P S -V Ia -1

DESIGN, STRUCTURE AND ECONOMICS OF A NATIONAL DAIRY CATTLE IMPROVEMENT SCHEME

Diserio, estructura y economla de un esquema nacional de me j ora. en vacuno lechero

E. B. BURNSIDE*L.R. SCHAEFFER*B.W. KENNEDY*

CANADAThe d e s i g n and economy o f v a r i o u s schemes f o r improvement o f

d a i r y c a t t l e p r o d u c t i o n u s i n g a r t i f i c i a l i n s e m i n a t i o n <AI> was s u b j e c t

to a r a s h o f r e s e a r c h a c t i v i t y in the m i d - 6 0 ' s to m i d - 7 0 ' s . A t the

o u t s e t , an ima l b r e e d e r s s h o u l d note th a t w h i l e the m e t h o d o l o g y o f

a r t i f i c i a l i n s e m i n a t i o n went i n t o use in the immediate p o s t W or ld War

I I e r a , an im a l g e n e t i c i s t s were s e e m i n g l y " a s l e e p a t the s w i t c h " , and

d i d not compare the e f f e c t i v e n e s s o f d i f f e r e n t s t r a t e g i e s to u t i l i z e

the p r o g e n y t e s t , u n t i l the 1 9 6 0 ' s , n o r , in g e n e r a l , g e t them

e f f e c t i v e l y in use u n t i l the 7 0 ' s . I f we are to p r o f i t f rom p a s t

e r r o r s , i t seems t h a t g e n e t i c i s t s who have an i n t e r e s t in s i m u l a t i o n

t e c h n i q u e s s h o u l d be c o n s t a n t l y s p e c u l a t i n g abo u t f u t u r e b i o l o g i c a l

b r e a k t h r o u g h s and c o m p a r i n g schemes to make use o f the se d ev e lo pm en ts

w e l l in adv ance o f t h e i r t ime, so th a t the m et hod s are in p l a c e when

the b i o l o g i c a l s y s t e m s are r e a d y . Have we s p e c u l a t e d f u l l y about the

f u t u r e o f embryo m a n i p u l a t i o n , f r e e z i n g and t r a n s f e r , and gene c l o n i n g

a s they may a f f e c t the f u t u r e o f anima l improvement p r o g r a m s ?

The s u b j e c t m a t t e r r e l a t i n g to d e s i g n , s t r u c t u r e and e cono m ic s o f

an ima l b r e e d i n g schemes h a s been t h o r o u g h l y r e v i e w e d (Cunn ingham,

1973; B u r n s i d e , 1974; and M i l l e r , 1977) , t h u s t h i s paper w i l l a t tempt

•De par tment o f An ima l & P o u l t r y S c i e n c e , U n i v e r s i t y o f Gue lph , Gue lph, O n t a r i o , Canada N I G 2W1

357

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«11» <As itthey affffeertt rr ::■ mi

— Number toff pprocgenyysi irree «35)

— lDi isrtri itbutti icon toff si irree .progeny over herds C(LU)

— Randomness off progeny tteesrtiirrg systems twirth respect tto 'herds and Sterna! lees mated twitthiim -herds «LO)

— ilfqpadt off cseilarrtiicon tbias con eesrti hnatti icon off breed frog values and con eestti rntarti icon adf ogenert io cparamete r sfBellartiiuee accuracy corf siirrse ceua’ kisatti ion mode 1 s «LU)

— fftellartiiuee accuracy 'Off couw iimdexiirng models «10)

— i impact off rtiites bed ween sdb-tpop ul lart ions iwirth im ppppuilsrti icons and ttirrne iperi iods iwitthiim cpopni lath ions (C.li)

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<CS) f#te tttieey affffcecrrt ii:i

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- I n t e n s i t y o f p r e s e l e c t i o n o f yo ung b u l l s <S-U>

- Im pa c t o-f per forma nce a n d / o r p r o g e n y t e s t s f o r beef t r a i t s i n a d d i t i o n to s e l e c t i o n f o r m i l k y i e l d <S)

- Number o f t r a i t s in the b r e e d i n g go a l <S-U>

- B u l l dam s u p e r i o r i t y <S>

- S u p e r i o r i t y o f s i r e s o f yo ung b u l l s <S>

- P r o p o r t i o n o f s i r e s o f s o n s r e p l a c e d a n n u a l l y <S>

