GENOTYPE ENVIRONMENTAL INTERACTIONS ON REPRODUCTIVE TRAITS OF BOVINE FEMALES. II1. SEASONAL VARIATION IN POSTPARTUM REPRODUCTION AS INFLUENCED BY GENOTYPE, SUCKLING AND DIETARY REGIMEN 1'2'3 P. J. Hansen and E. R. Hauser University f Wiscnsin, Madisn 53706 Summary We examined the effect f seasn f calving (winter = Octber t March, summer = April t September) n pstpartum reprductive traits by analyzing six data sets in which cws calved thrughut the year. In tw experiments, with suckled and nnsuckled cws, animals that calved in winter had lnger (P<.O1) intervals frm parturitin t first estrus (IE) than thse that calved in summer. Multiple regressin analysis [mdel; IE = y + bl (calving date) + b2 (calving date) 2 ] indicated that the relatinship between calving date and IE was less fr nnsuckled cws than fr suckled cws because there was hetergeneity f regressin (P<.O05) between grups. A similar seasnal difference 1 Dept. f Meat and Anim. Sci. Paper N. 782. Research supprted by the Cllege f Agricultural and Life Sciences, Univ. f Wiscnsin-Madisn, PHS Training Grant 5T01H00104-03, SEA-USDA, NICHHD, Frd Fundatin Grant N. 63-505 and a grant frm the Upjhn C. Experiments 1 t 4 were dne in cperatin with the Dairy Cattle Research Branch, USDA. Semen fr Exp. 5 and 6 was dnated by American Breeders Service, DeFrest, WI; American Int. Charlais Assc., Hustn, TX; Carnatin Genetics, Hughsn, CA; Curtiss Breeding Service, Cary, IL; East Central Breeders Cp., Waupun, WI ; Midwest Breeders Cp., Shawan, WI; Nba, Inc., Tiffin, OH, and Tri- State Breeders Cp., Westby, WI. 2The experiments were cntributins frm the Nrth Central Reginal Cperative Research Prject NC-1, "The Imprvement f Beef Cattle Thrugh Breeding Methds," r NC-2, "The Imprvement f Dairy Cattle Thrugh Breeding." 3The authrs wish t thank Drs. W. J. Tyler and L. E. Casida fr allwing us t use prtins f their data. The data fr Exp. 1 t 4 were cllected by Drs. W. E. Graves, J. W. Lauderdale, J. W. Riesen and S. Saiduddin. The data fr Exp. 5 were cllected by G. F. Henning. The authrs thank S. Kading, W. Winkter and J. Busby fr typing thismanuscript, E. Hch, P. Bringle and J. Kane fr their excellent technical assistance and D. Hansen fr preparing the figures. and calving date suckling status interactin (P<.IO) was bserved in a third experiment. Anther experiment invlved Hlstein cws ranked fr genetic ability fr milk prductin. Cws with genetic ability fr high milk prductin and calving in summer tended (P<.12) t have shrter IE than winter-calving cws. Genetically lw prducing cws did nt shw this tendency and there was hetergeneity f regressin (P<.IO) between grups. In tw experiments cws were fed diets either lw r high in energy and data were analyzed fr three cnsecutive pstpartum perids (PP1, PP2, PP3) beginning after first calving. In PP1 f ne experiment, intervals t first vulatin, estrus and cnceptin were lnger in winter than in summer. The multiple regressin equatin between calving date and IE was significant (P<.IO) nly fr cws fed diets lw in energy. There was n significant relatinship between calving date and IE in PP2 r PP3. In the ther experiment, there were nt