TECHNICAL BULLETIN 131

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SELECTION INDEX AND EXPECTED GENETIC PROGRESS IN CHIOS SHEEP

A. P. Mavrogenis and A. Constantinou

AGRICULTURAL RESEARCH INSTITUTE MINISTRY OF AGRICULTURE AND NATURAL RESOURCES

NICOSIA CYPRUS

APRIL 1991

ISSN 0070-2315

All responsibility for the information in this publication remains with the author(s). The use of trade names does not imply endorsement of or discrimination against any product by the Agricu ltural Research Ins titute.

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SELECTION IND EX AND EXPECTED GENETIC PROGRESS IN CHIOS SHEEP

A.P. Mavrogenis and A. Constantinou

SUMMARY

Data on 1506 female lambs of the Chios breed, collected from 1979 to 1989, were used to estimate genetic and phenotypic parameters required for the development of selection index­es combining milk production and growth. The lambs were the progeny of 93 sires an d had complete info rmation on birth , weaning (42±3 days post-partum) and l05 -day weight , as well as first lactation milk production following weaning. Indexes combining eith er weaning weight or l05 -day weight with part (90-day) or total milk yield were develo ped and genetic progress in one or both traits was estimated. A simplified selection index combining individu­al lamb performance (live weight at 105 days of age) and it 's dam's post-weaning milk pr o­duction (90-day) was the most efficient (RH.I=0.48). The index utilizing lamb weaning weight and the part lact ati on milk yield of it 's dam, altho ugh useful to suckling lamb operations, was the least efficient (RH.I=0.33). When both traits can be measured on the same individual (fe­males selected at approxi mately 18 months of age), the index combining l05 -day weight and 90-day milk yield was as efficient as the one consideri ng total milk yield (RH.I=O.64 vs RH.I=0.66 ). It may be concluded that a selection index combining lamb weight at 105 days of age and 90-day milk yield can be used effectively as a selection criterion for the simultane­ous improvement of both traits, particularly for dual purp ose sheep breeds.

IIEPIAH'PH

Lt OLXdu nUQuywYrl£ 1506 8r]Auxwv aQvLwv Xtou rtou OuyXEvtQw8r]xav OE KU ~EQvrinx o u £ Ltu8flOU£ ana to 1979 flEXQL to 1989 XQl]OLflon oLi]8l]xUV yta t OY unoAoy wflO qJatVOtuJttXWV xat vevorumxo v nUQuflEt QWV ytU rnv a vcmt uSl] EVOe; OAtXOU bd xt l] EJttAoyi]£ yta rnv YUAaxt On a Qa ywyi] Xat to 1;;wvtuvO ~aQo£. Tc otOLxd u UVUqJEQOvtUv OtO ~aQo e; xa ta tl] YEWl]Ol], t OY anoya AUxt WflO xat tl]V l]AtX(U rcov 105 l]flEQWV, xat r nv nUQuywyi] yaAUxtOe; tl]£ nQwt l]e; YUAUxttxi]e; n EQtOOoU. Oi OAtXOL OdXt EC; rntAoyi]£ rtou EsEta Otl]xuv orpopouocv t o MQoe; tou 1;;wou (orov anoyuAaxn oflo i] or«; 105 l]f.lEQE£) Xat t l]£ ya AuxtOn uQuywyi]e; tou i] tl]£ YUAuxtOrtUQuywyi]e; tl]£ fll]t EQa £ rou (flEQtxi]£ i] OAtxi]£). ESEt a Ot l]xE rn(ol]£ l] yEVEttxi] nQoooo£ n ou f.lJt0QE( va emtEux8d orov xa8l' xaQuxt i]Qa s l'xwQWta i] xat OtOU£ ouo xa Qaxti] Ql'£ flu1;;( ztou anOt l'AOuV tOY OAtXO odxt l]. 0 rn o cnoocnxo; odxtl] e; nrcv exetvo; nov ovv6Uu1;;l' to 1;;wvtuvO ~aQo e; one; 105 l]flEQl'£ xat rnv yaAUxtOnuQaywyi] rcov nQwtwv 90 l]fll'QWV fll't a tOY anoyuAUxt WflO (RH.I=0.48 ). 0 odxtl]£ rrou ovv6Ua1;;l' to ~a Qo£ r ou 1;;wou orov anoyaAaxnoflo Xat t nv flEQtxi] nUQuywyi] yaAuxtoe; t l]e; fll]tEQUe; rou, aYXat XQi]oLfloe; yLU OUa ti] flUtU nUQaywYrlC; aQvLau yaAUxto£, rir ov 0 ALYOt l'QO cnooot txoc. 'Otnv Xat OL ouo XUQuxti]Ql'£ unooouv v« flEtQl]80uv OtO (OLa ato/10 (or e 8l]AUXa 1;;wa ), 0 odxtl]£ nou ovvoua1;;EL to 1;;wvtuvO ~aQo £