- Number o f f i r s t s e r v i c e s to young b u l l s to a c h e i v e on t e s t e d da ugh te r <S>

- H e a l t h s t a t u s o f b u l l s "on the s h e l f " a s r e l a t e s to m o r t a l i t y r a t e <U>

- F r e e z i n g up l a r g e q u a n t i t i e s o f semen p r i o r to b u l l s b e i n g proven and s l a u g h t e r i n g b u l l s v s . " t e s t and r e s t " ( S )

- A c c u r a c y o f m i l k r e c o r d i n g and cow c u l l i n g p r o c e d u r e s <U>

- Im pa c t of embryo t r a n s f e r on b u l l - d a m s e l e c t i o n and number o f s i r e s samp 1e d / b u 11-dam <U>

- Im pa c t o f embryo t r a n s f e r on r e p l a c e m e n t female s e l e c t i o n <U)

- Impac t ( p o t e n t i a l l y n e g a t i v e ) o f embryo t r a n s f e r on p e r c e n t o f f e m a l e s mated to young b u l l s < U ) .

<3) A s the y a f f e c t 0 g s

- H e r i t a b i l i t y o f the t r a i t < s ) in b r e e d i n g go a l < S ) .

- Im pa c t o f chang e s o f g e n e t i c v a r i a n c e e i t h e r due to l i n k a g e d i s e q u i l i b r i u m or i n b r e e d i n g <U)

- Conse que nce s o f f i n i t e number o f l o c i and ma jo r gene e f f e c t s <ll)

(4 ) A s they a f f e c t L:

- P e r c e n t o f p o p u l a t i o n br e d to proven v s . unproven s i r e s <S>

- Speed w i t h which young b u l l s g e t s a m p le d and t h e i r age <S-U>

- Ra te o f t u r n - o v e r o f proven b u l l s t u d <S>.

- Rate o f t u r n - o v e r o f cow p o p u l a t i o n (U)

- Age o f b u l l dams when mated to pr odu ce yo ung b u l l s <U>

- Impac t o f embryo t r a n s f e r and use o f r e c o r d s in p r o g r e s s on age o f b u l l - d a m s when sons e n t e r A I s t u d s <U>

T h i s l i s t i s not a l l i n c l u s i v e , ye t i t shows many a r e a s f o r

s i m u l a t i o n r e s e a r c h on r a t e s o f g e n e t i c p r o g r e s s a s r e l a t e s to d e s i g n

o f p r o g e n y t e s t schemes. These s t u d i e s may not have the impact o f

e a r l i e r work in c h a n g i n g d i r e c t i o n s o f p l a n s , but a re c o n s i d e r e d to be

v e r y n e c e s s a r y in the f i n e t u n i n g o f p ro g ra m s. A s ex am p le s , the work

of L i nd h e (1968) and P e t e r s e n et a l (1974) d e m o n s t r a te d th a t much

l a r g e r q u a n t i t i e s o f semen <40,000-50,000 d o s e s / b u l l ) than s u g g e s t e d

by S k j e r v o l d (1963) i s g e n e t i c a l l y and e c o n o m i c a l l y a d v a n t a g e o u s a s

opp osed to s a m p l i n g l a r g e r numbers o f young b u l l s and f r e e z i n g o n l y

15,000 d o s e s / b u l l b e f o r e s l a u g h t e r i n g . Recen t work i n d i c a t e s th a t

w h i l e a cow index ( B u r n s i d e 1970) ba se d on a w i t h i n h e rd c o n te m po r a r y

c o m pa r i s o n p l u s s i r e ' s b e s t l i n e a r u n b i a s e d p r e d i c t i o n (BLUP) ( o f

t r a n s m i t t i n g a b i l i t y ) was h i g h l y c o r r e l a t e d <r = 0 . 8 9 ) w i t h i n t r a h e r d

BLUP cow i n d e x e s , the BLUP i nd e xe s were much b e t t e r p r e d i c t i o n s o f

f u t u r e p r o g e n y t e s t s o f young b u l l s ( r = 0 . 4 6 f o r BLUP v s . r = 0.11

f o r the c o n t e m p o r a r y i n d e x e s ) .