enugh wintercalving cws t make seasnal cmparisns in PP1 and seasn did nt significantly influence IE in PP2 r PP3. We cncluded that, in general, winter-calving cws appeared t take lnger t regain reprductive cmpetence after calving than summer-calving cws. Differences between seasns tended t be greater fr suckled cws, cws with genetic ability fr high milk prductin, primiparus cws and cws fed diets lw in energy. (Key Wrds: Pstpartum, Seasn, Cattle, Suckling, Milk Prductin, Nutritin.) I ntrductin Because the prepubertal and pstpartum perids bth require the transitin frm anestrus t estrus, the bservatin that seasn effects age at puberty in heifers (Hansen et al., 1981; Grass et al., 1982) led t the questin f wheth- 1362 JOURNAL OF ANIMAL SCIENCE, Vl. 56, N. 6, 1983
SEASON AND REPRODUCTION IN CATTLE 1363 er pstpartum reprductin may als be influenced by seasn. Several reprts (Hammnd, 1927; Buch et al., 1955; Thibault et al., 1966; de Kruif, 1975) indicate that pstpartum anestrus perids and calving intervals are lnger fr cws calving in the autumn and winter than fr thse calving in the spring and summer. Data frm six experiments invlving cws calving thrughut the year were analyzed t determine the influence f seasn f calving n pstpartum reprductin and the interactin f seasn with suckling status, genetic ability fr milk prductin, nutritinal level, breed and parity. Interactins between seasn and these factrs wuld indicate that the influence f seasn culd be reduced by varying management r gentype. Materials and Methds Exp. 1 and 2 were designed t test the effect f suckling and nnsuckling n pstpartum reprductive traits; the prcedures were detailed by Graves et al. (1968). Twenty-eight Herefrd cws (6 t 8 yr ld), cnsisting r eight sets f identical twins, fur sets f fraternal twins and fur unrelated cws, were used in Exp. 1. Tw suckled cws were remved frm the prject because their calves died. Cws within twin sets and unrelated cws were randmly assigned t the tw treatments (suckled r nnsuckled). The data in Exp. 2 were prvided by 64 Angus cws (mean age = 6 yr) randmly assigned t be suckled r nnsuckled. In bth experiments, cws were pastured frm early spring t late fall and were maintained in drylt during the winter. While in drylt, cws were fed alfalfa-brme hay r alfalfa-brme hay and crn silage ad libitum. Animals were bserved thrughut the year fr estrus behavir in a standard manner fr 30 min each mrning and afternn. TABLE 1. INFLUENCE OF SUCKLING AND SEASON OF CALVING ON DURATION OF POSTPARTUM ANESTRUS a Mean interval (d) frm parturitin t first estrus Exp. Seasn Suckled Nnsuckled Mean avalues are means SE. Winter 91 23.4 16 4.2 64 cr (7) b (4) Summer 35 4.4 17 4.3 23 d (5) (10) Mean 67 c** 17 d 39 Winter 107 19.2 41 11.3 78 c* (10) (8) Summer 65 6.6 26 4.4 45 d (22) (24) Mean 78 c** 30 d 54 Winter 58 5.1 36 -+ 5.6 47 c* (10) (10) Summer 40 +- 3.9 27 4.0 33 d (8) (10) Mean 50 c** 32 d 41 bnumber in parentheses is number f bservatins. C'dMeans within a clumn r rw with different superscripts differ at the level f significance indicated next t superscript c. All interactins were nnsignificant. tp<.10. *P<.01. **P<.O05.
