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Is the Cow a Seasonal Breeder?
Br. vet. J
(19R2).
138, 533
IS THE COW A SEASONAL BREEDER?
Bv A . R. PETERS• ANO G. M . RIL EYt
Agricultural Resea rch Council Research Group on Hormones and Fann Animal Rej;roduction, UniversifJ' of Nollingham School of Agriculture, Sutton Boning/on, Loughborouglz, Leics
SUMMAR Y
The tim e to the first ri se in milk progeste ro ne concentrations (acy cli c period ) w as d e termin ed in 18 1 post- part um beef cows a nd a m ea n pe r iod o f 53·5 d ays was found . Th ere was a sig nifi ca nt negativ e co rre la tion b etw ee n bod yw eight dur in g the firs t wee k j;ost partum a nd th e acycl ic p eriod (r = -0·3 22; P
INT RODUCT ION
The pe ri od o f tru e anoestrus or acycl icit y is often prolonged in pos t-pa rtum suck lin g beef cows (Ox enreid e r, 1968; In skee p & Lishman, 1979). Thi s may lead to a n exte nd ed ca lving interva l in indi vidua l cows a nd a n exte nded h erd ca lvin g seaso n both of which arc eco no mi ca ll y undesira bl e (A ll en , 1979). T he length of th e acy cli c pe riod m ay be reduced by prov ision of a d eq ua tc di e ta ry en ergy (\IV ilt bank eta/., 1964; Dunn et al., 1969). Seasonal diffe ren ces in th e acyc li c pe ri od h ave bee n reported fo r d a iry cows (Bulman & L a mmin g, 1978) and for beh av iou ra l anoes tru s in beef cows (K ing & Hurni k, 1980; M o n tgo m e ry, D av is & Hurre ll , 1980). H o weve r, severa l authors have co mm ented th a t seaso n a l effects on a noes tru s m ay be re la ted purely to differen ces in nutrit io n a nd m a n age m e nt ( Boyd , 1977; D e Kruif, 1978; T ucke r, 1982). It has been r epo rted r ece ntl y th a t the acyc li c pe riod in bee f cows was negat ively co rrelate d w ith bodyweight a t ca lvin g a nd th a t cow s ca lving in th e spring m o n ths were acyc li c fo r lo nger peri od s th a n those ca lving in the a utumn (Pe ters & Ril ey, 1982). Presen t address: *Mea t an d Li vestoc k Commi ssio n, P.O . Box 44, Quecnsway H ouse, Bletch ley, Milt on K ey nes. tDe pa nm en t o f Agri culture, U ni ve rsi ty o f Oxford, Pa rk s Road, Oxford .
BRITISH VETERINARY JO URNAL, 138,6
534
This study has been extended ro includ e s umm er-ca lving cows a nd furt her s tat is ti ca l ana lysis has been carried out.
MATERIALS AND METHODS
Progeste rone co n ce ntra ti ons were measured in who le milk sa mpl es ta ke n twi ce or three tim es weekly from 18 1 predomin a ntly H e refo rd X F ri es ia n s u ck lin g beef cows in fou r h erd s. Samp ling comme nced sho r tly a ft e r part u rit ion a nd co ntinu ed unt il nor m a l ovarian cycles w e re r e-estab li s hed . The d eta il s o f three h e rd s ha ve bee n d escribed b y Pe ter s & Ril ey ( 1982). The fourth co nsisted o f 24· M ay-calving s ing le-s uc klin g H e re ford X Fries ian cows. The co ws were w eig hed w ithi n one week of part uriti o n and subseque ntl y a t a pprox im ate ly two-wee kl y in tervals. The progeste rone assay was performed as descr ibed by Bulm a n & Lamming ( 1978) a nd the acycl ic period was defined as th e tim e to the first ri se in milk progeste ro ne co nce n trat ion of ~3 ng/ ml fo ll owed by reg ular progeste ro ne cycles . The clara were a naly sed using lin ea r regr ess ion , a n a lys is of va rian ce a nd cova ri a nce tec hni ques us in g th e Genstat V co mpute r package (R ot hampsted Expe rim e nt Station, H a rp c nd e n , Hertford shi re).
120
•
• • •• •••...2
100
>-
0 "0
"0 0
~
a.
u
2 • 2
• • ••
80
•2
60
u
<1
•••
40
• •
•
•• 2 •• • 2 2•.