B u r n s i d e (1974) drew a number o f c o n c l u s i o n s a f t e r r e v i e w i n g the

main s i m u l a t i o n s t u d i e s , and i t seems p r o f i t a b l e to comment on t h e i r

v a l i d i t y in l i g h t o f more r e c e n t work. B u r n s i d e (1974 ) c o n c l u d e d th a t

g e n e t i c g a i n w ou ld not i n c r e a s e l i n e a r l y f o r i n c r e a s i n g p o p u l a t i o n

361

cei i-rcpcg andtar numbers aF young: bull 1 Is tested!. TUti is eea-rtsi in-illy se&ms to)

ttcrtidJ today,. wiitoi eeegsrsi am deyamdi aa papnjl lantti iam siizz® aFf say,, deal Iff aanriilllliiam caw®,, yiieldlng Uiittttll® pattaarittiwell iirraresse? iim gen®ttiic ppragresspear year Far most crft to® ®Uterm®tti iu® gllam® iim terms aFf numbers aFfbuillUs sampled),, progeny group siiza®,, imdtti ngs too yaung but'11s ®ttc.. fits

papuj)iaettiiar siizes iirrcrass®,, iitt iis adVisabU® to) expancfii toe number a#ftestt progeny p ear bull 11 to max i mi :z® r rat+ter - titan, si mpl 'y- expand! irrg

Tilnumbers aFf bull Us tested) wttto, toe:- assoc, rated) dramatic: i in cresses iim

h au s i in g, ar semen tr eez- i in g. cast's.Burn&idfe ('1195746, camrliudfedi that iitt was bestt tty tturrm to® group aFf

bu i t s sir i ng. yaung. bud 11 Isa aver si awl iy (5OKty ear)’ ■. SB in® r an tti n g: p r qc edur e s based oiti BU UP me tti o ds (Hender son , < li91 3J)„ wh i ch sst® n ow iimiiaa, should1 designate to as® built 11® wbiiiatti am® most super i iar iim aa ©ivesm p op u l aeti iam.. Our expeni enae- i is to at to® topr-ranKi ingi si res Far miiltM< arr butterFistt yi®1 d1 aar® ■freguerTtlly drawn Ffranr diiFFenemtt bujllll “teams" . Tih;i,3 abservati an was ornrfti rmedi by ScltaeFFer 0119>7&>> iim aa BUUF ansUysus aFf Swedish) bull Us.. Fur toer ,. BUUP pronfts. should) desi gnat?® those si res ttx be used) mast ex tens) v®lly,, rattier toan to® age- aF toe buiUU,, as- iitt appear a that “ teams" a* buUUs Freguem telly vary upward® ar downwards iim geneti ic meriit,, aUtoaughr to® a v e r all 11 to end, wi lilt be. upwards im *nr

soundliy desi great pr ogram.Burnside <119-746’ can eluded' to at: * desi inattil 1® progeny group si ze was

between 30 and: 50) daughter® wiittti smal ler numbers being desirabU® Far smal l! test papulatti an® which) neied to) test prapar tianat®lly more buiUUs..

Tib® 1 larger numbers at records, requii red) Far accurst® si in® rarKi ng Fartraits sued as cal vi ng: nnnnnrr IfTT"1 " --------reproduction wh i;ctt ar® now aFfi nterest:,, necessi tat® pi ann,iing Far UOOOC to) 125; test bedFens iim 1 large-

362

fpgpullattiiams twiiTh thiicQh fpsrrBsnttsges orf rows nriilllk rneoonded. These numbers corf .daughters ane,, 'however,, mart ee*si!l>y abhi-evstblce Tor small Iter cow fpcqpullaafcti icons cur rpcqpUlaattiicons wtiTh llcow cpsr.cenTages corf now® rreacordecd cor timed ttoo >ymmg tbuillls..

The success corf a desi gned breedimcg program depenas con our cospatfciilliitt-y To rmoniittor Whstt iis 3arTifeJlll>y amcurTriirng,, ggenettihcalllby.. The -use corf swpngpri iaTe lliimear stfeettiisttiiccall mode I s sand The mouth imeavallunar*iom corf rmalltes and rfamalIces iim The fpcgpullattiicon cor sub-tpopu 1 sttiions 1 is Cessnanttiieell.. rfgjpi 11icoattiicon corf EBlilfP To These 11: i;near mode Is iim Canada 'has allllasMBd The eesTihnettiicon corf cgenettice ‘treands iim si)) '.proven -sires., tb> young hullll esnttriites, oO) asilll .young thulllte megistteered iim (herd 'books cb> edams corf -young hull 11s,, ee)> ttwD—yeai— oId cows cWhiiath call we eeeadh year,, and ■rf» aeSlll cows Whiicdh asall've teach -year.. temper i sons among A1 sttuds, among tpmouiirrmes,, cor among hands are aeJlll tby-fpnadurtfcs corf The same cpmigjnams tthafct fpraawiicde cgenectiice cewa'tcuattiicorts corf sirrees and cows. CDi ssemirnerticon corfThis iimrformasticon armuelllby To Those irrutdlcueti iim The (program cneaTes an awareness au.uing rparttiiciicpanTs corf The cdirnactbi on corf The program.