1364 HANSEN AND HAUSER Details f Exp. 3 and 4 have been presented by Saiddudin et al. (1968). Data frm 38 primiparus Hlstein cws were used in Exp. 3, designed t determine the influence f suckling vs nnsuckling n pstpartum reprductive traits. The data in Exp. 4 were btained frm 121 Hlstein females. These cws were frm a herd evaluated and ranked n genetic ability fr milk prductin and divided at the median int grups with genetic ability fr high r lw milk prductin. Cws within grups were randmly assigned t either a high r an average level f cncentrate feeding. In bth Exp. 3 and 4, the cws were hused in a stanchin-type dairy barn all year and allwed t run free in drytt fr 15 t 30 min twice daily at 0630 and 1530 t be bserved fr behaviral estrus. Temperatures in the barn were never belw freezing, and all animals were expsed t a minimum 14 h f light/d, mre when natural day-lengths exceeded 14 h. Details f Exp. 5 and 6 were prvided by Grass et al. (1982) and Hansen et al. (1982). Experiment 5 was designed t determine the influence f breed, level f dietary energy and their interactin n pstpartum reprductive traits. Data frm ne member f each f 31 sets f identical r fraternal twins (15 Hlstein and 16 Herefrd) were analyzed. The females were in drylt cntinuusly frm the time f purchase as calves until they had weaned three calves. At 210 d f age, heifers were randmly assigned within breed t either high r lw dietary regimens. Estrus was determined by frequent daily visual bservatins. Observatins f reprductive traits were recrded fr three cnsecutive pstpartum perids (PP1, PP2 and PP3) beginning with the pstpartum perid after first calving. All heifers were artificially inseminated at their first estrus after 15 m f age and at all subsequent estrus perids until cnceptin. After first, secnd and third calving, they were als bred at each estrus. Experiment 6 was designed t examine the influence f breed f sire and level f dietary energy n pstpartum reprductin. Data were btained n 58 females brn between August and Octber, 1974 t Hlstein dams and sired by Angus, Herefrd, Simmental r Chianina bulls. Prcedures were the same as in Exp. 5, with the fllwing exceptins. These females first received the experimental diets at 168 d f age and changes were made at 308 and 420 d f age. Vasectmized bulls were used in detectin f estrus and all heifers were bred at their pubertal estrus. Each animal was artificially bred t ne f 58 Jersey bulls fr their first calf and t ne f 58 Charlais bulls fr tw subsequent calvings. The cws were palapated weekly beginning at d 14 pstpartum fr uterine invlutin, detectin f vulatin and pregnancy. Uterine invlutin was defined as the return f the uterus t a pregravid cnditin as indicated by size, lcatin in the bdy cavity and tnicity f the uterine hrns. Statistical Prcedures. In all experiments, winter was defind as Octber thrugh March and summer as April thrugh September. Data n pstpartum intervals were evaluated by least-squares analyses f variance. Lg transfrmatins were used when necessary t adjust fr hetergeneity f variance. In Exp. 4, the data were pled acrss level f feeding because interval t estrus was nt significantly affected by level f feeding r by level f feeding seasn. In Exp. 6, intervals t first vulatin, first estrus and cnceptin were nt significantly influenced by breed f sire r the interactin f breed f sire with seasn; therefre, the data are presented as pled acrss breed f sire. In Exp. 6, the significance f the seasnal influence n frequency f quiet vulatins (vulatins nt accmpanied by an bserved estrus) and services per cnceptin was tested by chi-square analysis. Multiple regressin analyses were used t determine the influence f calving date n interval frm calving t first estrus. The equatin used was y = i + bl x + b2x 2 where y is the pstpartum trait and x is days f the year, numbered frm 1 (January 1) t 365 (December 31). Results Exp. 1, 2, 3. Suckled cws had lnger (P<.005) intervals frm parturitin t first estrus than nnsuckled cws in all experiments (table 1). Cws calving in the winter had lnger intervals t first estrus than cws calving in the summer in Exp. 1 (P<.10), 2 (P<.01) and 3 (P<.01). Interval t estrus was regressed n the linear and quadratic functin f calving date within suckled and nnsuckled grups fr each experiment (figure 1). Suckled cws that calved in the winter had lnger intervals t first estrus than thse calving in the summer in Exp. 1, 2 and 3. The multiple regressin e2quatins were significant (P<.005) in Exp. 2 (R =.32) and 3 (R 2 =.55). The same tendency was fund fr