•• • • • 2
2 •• 2
•• 2
•
•
3 22
••2 42
•• z •
May
July
•• ••••
... ..
• •
••
Mar
•
• 2 •• 342 e 2 e2
•
20
Jon
••• •2 •• •• • •
2 ••
•• • • • •
•
>-
u
• •
.. e2
•
V>
• • •3 ••
• • ••
Sept
••
•
•
• •• • Nov
Do le o f calvin g
Fig. I . Rel ations h ip between date of ca lv ing a nd t he acy cli c period.
RESULTS
Th e overa ll mea n acycl ic pe r iod was 53·5 days ( ra nge II to 11 8 day s). N e ithe r age, breed , b odyw e ight ch ange post partum , no r suck lin g of two as co mpa r ed to one ca lf a ffec ted th e leng th o f the acyclic pe ri od (see Pe ters & Ril ey, 1982).
SEASONAL BREED I NG IN BEEF COVVS
535
TABLE I THE EFFECT OF MONTH OF CA LVI NG ON BODYWEIGHT AND THE LENGTH OF T HE ACYCLIC PERIOD
Month ofcalving
Nov-Apr
11
Mean bodyweight kg
91
5 11 ·9
SED
70·8
2·6 (P< O·OO I)
12· 1(NS)
May-Oc t
488·0
90
SED
Mean acyclic period* days
35·9
• l'vl ea n acycl ic peri ods a rc adjusted ror the c rrcct o r the bod ywcight cova ri a te .
There was a sig nifi ca nt negative correlat io n between the firs t bodyweightjJoslparlum and the acycl ic period (r = 0·322; P< O·OO I ). The relat ion between d ate of ca lving a nd the lengt h of th e acycl ic period is shown in Fig. I. Cows calv ing between Novembe r I and April 30 we re acycl ic sig nifi cantly longer tha n those ca lvin g between M ay I a nd October 3 1 (Tab le I). The re was no sig nifi ca nt differe nce between mea n bodyweight j;osl j;a rlum a t th e two periods of the year. There was no corre la tio n between photoperiod (corrected fo r Ia ti tude a nd supp lementa l li g h ting) o n the day of ca lvin g and the lengt h of the acycl ic period . H owever, there was a hi g hl y sign ifi cant negat ive co rrelation between the d a il y
,,_0·737; P
•• •
••
• •
• •
500
600
700
800
900
1000
1100
Day length m1n
Fig. 2. Relationship between daily phowperiod (day length) one month berore ca lvin g and the acyc lic period . Da y length va lues a rc adjusted ror latitud e a nd supp lcm ct1la ll ighting .
536
BRITISH VETERINARY .J OURNAL, 138,6
photoperiod one month befo re calving and the leng th o f the acycl ic period (r = - 0·737; P< O·OOl; Fig. 2). The regress ion equa tion for th e above re la ti o ns hip was: acyclic period in days = 181·6- (0·08 X first bodywcight post par/tun) -0· 12 x (photoper iod one month before calvi ng).
D I SCUSS I ON
The prese nt d ata, a long with that a lready pub li sh ed (Peters & Ril ey, 1982), suggest th at bodyweight a ro und the time of pa rturiti on is an importa nt determ in a nt of th e acycl ic period. This is cons iste nt w ith other findings that pre- p a rtum nutr iti on is m o re impor tan t th an that J;ost par/tun in d e te rminin g th e le ngt h o f the ac ycl ic period (Wilt ba nk , 1970; Corah, Dunn & K a lt e n bac h , 1975; M o nt gome ry el aL., 1980) . Although bodyweight p rovides only a n imprec ise ind ex of nutriti o n a l stat us, it was used in the present st udy in preference to th e techniqu e of body -cond ition sco ring (Lowman, Somervi ll e & Scott, 1976), since the latte r w as found to be hi g hl y va ri ab le between o perato rs. Th e prese nt findin g that cows exposed to longer phoLOper iod durin g late preg na ncy tend to und ergo s ho rte r acyc li c periods a nd v ice versa is co nsis te nt with r epo rt s of seaso na l differe nces by Bu lma n & La mmin g ( 1978), Kin g & Hurnik ( 1980) a nd Montgo m er y et aL. ( 1980) a lthou g h these st udi es d id no t a na lyse possib le nutriti ona l innu e nces. A poss ible effec t of phoLOpe ri od o n pos t-pa rtum a noest ru s was fir s t s ugges ted by Thibault et aL. ( 1966) a nd th e prese nt d a ta s tro ng ly s uppo rt thi s sugges ti on. M a n y wild spec ies of Bovidae a rc seasona l breeders (Asde l! , 1964·), cha nges in da il y photope ri od bein g th e cue fo r the o nse t o r termin at io n of b reed ing act iv it y. D o m es tica ti o n of ca t tle has tended to se lec t aga in s t seaso nal breedi ng bu t it is perhaps no t surprising if a ves tig ia l sens it ivi ty to ph otope riod rema in s. Our da ta s uggest th a t th e beef cow is simil a r in thi s respec t to th e m arc, in w hi c h breeding act iv it y is inhibited during the tim e ofs ho rt ph oto per iod (Ginther, 1974·). In ca tt le, thi s pa tt ern of seaso na l ac tivit y wou ld tend to p red is pose to s pring-s umm er ca lvin g th e mos t ap prop ri ate ti m e of yea r fo r the fera l a n im a l. T he pos sibility of a n effect of e nv ironmenta l te mpe rat ure ca n not be ru led out as a mbi e nt temperature and ph oto period a re hi ghly pos it ively co rrelated . Obv iously the re lat ive importance of these fac tors requires tes tin g under s tri ct ly co ntro ll ed co ndi tio ns before a d efi nite concl usion ca n be reached . From a m o re prac tica l sta ndpoi nt , it is clea r tha t adequate nutriti o n is importa nt in d ete r m ini ng the leng th of the bovine acycli c p e riod a nd hence the ava il ab ilit y for reb reed in g. I t is suggested that und er- feedi ng of pregnant beef co ws due LO ca lve in early s pring, a t a tim e of sho rt photoperiod, is lik ely to be more d e leterious to the o nse t of ovarian act iv ity a ft er ca lv ing th a n for th ose ca lvi ng in the sum m e r a nd au tumn .
:\ C K NOWLEDGEMENTS
The authors a r e g ra te ful LO the East o f Scot la nd Co ll ege of Agricu lture, the H ill Farm in g R esearch Organ isa tion and the Ag ri cu lt ura l D eve lo pm e nt a nd Adv iso ry Se rv ice fo r the prov isio n of milk sa mpl es.
SEASONA L BREEDI NG I N BEF.r CO WS
537
REFERE NCES
AL LEN, D . M . ( 1979). Proceeding of the Second Stud)' Conference of the Scollish t1griculturnl Colleges, 17. ASD ELL , S. A. ( 1964). Pal/ems of Mammalian Reproduction, 2nd cdn . London : Constable. 130Y D, H . ( 1977) . Jleterinao• Record 100, ISO. BULM AN, D . C. & L A~ I M INC, G. E. ( 1978). J oumal of Refno duction and Fertility 54, 4-4 7. COR!\H , L. R ., D UNN , T. G . & K ALTENBAC H, C . C . ( 1975) . j oumal of Animal Scimce 41, 8 19. DE K RU I F, A . ( 1978) . J oumal of RejJroduction and Fert ility 54, 507 . DU NN, T. G. , I NGALLS, j . E., Z IMMER MAN , D . R. & WILTB.-\ NK, .J . N . ( 1969 ). J ournalo.f,1nimal Science 29, 7 19. G INTHER, 0 . .J . ( 1974·). American J ournal of Veterinao• Research 35, 11 73. I NSKEE P, E. K . & LI SHM AN, A . V•i. ( 1979). Beltsville SJ•IIlfJosia in Agricultural Research (cd . H . H aw k) 3, 277. M ontclair: Allanhc ld , O smun. K INC, G. J. & H URN I K, J. F. ( 1980) . Proceedings of the 9th Intemational Congress on Animal Reproduction and Artificial Insemination 3, 39 . L OWMA N, B. G ., ScoTr, N. A. & SOMERV ILLE , S. H . ( 1976). East of Scotland College of
Agriculture, Bulletin No. 6. M ON TGO MERY , G . W ., DAVI S, G . H. & H u RRf.LL, G . A. ( 1980). Proceedings of th e New Zea land Sociel:)• of Anima l Production 40, 280. O XEN RE ID ER, S. L . ( 1968). America n J ournal of Veterina ry Research 29, 2099. PETERS , A. R. & RIL EY, G . M . ( 1982). Animal Production 34, 145. THIB t\ ULT, C ., CouROT, M ., M ART INET, L. , MA UI..EON, P., DE M ESN IL ou Bu iSSON, F., ORTOV ANT, R. , PELLETI ER, J. & SICNORET, .J. P. ( 1966). J oumnl of Animal Science 25, suppl. p. 11 9. Tu CKER , H . A. ( 1982). Th eriogenolog;• 17, 53. WILTBi\N K, j . N. ( 1970) .Joumal of Animal Scimce 31, 755 . WI LTBi\NK , J. N., ROWDEN, V'•i . W ., I NGALLS, j . E. & Z I M~IF.RM .-\ N, D . R . ( 1964). J oumal of Animal Science 23, I 049 .