or«; 105 l]/1EQl'e; xrn rnv /1l'QLXi] yaAux tOnaQuywyi] tOU (or «; 90 l]/1EQl'£) nrcv to (OLa cnooouxo; /1l' exetvo onou XQl]OL/10noLi]8r]xl' n oALXi] yUAaXtonuQuywyi] tOU (RH.I=0.64 vs RH.I=0.66). E(vat qJUVl'QO Ott 0 odxt l1£ rnLAoyi]£ nou ovvoua t EL to t wvta vo ~aQo £ rou twou oru; 105 l]flEQl'£ tl]£ l1ALx(a£ tOU Xat rnv n UQuywyi] ya Aaxt oe; OtOUe; n QwtOu£ 3 /1i]vl'e; /1Eta tOY anoyaAuXn0/10 f.lJtoQd va XQl1<Jt/1onoLl]8d ue EJtLt Ux(a oo» XQLti]QLa rntAoyi]e; yLa r nv tUvt0XQOVl] ~l'A rtcoon Xat nov ouo XUQaxti]Qwv. Ml'yaA.'Ut l'Ql] Ol]flflua(U nQooAu fl~avEL

reroi« fll'8000Aoy(U orov n QoxELtat yLU qJuAtC; oLn A.i]e; EXfll'taAAEUOl]£ onwc; tLvat t o nQo~uto tl]£ qJUAi]£ Xtou.

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INTRODUCTION

In most Mediterranean countries, emphasis is given to both milk and meat production from sheep. One of the most frequently used criteria of selection for meat is lamb live weight at weaning. Weaning weight, however, is influenced by the dam performance and does not probably reflect the lamb's true genet ic potential for growth. Dam ef­fects, of either genetic or maternal nature, are mostly manifested as the ability of the dam to provide adequate milk for the growth of her young (Vesely and Robison, 1971; Robison, 1972). On the other hand, lamb growth following weaning express­es the lamb's true genetic potential, being mostly free of maternal effects (Mavrogenis and Louca, 1979).

Selection based on an index or total score (Smith, 1936; Hazel, 1943) has been shown to be the most efficient method for the simultaneous im­provement of two or more economically important traits (Hazel and Lush, 1942). Selection indexes are used for traits that may be independent, antagonis­tic, influenced by only additive genetic effects or both additive and maternal effects (Robison, Chap­man and Self, 1960; Henderson, 1963; Van Vleck, 1970; Mart in and Smith, 1980). The development of selection indexes involving traits that are sex limit ­ed requires the estimation of genetic and phenotyp­ic variances and covariances between the traits .

The purpose of the present study was to utilize estimated parameters (Mavrogenis, 1988) in con­structing selection indexes for the simultaneous im­provement of lamb growth and milk production. Calculated indexes based on a measure of growth and either part or total milk yield for males and/or females, are discussed with respect to expected ge­netic progress and efficiency of selection.

MATERIALS AND METHODS

Data

The genetic and phenotypic parameters used in the present study were obtained previously (Mav­rogenis et aI., 1988) from 1506 female Chios lambs born between 1979 and 1989. The lambs were the progeny of 93 sires and had complete information on weight at birth, at weaning (42±3 days post­partum) and at lOS days, as well as first lactation milk production following weaning. Milk production

until weaning was not considered in the estimation of part (90-day) and total milk production, which were based on monthly individual test day records.

Se lection ind exes

Selection indexes considered in the present study utilized phenotypic and genetic relationships among weaning or l OS -day weight and part or total milk yield. Hence , the general from of the index was:

I = LbiPi or in matrix notation I = h: P

the equat ions for Q that maximize the correlation ,~ between the index and the aggregate genotype iwere:

P Q = R C ~ or Q= P_1R C ~

where:

b is a vector of estimated values

P is a phenotypic variance-covariance matrix

R is a matrix of relationships

C is a genetic variance-covariance matrix and

a is a vector of economic values (weights).

Therefore, genetic gain is maximized when the elements of matrices P and C follow a multinomial normal distribution and the predicted response from index selection can be written as:

~GH .I = h: R C ~ = i BH.I or= i or

where or = (Q' P Q)05

Direct and correlated responses resulting from in­dex selection are, therefore, maximized when the regression or correlation between the aggregate gen­otype and the selection index are maximize d. The aggregate genetic value, therefore, for any individual is the sum of the breeding values for each trait weighed by their appropriate economic values.