SEASON AND REPRODUCTION IN CATTLE 1365 the nnsuckled grup f Exp. 3, but nt fr Exp. 1 r 2, and nne f the regressins were [56 + 8.3 d (winter) vs 62 +- 9.6 d (summer)] r PP3 [56 + 7.4 d (winter) vs 58 + 10.1 d (sumsignificant fr nnsuckled grups. Date f mer)]. Neither breed nr dietary level interacted calving X suckling interactins were indicated with seasn f calving and multiple regresby hetergeneity f regressin between suckled and nnsuckled grups in Exp. 1 (P<.10), 2 (P<.005) and 3 (P<.005). sins f calving date n interval t first estrus were nnsignificant. Exp. 6. The effects f dietary regimen and Exp. 4. Animals with genetic ability fr high milk prductin had lnger (P<.025) intervals t estrus (-x = 41 d) than cws with genetic ability fr lw milk prductin t~ = 31 d). Neither the effect f seasn f calving nr 2~ seasn f calving x genetic ability fr milk prductin significantly affected interval t, \ estrus. Intervals t first estrus (~ + SE) were 45 \ / -+ 5.0 d (winter) vs 36 + 3.4 d (summer) fr \ / cws with genetic ability fr high milk prduc-, %. S'~ tin and 30 + 3.4 d (winter) vs 32 + 4.2 d ~',~,~ ~ i (summer) fr cws with genetic ability fr lw ~,~ S. milk prductin. Multiple regressin analysis 2 9 indicated that cws with genetic ability fr high 9 " 9 milk prductin that calved in winter tended (P<.12) t have lnger intervals t first estrus than thse calving in the summer (figure 2). I I I I I I This was nt true fr the genetic ability fr lw,, milk prductin grup. There was hetergeneity ~, I OR I ~ suckleo nn suckled 1 f regressin (P<.10), indicating that the ~,,r relatinship between calving date and interval 0 t first estrus was influenced by genetic ability / fr milk prductin. -~,,~ / Exp. 5. Because nly six animals gave birth \( / / t their first calves during the winter, the data u. ---- e ~ 9,I frm PP1 were nt tested fr an effect f \ I00 \ / seasn. The interval t first estrus (x + SE) was 0% I 76 + 18.5 d fr winter-calving cws cmpared %0 O / ~ with 62 +- 8.1 d fr summer-calving cws. Sea- -~ O"~ q e J / c a sn f calving did nt significantly influence - the duratin f the pstpartum anestrus in PP2 9 i% 9 9 I I I I I I Figure 1. Relatinship between calving date and interval t first estrus in Exp. 1 (tp panel), 2 (middle panel) and 3 (bttm panel). Circles represent individual bservatins and lines represent predicted intervals t first estrus frm the multiple regressin equatin: interval t first estrus = intercept + b 1 (calving date) + bz (calving date) 2. Regressin equatins fr suckled grups were y = 103.6 -.699x +.O019x 2 (Exp. 1, P>.20); y = 187.8 -- 1.392x +.0036x 2 (Exp. 2, P<.O05) and y = 76.2-.463x +.O013x 2 (Exp. 3, P<.O05). Equatins fr nnsuckled grups were y = 27.6 -.123x +.O003x 2 (Exp. 1, P>.20); y = 26.2 -.070x +.O004x 2 (Exp. 2, P<.20) and y = 42.5 -.219x +.O007x 2 (Exp. 3, P<.15). Hetergeneity f regressin between grups was present in Exp. 1 (P<.lO), 2 (P<.O05) and 3 (P<.O05). I00 '%`~ ~149 S S i I I + I 9 I I Feb I Ap; I JUrl I Aug I OC! I 04Pr I Calving Date
1366 HANSEN AND HAUSER 14C 0 I I high milk prductin 9 lw mi!-k prductin IOC 9 6C 9 ~ ~ 0 9 9 % 0 9 % 0 09 0 9 ^ I ~., "'" "..,6,,...^... ~.=,=,.~,==.,.~.,=,.- ~ 0 0 9 O0 2-. 9. :.... iu L-- -- I,I- O I-- 140 O > L= e- 100 0 - ~ J Iw nutritin 9 high nutritin 0 0 60 ~. ~ 9 f 0 9 9 ~ O 2C O 9 9 ee 9 ~ 9 Q i i i I i i Febl Apr I Junt Augl Octl Dec! Calving Date Figure 2. Relatinship between calving dates and interval t first estrus fr Exp. 4 (tp panel) and PP1 f Exp. 6 (bttm panel). Circles represent individual bservatins and lines represent predicted intervals t first estrus frm the multiple regressin equatin: interval t first estrus = intercept + b I (calving date) + b: (calving date) 2. Regressin equatins fr Exp. 4 were y = 57.2 -.191x +.0004x 2 (high milk prductin, P<.12) and y = 26.2 +.067x -.O002x 2 (lw milk prductin, P>.20). Regressin equatins fr Exp. 6 were y = 99.3 -.533x +.0013x 2 (lw diet, P<.10) and y = 64.3 -.235x +.O005x 2 (high diet, P>.20). Hetergeneity f regressin between grups was present in Exp. 4 (P<.IO).