(19R2).
138, 533
IS THE COW A SEASONAL BREEDER?
Bv A . R. PETERS• ANO G. M . RIL EYt
Agricultural Resea rch Council Research Group on Hormones and Fann Animal Rej;roduction, UniversifJ' of Nollingham School of Agriculture, Sutton Boning/on, Loughborouglz, Leics
SUMMAR Y
The tim e to the first ri se in milk progeste ro ne concentrations (acy cli c period ) w as d e termin ed in 18 1 post- part um beef cows a nd a m ea n pe r iod o f 53·5 d ays was found . Th ere was a sig nifi ca nt negativ e co rre la tion b etw ee n bod yw eight dur in g the firs t wee k j;ost partum a nd th e acycl ic p eriod (r = -0·3 22; P
INT RODUCT ION
The pe ri od o f tru e anoestrus or acycl icit y is often prolonged in pos t-pa rtum suck lin g beef cows (Ox enreid e r, 1968; In skee p & Lishman, 1979). Thi s may lead to a n exte nd ed ca lving interva l in indi vidua l cows a nd a n exte nded h erd ca lvin g seaso n both of which arc eco no mi ca ll y undesira bl e (A ll en , 1979). T he length of th e acy cli c pe riod m ay be reduced by prov ision of a d eq ua tc di e ta ry en ergy (\IV ilt bank eta/., 1964; Dunn et al., 1969). Seasonal diffe ren ces in th e acyc li c pe ri od h ave bee n reported fo r d a iry cows (Bulman & L a mmin g, 1978) and for beh av iou ra l anoes tru s in beef cows (K ing & Hurni k, 1980; M o n tgo m e ry, D av is & Hurre ll , 1980). H o weve r, severa l authors have co mm ented th a t seaso n a l effects on a noes tru s m ay be re la ted purely to differen ces in nutrit io n a nd m a n age m e nt ( Boyd , 1977; D e Kruif, 1978; T ucke r, 1982). It has been r epo rted r ece ntl y th a t the acyc li c pe riod in bee f cows was negat ively co rrelate d w ith bodyweight a t ca lvin g a nd th a t cow s ca lving in th e spring m o n ths were acyc li c fo r lo nger peri od s th a n those ca lving in the a utumn (Pe ters & Ril ey, 1982). Presen t address: *Mea t an d Li vestoc k Commi ssio n, P.O . Box 44, Quecnsway H ouse, Bletch ley, Milt on K ey nes. tDe pa nm en t o f Agri culture, U ni ve rsi ty o f Oxford, Pa rk s Road, Oxford .
BRITISH VETERINARY JO URNAL, 138,6
534
This study has been extended ro includ e s umm er-ca lving cows a nd furt her s tat is ti ca l ana lysis has been carried out.
MATERIALS AND METHODS
Progeste rone co n ce ntra ti ons were measured in who le milk sa mpl es ta ke n twi ce or three tim es weekly from 18 1 predomin a ntly H e refo rd X F ri es ia n s u ck lin g beef cows in fou r h erd s. Samp ling comme nced sho r tly a ft e r part u rit ion a nd co ntinu ed unt il nor m a l ovarian cycles w e re r e-estab li s hed . The d eta il s o f three h e rd s ha ve bee n d escribed b y Pe ter s & Ril ey ( 1982). The fourth co nsisted o f 24· M ay-calving s ing le-s uc klin g H e re ford X Fries ian cows. The co ws were w eig hed w ithi n one week of part uriti o n and subseque ntl y a t a pprox im ate ly two-wee kl y in tervals. The progeste rone assay was performed as descr ibed by Bulm a n & Lamming ( 1978) a nd the acycl ic period was defined as th e tim e to the first ri se in milk progeste ro ne co nce n trat ion of ~3 ng/ ml fo ll owed by reg ular progeste ro ne cycles . The clara were a naly sed using lin ea r regr ess ion , a n a lys is of va rian ce a nd cova ri a nce tec hni ques us in g th e Genstat V co mpute r package (R ot hampsted Expe rim e nt Station, H a rp c nd e n , Hertford shi re).