HI = L aj A:

where aj and A] are the econom ic weights and genetic values for the jth trait , respectively. Hence, the genetic gain for the

jth trait based on index selection would be:

~Gj.I = i BAj:I <Ji = i Cov (Au ) / Oi

where Cov (Aj.l)=L bi a Nj

and or = 2: bi (J2pi = (Q'

The assumptions made for the index were that there was

= Q' A ]

P Q)O.5

the construction of no inbreeding, there

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Table 1. Estimates of heritability, genetic and phenotypic variances economic weights used in computing se­lection indexes

Parameter Economic

/\ /\

Character 0 2 A 0 2

p weights

Live weight (kg) weaning 0.46 2.9486 6.4084 1.00 l OS-days 0.68 9.3804 13.7466 1.00

Milk yield (kg) 90-day 0.31 1436.8981 4635. 1487 0.15 total 0.34 1700.OS 92 5024.3544 0.15

were no genotype by environment interactions, that there were no import ant maternal effects and that mating was not assort ative. In addition , economic values were defined as the increment in profit oc­curring from the increase in any particular trait by one unit. The relative economic weights used in constructing all indexes were based on the pheno­typic standard deviat ions of the individual traits and the current market prices per kg milk or Iamb live weight.

Indexes developed were based on individual lamb performance for live weight and dam performance for milk prod uction, or on the performance of the individual itself. Hence, the latter can be considered for female selection alone.

RESULTS AND DISCUSSION

Est imated heritabilities, genetic and phenotypic variances and economic weights used in construct­

ing the indexes are given in Table 1. Genetic and phenotyp ic covariances and correlations amon g live weights and milk production are presented in Table 2. All parameters used in the construction of selec­tion indexes have been adjusted for known environ­mental affects and were estimated using the paternal half-sib correlations method (Mavrogenis, et ei., 1988).

Based on the correlation between the aggregate genotype and the index (RH.I), which is a measure of the relative efficiency of different indexes, it can be seen in Tables 3 and 4 that the most efficient in­dexes that maximize genetic gain in both traits were those utilizing the lambs lOS-day weight and 90-day milk yield. For the latter trait it was irr ele-: vant whether the dam's (Table 3) or the individual's (female) own production (Table 4) was considered . Indexes combining weaning weight and part or total milk production were less efficient . Although wean­ing weight and milk production were essentially ge-

Table 2. Estimates of genetic and phenotypic covariances and correlations among traits used in computing selection indexes

Relationship/ 90-day milk yield Total milk yield

Character Genetic Phenotypic Genetic Phenotypic

Covariances: weaning weight -0.56 -7.49 -0.34 -11.58 l OS-day weight 2.80 -2.71 3.37 - 8.22

Correlation s: weaning weight -0.07 -0.06 -0.02 - 0.06 l OS -day weight 0.19 -0.02 0.11 - 0.03

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Table 3. Selection indexes, response and relative efficiency of selection based on individual lamb performance for weight and dam performance for milk

Response to selection Efficiency

Index ~GH .I ~G l. H ~Gz . H RH.I

111 =0.474 (WI-Il)+0.024 (MI-Il) 1.979 i 0.669 i 17.218 i 0.33 liz =0.497 (WI-Il)+0.026 (Mz-Il) 2.183 i 0.668 i 20.387 i 0.34 I2J =0.718 (WZ-Il)+0.024 (MI-Il) 3.106 i 2.189 i 11.735 i 0.48 I22 =0.734 (Wz-Il)+O.027 (Mz-Il) 3.272 i 2.133 i 14.674 i 0.47

WI and Wz are weaning and lOS-day weight and MI and Mz are 90-day and total milk yield, respectively

netically independent the sign of the estimated cor­relations was negative. On the other hand estimated genetic correlations between lO5-day weight and milk production were positive, but also low and in­significant.

Part lactation milk pro duction was highly corre­lated genetically (0.98±0.17) and phenotypically (0.81) with total milk production (Mavrogenis, 1988). Furthermore, 90-day milk yield may be con­sidered as a measure of yield standardized for lacta­tion length and can be obtained much earlier com­pared to total yield. Hence, despite the somewhat lower efficiency. (RH.I) of the indexes involving part lactation it may be more advant ageous to use in­dexes involving part lactation yield.

Expected correlated responses from index selec­tion were higher for lOS-day weight or total milk production. In all cases, selection intensity may be disregarded. Generation interval, however, would be

somewhat shorter when the dam's production is considered instead of the performance of the indi­vidual itself (female selection only), despite the pre ­cocity of the breed in question. First lambing in Chios can occur at the early age of 13 months, and conception rates reach 85 to 90% at mating as lambs.