FRnad iarti img The outcome corf rfUtune rmaTiirrtgs shod led ail so theimp lemented tberfone The rmattiimegs arttualllyy take cpllace.. 11m This way , cwa'l-uab lea cgeneT ice iirntiiwiicduailts omul led 'ire timed more -wise'l-y tby i-know img The ipOtentt ial 'genet; ice and ettonomicc walcue corf spec; i-ff ice matt i ngs. Such rprediccttiicons sroullcd become an iimTagraail ppearT corf a edaiimy .operart i on Through ♦the use corf imicrcnccnjputens Thatt can the lliimkeri tto mattikonall data bases erf i imrfonntetti icon..

Eaneffull mon iTor-img aorf The rhesullTs corf a ttmeectiimg program may al so uncover cp rob learns cor .weaknesses iim The program which asm Then he silTeneti and irmprowed. fRor eexarpp.lce,, iirf tthe assoc iattlion between

B63

p e d i g r e e in d e x e s and pr o g e ny t e s t s on b u l l s s t a r t s to d e c l i n e over

t i m e, then p o s s i b l y the model u sed to a n a l y z e the d a t a may be

i n a p p r o p r i a t e and ch a ng e s s h o u l d be made. We s h o u l d not assume th a t

once a prog ram i s e s t a b l i s h e d , i t i s c o r r e c t and s h o u l d never be

c h ang e d. We must obs erv e c o n t i n u o u s l y and make c h a n g e s -from t ime to

time.

I t may be d e s i r a b l e to dev e lo p a g l o b a l s t r a t e g y -for d a i r y c a t t l e

b r e e d i n g . The o v e r a l l p o p u l a t i o n o f d a i r y cows s h o u l d be broken i n t o

a number o f s u b - p o p u l a t i o n s , each w i t h i t s j i n i q u e br eed i ng g o a l s , and

a c h i e v i n g near maximum g e n e t i c p r o g r e s s .

A ma jo r prob lem a r e a in d e v e l o p i n g a g l o b a l d a i r y b r e e d i n g

s t r a t e g y j s ' ^ t h e amount o f exchange o f semen f r om b u l l - s i r e s wh ich

s h o u l d take p l a c e . P r e s e n t l y , top proven s i r e s o f the N o r t h Amer ican

Hoi s t e i n - F r i e s i a n are b e i n g used a s b u l l - s i r e s in many p o p u l a t i o n s ,

w i t h a s u b s t a n t i a l i n c r e a s e in the r e l a t i o n s h i p among these

p o p u l a t i o n s an u l t i m a t e r e s u l t . S h o u l d such e x c h a n g e s be e n c o u r a g e d ?

Land (1 981) s u g g e s t s th a t the h i s t o r i c a l a pp r o a ch o f i n t r o d u c i n g

s u p e r i o r e x o t i c s t r a i n s to improve l o c a l ' p o p u l a t i o n s m i g h t be

r e p l a c e d , a t l e a s t in p a r t , by a p o l i c y o f deve lo pm en t o f s t r a i n s f o r

d i f f e r e n t b i o l o g i c a l s p e c i a l t i e s . For example we may w i s h to deve lo p

d a i r y cows o f d i f f e r e n t s i z e s , l a r g e to s m a l l , h i g h b u t t e r f a t , h i g h

m i l k , h i g h p r o t e i n y i e l d s , a s w e l l a s dua l p u r po s e s t r a i n s . T h i s

app ro a ch c o u l d d e v e lo p g r e a t e r g e n e t i c f l e x i b i l i t y . However i t i s not

a p p a r e n t who s h o u l d bear the c o s t s o f the lower m i l k y i e l d s and

d e c r e a s e d p r o f i t a b i l i t y under p r e s e n t - d a y c o n d i t i o n s wh i ch may be

a s s o c i a t e d w i t h some o f these new s t r a i n s . F u r t h e r m o r e , i t i s

364

d e s i r a b l e f o r each such s t r a i n to have a m i l k r e c o r d i n g prog ram and a

p o p u l a t i o n s i z e o f a t l e a s t 200,000 to 500,000 cows to app ro ach the

r a t e s o f g e n e t i c p r o g r e s s a c h i e v a b l e today in improved d a i r y c a t t l e .

