cong. Anita. Reprod. Artif. lnsem. 3 : 117 (Abstr.). Brooks, P, D. and D. J. A. Cole The effect of the presence of a boar on the attainment of puberty in gilts. J. Reprod. Fertil. 23:4-35. Bruce, H. M An exteroceptive block to pregnancy in the mouse. Nature 184:105.

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