SEASON AND REPRODUCTION IN CATTLE 1367 seasn f calving n pstpartum reprductive traits are presented in table 2. During PP1, cws fed the high energy diet that calved in the winter had lnger inervals t uterine invlutin than cws calving in summer, while the ppsite was true fr thse receiving the lw energy diet, resulting in a diet seasn interactin (P<.10). Neither seasn f calving nr seasn diet interactins significantly affected the time frm parturitin t uterine invlutin in PP2 and PP3. During PP1, cws fed the high energy diets had shrter intervals t first vulatin (P<.lS) and first estrus (P<.10) than cws fed the lw energy diets. During PP2, dietary regimen affected (P<.05) interval t first vulatin, but nt intervals t first estrus r cnceptin. Dietary regimen did nt affect any f these traits during PP3. Cws calving in the summer had shrter intervals t first vulatin (P<,005), first estrus (P<.005) and cnceptin TABLE 2. EFFECT OF LEVEL OF NUTRITION AND SEASON OF CALVING ON POSTPARTUM REPRODUCTIVE TRAITS (EXP. 6)a, b Interval (d) frm parturitin t: Dietary Seasn f Uterine level calving invlutin Ovulatin Estrus Cnceptin First pstpartum perid High Winter 33 -+2.7 53 +- 8.8 59 +, 10.6 73 +, 15.9 (8) (8) (8) (8) Summer 28-+ 1.7 31-+ 5.4 39+, 6.5 70+- 9.8 (21) (21) (21) (21) Lw Winter 29 + 1.8 67 + 5.5 87 -+ 6.7 101 +, 10.0 (19) (20) (20) (20) Summer 33 -+ 2.7 40 -+ 8.6 41 +, 10.4 55 +, 15.6 (9) (9) (9) (9) Secnd pstpartum perid High Winter 36 +- 2.9 33 +, 4.6 44 +, 5.1 85 +, 8.9 (11) (12) (12) (12) Summer 34 +, 2.5 22 +, 4.0 41 +, 4.4 54 + 7.6 (16) (17) (17) (17) Lw Winter 28 -+ 2.6 32 -+ 3.6 45 -+ 3.9 66 +, 6.8 (15) (21) (21) (21) Summer 24 +, 3.7 43 -+ 5.9 50 +, 6.4 69 +, 11.3 (8) (8) (8) (8) Third pstpartum perid High Winter 35 +- 2.7 33 + 4.2 44 +, 5.4 83 +- 9.4 (15) (16) (16) (16) Summer 32+ 3.4 28+4.8 37+, 6.3 59+ 10.8 (12) (13) (13) (13) Lw Winter 28-+ 1.8 35 + 4.1 47 + 5.4 67 +- 9.3 (17) (18) (18) (18) Summer 33 + 3.3 33 +, 5.7 44 7.4 52 +, 12.9 (9) (11) (11) (11) avalues are least-squares means + SE. Number f bservatins are in parentheses. bduring the first pstpartum perid, diet influenced (P<.10) interval t estrus. Seasn influenced intervals t vulatin (P<.005), estrus (P<.005) and cnceptin (P<.08) and there was a diet X seasn interactin fr interval t invlutin (P<.10). During the secnd pstpartum perid, diet influenced interval t vulatin (P<.05) and invlutin (P<.005) and there were diet seasn interactins fr interval t vulatin (P<.025) and cnceptin (P<.07). During the third pstpartum perid, seasn influenced interval t cnceptin (P<.09).