Land (1981 ) a r g u e s t h a t t h i s i s u n n e c e s s a r y ! t h a t the l i n e s c o u l d be

t r e a t e d l i k e sm a l l c o n t r o l p o p u l a t i o n s , however i t i s q u i t e p o s s i b l e

to d e f i n e a number o f d i v e r g e n t s e l e c t i o n g o a l s and p ro c e e d tow ards

them i f p o p u l a t i o n s o f 200,000 or more can be c r e a t e d . Ex am ples o f

such s u b - p o p u l a t i o n s wh ich are e c o n o m i c a l l y v i a b l e are the J e r s e y and

A y r s h i r e b r e e d s , and the S c a n d i n a v i a n b r e e d s . The c r e a t i o n o f new

s u b - p o p u l a t i o n s by c r o s s b r e e d i n g f o l l o w e d by s e l e c t i o n , a s s u g g e s t e d

by S k j e r v o l d <1981) , i s both f e a s i b l e and d e s i r a b l e .

Cunn ingham <1973) d i s c u s s e d a l t e r n a t i v e b r e e d i n g s t r a t e g i e s f o r

d a i r y and d a i r y - b e e f cow p o p u l a t i o n s . In d i s c u s s i n g the a d v a n t a g e s o f

p l a n n i n g to take a d v a n ta g e o f h e t e r o s i s in d a i r y p r o d u c t i o n , he

e m p h a s i z e s t h a t each 1 ine or breed s h o u l d be l a r g e enough (100 ,000

cows or more) to p r o v i d e f o r a near opt imum A I p r o g e n y - t e s t i n g

prog ram . T h i s do e s not seem at v a r i a n c e w i t h our c o n c e p t o f a g l o b a l

s t r a t e g y f o r d a i r y c a t t l e b r e e d i n g . One f a c e t wh ich wou ld r e q u i r e

e x t e n s i v e p l a n n i n g and r e s e a r c h i n v o l v e s d e f i n i t i o n o f b r e e d i n g g o a l s ,

c o n s i d e r a t i o n o f b r e e d s which wou ld be most a p p r o p r i a t e to d i f f e r i n g

g o a l s and deve lo pm en t o f g e n e t i c a l l y d i f f e r e n t s u b - p o p u l a t i o n s e i t h e r

by f o r m i n g a h e t e r o g e n e o u s c r o s s b r e d f o u n d a t i o n f o l l o w e d by p e r i o d i c

i n t r o d u c t i o n o f new germ p l a sm or s i m p l y by i s o l a t i n g l a r g e

p o p u l a t i o n s and s e l e c t i n g to ward s s p e c i f i c b r e e d i n g g o a l s . Some

e v id e n c e i s a c c u m u l a t i n g o f d e c r e a s e d f i t n e s s in s e l e c t e d H o l s t e i n -

F r i e s i a n p o p u l a t i o n s a s m i l k y i e l d i n c r e a s e s , t h u s p l a n s f o r

s y s t e m a t i c i n t r o d u c t i o n o f d i f f e r e n t germ p l a sm w ou ld be d e s i r a b l e .

365

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a p p e a r iim tttae lliittsenattime ttrcom XYWK6 ttn im BZ con tttiee estibijeerEtt ooff eecconomicc

csrnujllatt!icon ccff A ll tbneeectiim® p ro g ra m s. We axonrlludte tthstt tttiee eeearlliisrr

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tbreecders acre "’Vmefstti imp con ipsasft 11 sam el t^'".. SBirree rprcmuramentt an ti

m*air ntt-enanaxe nos'cts Hmus rriicsen tiramact iccalll>y iim tttiee fpeesrt 46 /ye ars,; a n ti

tttorttrh Anwriiaaffn (All <sttutis arse -ffulllby anmgxuttBeriizaati now,, W itth nrnnlti m ore

•accurate tt.imanrii-a) 1 iimHonnreartiioon (than ex i s te t i iim ttttiee ddO"®.. (Annual

ib re e tie rs a n ti ffi im ano is] 1 m a n a g e rs xff s iiu t is stro iii led cdeieel lcgp aconom kr

cgptti tm izcarti icon rpaas-tcages ,Wtii iath m ay ttie rroutt; imelby Lfptiatteti a n ti luseti,. 'We a r e/ar.