Expected genetic progress from direct selection for milk production alone was estimated between 21.11i (90-day) and 24.14i (total) per generation. Expected response to individual selection for wean­ing weight would be 1.16i and for lOS-day weight 2.52i per generation.

Selection based on the index combining lO5-day weight and 90-day milk .yield would result in an ex­pected genetic progress of 3.106i (dam performance for milk) or 4.177i (lamb performance itself). Corre­lated responses for lOS-day weight and 90-day milk yield (when z!b=la) from index selection would be

Table 4. Selection indexes, response and relative efficiency of selection based on individual performance for both traits

Response to selection Efficiency

Index ~GH .I ~G l.H ~Gz.H RH.I

111 =0.501 (Wl-Il)+0.047 (MI-Il) 3.395 i 0.427 i 19.841 i 0.17 112 =0.543 (WI-Il)+0.OS2 (Mz-Il) 3.824 i 0.414 i 22.854 i 0.60 I2J =0.727 (Wz-Il)+0.048 (Ml-Il) 4.177 i 1.654 i 16.830 i 0.64 12z =0.750 (Wz-Il)+0.OS2 (Mz-Il) 4.574 i 1.576 i 20.027 i 0.66

WI and Wz are weaning and lOS-day weight and MI and Mz are 90-day and total milk yield, respectively

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expected at 2.19 kg (lOS-day weight) and 11.74 kg (90-day milk yield) in the first case, and 1.65 kg and 16.833 kg (for lOS-day weight and 90-day milk yield, respectively) in the second case.

Considering that Chios sheep is a dual purpose breed, index selection presents a rational choice for the simultaneous improvement of both traits, al­though expe cted genetic progress in milk production would only represent about half the expected gain from direct selection for milk production alone .

ACKNOWLEDGEMENTS

The authors acknowledge the valuable assistance of Mr Ph. Lysandrides and Mr C. Heracleous for data collection and statistical analyses.

REFERENCES

Hazel, L.N. 1943. The genetic basis for constructing selection indexes. Genetics 28: 476-490.

Hazel, L.N., and r.i, Lush. 1942. The efficiency of three methods of selection. Joum al of Her edi ty 33:393-399.

Henderson, C.R. 1963. Selection index and expected genetic advance. In Statistical Genetics and Plant Breeding, NAS-NRC 982:141-163.

Lawlor, MJ., A Louca, and A Mavrogenis. 1974. The effect of three suckling regimes on the lacta­tion performance of Cyprus fat-tailed, Chios and Awassi sheep and the growth rate of the lambs. Animal Production 18:293-299.

Martin, T.G., and C. Smith. 1980. Studies on a se­lection index for improvement of litter weight in sheep. Animal Pro duction 31:81-85.

Mavrogenis, AP., 1982. Environmental and genetic factors influencing milk pro duction and lamb out­

put of Chios sheep. Livestock Pr oduction Science 6:519-527.

Mavrogenis, AP. 1988. Genetic and phenotyp ic re­lationships among early measures of growth and milk production in sheep and goats. Technical Bulletin No. 103. Agricultural Research Institu te, Nicosia. 8p.

Mavrogenis, AP., and A Louca. 1979. A note on some factors influencing post-weaning perfor­mance of purebred and crossbred lambs. Animal Producti on 29:415-418.

Mavrogenis, A.P., A Louca, and a.w. Robison. 1980. Estimates of genetic parameters for pre­weaning and post-weaning growth traits of Chios lambs. Animal Production 30:271-276.

Mavrogenis, A P., C. Papachristoforou, P. Lysan­drides, and A Roushias. 1988. Environmental and genetic factors affecting udder characters and milk production in Chios sheep. Genetique, Se­lection, Evolution 20:477-488.

Robison, a.w. 1972. The role of matern al effects in animal breeding. V. Maternal effects in swine. Joumal of Animal Science 35:1303-1315.

Robison, a.w., A B. Chapman, and H.L. Self. 1960. Swine selection indexes including live animal measur ements as indicato rs of carcass merit. Joum al of Animal Science 19:1024-1030.

Smith, H.F. 1936. A discriminant function fer plant selection. Annalles Eugenics 7:240-250.

Van Vleck, L.D. 1970. Index selection for direct and maternal components of economic traits. Bi­ometrics 26:447-453.

Vesely, l A, and a.w. Robison. 1971. Genetic and materna l effects on preweaning growth and type score in beef calves. Genetics 52:563-576.

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