120
•
• • •• •••...2
100
>-
0 "0
"0 0
~
a.
u
2 • 2
• • ••
80
•2
60
u
<1
•••
40
• •
•
•• 2 •• • 2 2•.
•• • • • 2
2 •• 2
•• 2
•
•
3 22
••2 42
•• z •
May
July
•• ••••
... ..
• •
••
Mar
•
• 2 •• 342 e 2 e2
•
20
Jon
••• •2 •• •• • •
2 ••
•• • • • •
•
>-
u
• •
.. e2
•
V>
• • •3 ••
• • ••
Sept
••
•
•
• •• • Nov
Do le o f calvin g
Fig. I . Rel ations h ip between date of ca lv ing a nd t he acy cli c period.
RESULTS
Th e overa ll mea n acycl ic pe r iod was 53·5 days ( ra nge II to 11 8 day s). N e ithe r age, breed , b odyw e ight ch ange post partum , no r suck lin g of two as co mpa r ed to one ca lf a ffec ted th e leng th o f the acyclic pe ri od (see Pe ters & Ril ey, 1982).
SEASONAL BREED I NG IN BEEF COVVS
535
TABLE I THE EFFECT OF MONTH OF CA LVI NG ON BODYWEIGHT AND THE LENGTH OF T HE ACYCLIC PERIOD
Month ofcalving
Nov-Apr
11
Mean bodyweight kg
91
5 11 ·9
SED
70·8
2·6 (P< O·OO I)
12· 1(NS)
May-Oc t
488·0
90
SED
Mean acyclic period* days
35·9
• l'vl ea n acycl ic peri ods a rc adjusted ror the c rrcct o r the bod ywcight cova ri a te .
There was a sig nifi ca nt negative correlat io n between the firs t bodyweightjJoslparlum and the acycl ic period (r = 0·322; P< O·OO I ). The relat ion between d ate of ca lving a nd the lengt h of th e acycl ic period is shown in Fig. I. Cows calv ing between Novembe r I and April 30 we re acycl ic sig nifi cantly longer tha n those ca lvin g between M ay I a nd October 3 1 (Tab le I). The re was no sig nifi ca nt differe nce between mea n bodyweight j;osl j;a rlum a t th e two periods of the year. There was no corre la tio n between photoperiod (corrected fo r Ia ti tude a nd supp lementa l li g h ting) o n the day of ca lvin g and the lengt h of the acycl ic period . H owever, there was a hi g hl y sign ifi cant negat ive co rrelation between the d a il y
,,_0·737; P
•• •
••
• •
• •
500
600
700
800
900
1000
1100
Day length m1n
Fig. 2. Relationship between daily phowperiod (day length) one month berore ca lvin g and the acyc lic period . Da y length va lues a rc adjusted ror latitud e a nd supp lcm ct1la ll ighting .
536
BRITISH VETERINARY .J OURNAL, 138,6
photoperiod one month befo re calving and the leng th o f the acycl ic period (r = - 0·737; P< O·OOl; Fig. 2). The regress ion equa tion for th e above re la ti o ns hip was: acyclic period in days = 181·6- (0·08 X first bodywcight post par/tun) -0· 12 x (photoper iod one month before calvi ng).