1368 HANSEN AND HAUSER (P<.08) than winter-calving cws during PP1 and shrter (P<.09) intervals t cnceptin during PP3. Nne f these traits were significandy influenced by seasn during PP2. Thugh the interactins were nnsignificant, the influence f seasn f calving n intervals t first vulatin, first estrus and cnceptin tended t be greater fr cws receiving lw energy diets than thse receiving high energy diets during PP1 (table 2). Evidence fr dietary regimen x seasn interactins were als apparent frm multiple regressin analyses. Cws fed diets lw in energy and calving in the winter had lnger intervals t first estrus than cws calving in the summer (figure 2, R 2 =.20). While the same relatinship appeared t ccur fr cws fed high energy diets, differences in predicted intervals t estrus between seasns were nt as great and the regressin mdel did nt explain a significant surce f variatin in interval t estrus. During PP1, there tended (P<.I 5) t be mre vulatins withut bserved estrus amng winter calving cws (.64/animal) than amng summer-calving cws (.37/animal). Seasn f calving did nt influence the frequency f quiet vulatins in PP2 r PP3. Services per cnceptin were nt influenced by seasn f first breeding in any pstpartum perid. Discussin In three f six experiments, cws that calved between Octber and March had significantly (P<.10 r less) lnger intervals frm parturitin t first estrus than cws that calved between April and September. A similar relatinship between seasn and interval t first estrus appeared t ccur in sme grups f tw ther experiments. The winter-summer classificatin placed discrete and arbitrary limits n a cntinuus variable, and a secnd analysis that related a parablic functin f calving date t pstpartum traits indicated similar cnclusins. Influences f seasn have been bserved in ther temperate regins (Hammnd, 1927; Buch et al., 1955; Thibault et al., 1966). Therefre, these results indicate that under certain cnditins pstpartum anestrus is affected by seasn f calving. The relatinship between calving date and interval t first estrus was greater fr suckled cws than fr nnsuckled cws (Exp. 1, 2 and 3) and greater fr primiparus cws than fr multiparus cws (Exp. 6). There were als tendencies fr the assciatin f calving date and interval t estrus t be greater fr cws with genetic ability fr high milk prductin (Exp. 4) and fr cws fed lw energy diets (PP1 f Exp. 6). There were n diet x seasn interactins in PP2 r PP3 f Exp. 6, but nutritin did nt significantly influence interval t first estrus fllwing secndr third parturitins. It is pssible that seasn had a greater influence when pstpartum anestrus was lengthened by ther causes. Althugh cattle breed thrughut the year, these data suggest mechanisms by which calving wuld ccur mre frequently in spring and summer than in winter if cws were bred at each estrus. If cws calving in the winter have lnger intervals frm parturitin t first estrus and cnceptin, their calving times wuld tend t mve tward spring with advancing age. Cws calving in spring and summer, having shrter calving intervals, wuld tend t cntinue calving in spring and summer. Hewett (1968) reprted that the percentage f cws calving frm January t June increased with age. The cmpnents f seasnal variatin that affected duratin f the pstpartum anestrus are nt knwn. Seasnal variatins in management r diet were nt respnsible fr the effect f seasn n pstpartum anestrus in Exp. 3, 4 and 6 because management prcedures were similar thrughut the year. Diet varied between seasns in Exp. 1 and 2, but mst animals