m s

begj irrmi irrg to dfeuepllapj as. Uiimearr prragr«mnri ;rrg peck-age- fair (¥il caraperafti iwees. iim Caarsatte,, bautt bB.>1 i i iev.ee abhejr aBUtiBsrrraatti iuss, sbaujl 'cti bee- eecfptlarreectt.Hi snountti rrg anrdd fflkww off gpereetti ic i mpravemenb ffmamn generabi am ttao gjBsneeraebiicon musb bee iimxanpjarattesdi iimto tfrtasec programs.,

FFiimealUly',, we- musb rreecnnrsi icterr paesb neesseearab am economic:apjbinriizzaabiiam iim lliigjtib off SSirrctt and: MlittoJteelUK'* <(119003); cUisaussiiam off attaniaee off dJiscmimttimg raettes,. Eteuaaiaae many, off tbs sbudiiss fraeuee abasemcbiscoaumbiiiTg recto® substanb i lael 11 l>y itii igtiar ttraaii ttreesae? workers en dor see and: name- Havee ussdi biigj+ear di iscxaurb rectos ffarr bssreeffiito bsey, sett tbean ffam enste (-.say sett 3X0),, wee are ffeeuaurri irrg sch enies wi i tilt: thiigftear sttomb-torm, bKrreeffiito.. TUNis may, eocglleeiim adean bages" off dtia-li purpose sattemesaibiilliizziirrg growth per-farmamre- tosbiirrg iim addiittiiam to> progeny/ tosbi;ng: far mriillkc,, as bwreeffiito from; impr ovemeu ts iim growth rraato bendl tto aacrue- eeearlly..

SUMMARY

Tide impact! off rreuenb deveeliopmenbs iim siirree eardi cow eeweelluaebiiami,, presenb day costs, arret berreffiibs,, 1 awer di scaur bi img rabes arrdi gene- ffllawo sbcnjjlldJ be? eeveeJluectodi iim 1 iirear cur dynami c prograsmriimg an simuliabi am mode l s;.. These/ gllaarmiimg packages shou 1 Id: be? ubiilliizzeedi by/ studs: as,poutfime iim forward; ppllearmimg>. gl:obal1 sbrabegy for dairy- caebblleebreaediimg iis sugges t ed?; bak-.i ;ng i into con s i dera-bi on papujl’ab.iiam siizeess,, br eedi rrg goal's,, eyctlange- off semen from bul l1-si res between papurlleebiions,, inbreeding,, Heetorasiis,, aardi development' off symbtteettiices by- crossbreed! ng>.

Progeny-- tosbiirrg pxragnetm® shauUdi be rreeffi irresdi to apbinriizffie gensbi ;c progress aardi prroffiibs as eeearfti subpapu 1 -abian advances, towards: iibsdesi gne-tod: brreeeediimg goaf..

RESUMEN

El impacto de r e c i e n t e s d e s a r r o l l o s en e u a lu a c i on de

r e p r o d u c t o r e s y hembras , a c t u a l e s c o s t o s y b e n e f i c i o s , r a t a s de

d e s c u e n t o y f l u i r de ge n e s , deben e v a l u a r s e en p r o g r a m a s l i n e a l e s

d i n a m i c o s o model o s de s i m u l a c i o n . E s t o s p a q u e t e s de p 1 an i f i cac i on

deben u s a r s e por 1 os c r i a d o r e s en -forma r u t i n a r i a p a r a p 1 aneam i en t o .

Una e s t r a t e g i a g l o b a l p a r a mejo ' ramiento del ga nado l e c h e r o es

s u g e r i d a , tomando en c o n s i d e r a c i o n tamanos de p o b l a c i o n , m e ta s de

m e j o r a m i e n t o , i n t e r c a m b i o de semen de t o r o s e n t r e p o b l a c i o n e s ,

e n d o c r i a , h e t e r o s i s , y d e s a r r o l l o de l i n e a s s i n t e t i c a s por med io de

c r u z a m i e n t o s . L o s pr o g ra m as de t e s t de p r o g e n i e deben r e - f i n a r s e pa ra

o p t i m i s a r p r o g r e s o g e n e t i c o y b e n e f i c i o s a me d i da que cada p o b l a c i o n

ava nz a h a c i a su d e s i g n a d a meta de m e j o r a m i e n t o .

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