D I SCUSS I ON
The prese nt d ata, a long with that a lready pub li sh ed (Peters & Ril ey, 1982), suggest th at bodyweight a ro und the time of pa rturiti on is an importa nt determ in a nt of th e acycl ic period. This is cons iste nt w ith other findings that pre- p a rtum nutr iti on is m o re impor tan t th an that J;ost par/tun in d e te rminin g th e le ngt h o f the ac ycl ic period (Wilt ba nk , 1970; Corah, Dunn & K a lt e n bac h , 1975; M o nt gome ry el aL., 1980) . Although bodyweight p rovides only a n imprec ise ind ex of nutriti o n a l stat us, it was used in the present st udy in preference to th e techniqu e of body -cond ition sco ring (Lowman, Somervi ll e & Scott, 1976), since the latte r w as found to be hi g hl y va ri ab le between o perato rs. Th e prese nt findin g that cows exposed to longer phoLOper iod durin g late preg na ncy tend to und ergo s ho rte r acyc li c periods a nd v ice versa is co nsis te nt with r epo rt s of seaso na l differe nces by Bu lma n & La mmin g ( 1978), Kin g & Hurnik ( 1980) a nd Montgo m er y et aL. ( 1980) a lthou g h these st udi es d id no t a na lyse possib le nutriti ona l innu e nces. A poss ible effec t of phoLOpe ri od o n pos t-pa rtum a noest ru s was fir s t s ugges ted by Thibault et aL. ( 1966) a nd th e prese nt d a ta s tro ng ly s uppo rt thi s sugges ti on. M a n y wild spec ies of Bovidae a rc seasona l breeders (Asde l! , 1964·), cha nges in da il y photope ri od bein g th e cue fo r the o nse t o r termin at io n of b reed ing act iv it y. D o m es tica ti o n of ca t tle has tended to se lec t aga in s t seaso nal breedi ng bu t it is perhaps no t surprising if a ves tig ia l sens it ivi ty to ph otope riod rema in s. Our da ta s uggest th a t th e beef cow is simil a r in thi s respec t to th e m arc, in w hi c h breeding act iv it y is inhibited during the tim e ofs ho rt ph oto per iod (Ginther, 1974·). In ca tt le, thi s pa tt ern of seaso na l ac tivit y wou ld tend to p red is pose to s pring-s umm er ca lvin g th e mos t ap prop ri ate ti m e of yea r fo r the fera l a n im a l. T he pos sibility of a n effect of e nv ironmenta l te mpe rat ure ca n not be ru led out as a mbi e nt temperature and ph oto period a re hi ghly pos it ively co rrelated . Obv iously the re lat ive importance of these fac tors requires tes tin g under s tri ct ly co ntro ll ed co ndi tio ns before a d efi nite concl usion ca n be reached . From a m o re prac tica l sta ndpoi nt , it is clea r tha t adequate nutriti o n is importa nt in d ete r m ini ng the leng th of the bovine acycli c p e riod a nd hence the ava il ab ilit y for reb reed in g. I t is suggested that und er- feedi ng of pregnant beef co ws due LO ca lve in early s pring, a t a tim e of sho rt photoperiod, is lik ely to be more d e leterious to the o nse t of ovarian act iv ity a ft er ca lv ing th a n for th ose ca lvi ng in the sum m e r a nd au tumn .
:\ C K NOWLEDGEMENTS
The authors a r e g ra te ful LO the East o f Scot la nd Co ll ege of Agricu lture, the H ill Farm in g R esearch Organ isa tion and the Ag ri cu lt ura l D eve lo pm e nt a nd Adv iso ry Se rv ice fo r the prov isio n of milk sa mpl es.
SEASONA L BREEDI NG I N BEF.r CO WS
537
REFERE NCES
AL LEN, D . M . ( 1979). Proceeding of the Second Stud)' Conference of the Scollish t1griculturnl Colleges, 17. ASD ELL , S. A. ( 1964). Pal/ems of Mammalian Reproduction, 2nd cdn . London : Constable. 130Y D, H . ( 1977) . Jleterinao• Record 100, ISO. BULM AN, D . C. & L A~ I M INC, G. E. ( 1978). J oumal of Refno duction and Fertility 54, 4-4 7. COR!\H , L. R ., D UNN , T. G . & K ALTENBAC H, C . C . ( 1975) . j oumal of Animal Scimce 41, 8 19. DE K RU I F, A . ( 1978) . J oumal of RejJroduction and Fert ility 54, 507 . DU NN, T. G. , I NGALLS, j . E., Z IMMER MAN , D . R. & WILTB.-\ NK, .J . N . ( 1969 ). J ournalo.f,1nimal Science 29, 7 19. G INTHER, 0 . .J . ( 1974·). American J ournal of Veterinao• Research 35, 11 73. I NSKEE P, E. K . & LI SHM AN, A . V•i. ( 1979). Beltsville SJ•IIlfJosia in Agricultural Research (cd . H . H aw k) 3, 277. M ontclair: Allanhc ld , O smun. K INC, G. J. & H URN I K, J. F. ( 1980) . Proceedings of the 9th Intemational Congress on Animal Reproduction and Artificial Insemination 3, 39 . L OWMA N, B. G ., ScoTr, N. A. & SOMERV ILLE , S. H . ( 1976). East of Scotland College of
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