were classified as being in a "gd" cnditin thrughut the year. There may have been ther unknwn factrs that varied with seasn, but t ur knwledge the nly seasnal variatins cmmn t all experiments were thse in phtperid and temperature. Phtperid has been shwn t affect reprductin in bvine females (Deas, 1971; Peters and Tucker, 1978) and temperature culd effect reprductin directly by altering hrmnal secretin r indirectly by influencing energy requirements fr maintenance. The results f these experiments are imprtant fr a mre cmplete understanding f the variatin in pstpartum reprductin. Because winter cnditins at calving were ften assciated with lng delays in the return t estrus and because the effect f seasn was greater when accmpanied by ther factrs lengthening the pstpartum anestrus, manipulatin f envirnmental r management cnditins may lead t increased reprductive efficiency amng wintercalving cws. In additin, interactins f seasn with ther variables affecting pstpar-
SEASON AND REPRODUCTION IN CATTLE 1369 turn reprductive functin culd bscure the results f experiments in which their presence was nt cnsidered. Fr these reasns, the results reprted herein shuld be the basis fr experiments t test the hyptheses suggested frm these bservatins. Literature Cited Buch, N. C., W. J. Tyler and L. E. Casida. 1955. Pstpartum estrus and invlutin f the uterus in an experimental herd f Hlstein-Friesian cws. J. Dairy Sci. 38:73. Deas, D. W. 1971. The effect f supplementary light n winter infertility in cattle. Vet. Rec. 89:242 (Abstr.). de Kruif, A. 1975. An investigatin f the parameters which determine the fertility f a cattle ppulatin and f sme factrs which influence these parameters. Tijdschr. Diergeneesk. 100:1089. Grass, J. A., P. J. Hansen, J. J. Rutledge and E. R. Hauser. 1982. Gentype envirnmental interactins n reprductive traits f bvine females. 1. Age at puberty as influenced by breed, breed f sire, dietary regimen and seasn. J. Anim. Sci. 55:1441. Graves, W. E., J. W. Landerdale, E. R. Hauser and L. E. Casida. 1968. Relatin f pstpartum interval t pituitary gnadtrpins, varian fllicular develpment and fertility in beef cws. Wiscnsin Res. Bull. 270:23. Hammnd, J. 1927. The Physilgy f Reprductin in the Cw. Cambridge Univ. Press, Lndn. Hansen, P. J., D. H. Baik, J. J. Rutledge and E. R. Hauser. 1982. Gentype envirnmental interactins n reprductive traits f bvine females. II. Pstpartum reprductin as influenced by gentype, dietary regimen, level f milk prductin and parity. J. Anim. Sci. 55:1458. Hansen, P. J., K. K. Schill, L. A. Kamwanja, E. R. Hanser and D. J. Dierschke. 1981. The influence f seasn n sexual develpment in the bvine female: Ovarian gtwth and bdy weight as related t puberty. In: N. B. Schwartz and M. A. Hunzieker-Dunn (Ed.) Dynamics f Ovarian Functin. pp 239-244. Raven Press, New Yrk. Hewett, C. D. 1968. A survey f the incidence f the repeat breeder cw in Sweden with reference t herd size, seasn, age and milk yield. Brit. Vet. J. 124: 342. Peters, R. R. and H. A. Tucker. 1978. Prlactin and grwth hrmne respnses t phtperid in heifers. Endcrinlgy 103:229. Saiduddin, S., J. W. Riesen, W. J. Tyler and L. E. Casida. 1968. Relatin f pstpartum interval t pituitary gnadtrpins, varian fllicular develpment and fertility in dairy cws. Wiscnsin Res. Bull. 270:15. Thibault, C., M. Curt, L. Martinet, P. Maulen, F. du Mesnil du Buissn, R. Ortavant, J. Pelletier and J. P. Signret. 1966.. Regulatin f breeding seasn and estrus cycles by light and external stimuli in sme mammals. J. Anim. Sci. 25(Suppl. 1):119.