BREEDING OBJECTIVES AND SELECTION TRAITS FOR EXTENSIVE BEEF CATTLE PRODUCTION IN THE TROPICS

D.J.S. HETZEL and G.W. SEIFERT, AUSTRALIA

C.S.I.R.O. Division of Tropical Animal Science,P.0. Box 5545, Rockhampton Mail Centre, 4702,

Queensland, Australia.

SUMMARY

Breeding objectives are largely determined by market requirements. In the tropics, the overa ll breeding objective can be considered in terms o f growth, reproductive rate, carcass quality, adaptation and temperament. Growth rate is a simple e f f ic ie n t selection tra it . Reasons why potentia l increases in mature size are not as disadvantageous as in temperate areas are discussed. Reproductive performance has been less important in harsh than benign environments because of the later age of turnoff and higher drought risk of f e r t i l e cows. Current market trends towards younger slaughter stock h igh light the need to id en tify marker tra its in bulls for cow f e r t i l i t y . Adaptive tra its have been defined for heat tolerance, parasite and disease resistance. I t is not yet clear under what conditions d irect selection fo r adaptation is l ik e ly to be advantageous. I t may be appropriate where genotype - environment interactions need to be considered.

INTRODUCTION

In p ractica l breeding programs, the rea lized rates of genetic improvement f a l l considerably below theoretical rates. The reasons for this are many and varied but one o f the major factors stems from a lack o f agreement by sc ien tists and breeders on the re la tive importance o f breeding ob jectives . A second factor is related to the problems associated with the translation of these objectives into practical breeding programs. R e la tive ly few selection tra its that are highly correlated to the ob jective and suitable for p ractical breeding programs have been defined, and ob jective measurement has been poorly accepted by the beef ca ttle industry, especia lly in the tropics.

I t is not possible to precisely define breeding ob jectives for the tropics as a whole since they w ill vary with the d iffe ren t physical environments and for each production and marketing system. Further, the marketing system in most industries is a dynamic one in which prices and costs are regularly changing. As a resu lt, the re la tive importance o f components o f the breeding ob jective and therefore of selection tra its w i l l vary, often in an unpredictable way. Consequently, in this paper breeding objectives w ill only be discussed in general terms. Differences between objectives in trop ica l and temperate areas w ill be highlighted.

PRODUCTION SYSTEMS

There is a wide variety o f production systems in the trop ics. Throughout the tropics, production systems vary from fu lly intensive systems where land s ize is very small and animals are handled da ily , to those based on extensive rangelands in which ca ttle are run at extremely low stocking rates and are harvested on an annual or even less regular basis. Some o f these systems

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u t i l iz e the dual or multi-purpose nature of c a tt le , particu la rly those that predominate in lesser developed countries in the trop ics. In these situations, ca tt le are not only kept for their meat, hides, horns and bones, but are often a v ita l source o f draught power, milk, fu e l, f e r t i l i z e r and are even u tilized fo r sport and recreation. Breeding objectives in such multi-product situations are extremely d i f f ic u l t to define since often l i t t l e is known of the re la tive magnitude o f inputs and outputs, including socio-economic factors which are often of overrid ing sign ificance. Although feed -lo t fattening operations are found in the trop ics, they w ill remain a minor component because the grain required is also used by humans, a factor which w i l l always keep prices high. In this paper discussion w il l be restricted to extensive or ranching beef production systems in the tropics using northern Australia as an example.

Environmental Factors

There are a number o f factors, such as le v e l o f nutrition , heat and humidity, parasites and disease which account fo r the wide range of environments found in the tropics of Australia.

The res tric ted ra in fa ll d istribution combined with the high temperatures resu lt in a short growing season. Rapid growth o f pasture species in so ils which are o ften low in f e r t i l i t y are responsible fo r low quality but usually ample quantity o f forage. Because of the variation in feed supply from one season to the next, conservative stocking rates are employed. The use o f improved pasture species, particu larly legumes, has had mixed success. Tree legumes such as Leucaena and G lyric id ia o ffe r much promise fo r the future because of the ir deep root systems and general hardiness.

h n p h n p h n p h n pDry Dry-Wet Wet Wet-Dry

Season

Fig. 1. Level o f Stress due to Heat (H ), Nutrition (N) and Parasites (P) during d iffe ren t seasons in the humid and sub-humid trop ics.

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Heat stress via the combined e ffec ts of temperature and humidity primarily a ffec ts appetite and a c t iv ity o f ca tt le . I t is highest during lactation thereby leading to e ffec ts on reproductive rate (Turner, 1982).

Ectoparasites e .g . ticks, bu ffalo f l y have a d irec t e ffe c t on animal production via blood loss, ir r ita t io n and the immune response. Ticks are also carriers o f diseases such as babesiosis and anaplasmosis while other vector born diseases include ephemeral fever and akabane. Endoparasites, p rincipally gastro in testina l helminths are widespread in the trop ics, as they are in temperate areas. However, the opportunity and economics of control are much reduced because o f the re la t iv e ly low value o f each animal unit, large herd size and handling problems associated with extensive range conditions.

The physical environment in the tropics is highly conducive to the survival o f pathogens and vectors o f diseases. Accordingly, many diseases occur fo r which preventative or therapeutic measures do not ex ist. Treatment is often not feas ib le or economic for the reasons given above, especia lly where control o f stock from neighbouring areas is poor.

The e ffe c ts o f the various stresses on the animal and therefore on its production during the year are represented in Fig. 1 . I t can be seen that the environmental stresses are not constant but vary independently. During the dry season when temperature and parasitic stresses are lowest, nutritional stress is high. During the wet season when nutritional stress is low, animals unadapted to heat and/or parasites are unable to u t i l iz e the feed and su ffer permanent losses in production. Prior to the wet season, growing animals w ill often maintain or even lose weight due la rge ly to the low qu ality feed. Adapted animals w i l l recover much of this loss via compensatory gains in the fo llow ing seasons. Adaptation to the environmental stresses is therefore an important requirement.

Marketing Systems

Market requirements have the major impact on breeding ob jectives . Beef breeders and producers must respond d irec t ly to both current and predicted market movements. In northern Australia, the grading system is based almost en tire ly on export market requirements. Components o f a single carcass are sold to many d iffe ren t overseas destinations as well as on the domestic markets. The bulk of the export market is fo r U.S. manufactured beef. Because of the high slaughter cost per head, a premium is paid fo r large c a tt le . Large cu ll bulls and cows therefore achieve premium prices. A very small lucrative export market to Japan exists for carcasses weighing greater than 320 kg.

At present, no premium is paid fo r young females compared to older females. There is therefore no incentive to market heifers in preference to mature cows. Cull cows, being heavier, rea lize up to 30% higher values than h eifers . Thus i t is economical to breed a higher proportion o f h eifers and cu ll heavier cows for sale. I t follows that longevity is a t r a it o f l i t t l e economic importance.

BREEDING OBJECTIVES AND THEIR IMPLEMENTATION

The common production goal fo r beef producers is to maximise the weight of beef o f acceptable quality per unit of cost. Because markets are f in it e and prices fluctuate more than costs, Dickerson (1982a) argued that cost reduction may be more relevant than increases in productivity and associated p ro fits .

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Minimizing costs per kilogram of beef produced is usually compatible with optimising productivity per hectare (Morris, 1979). A necessary qu a lifica tion at the producer leve l fo r any objective is 'subject to an acceptable degree of r is k '. The le v e l o f acceptable risk w ill depend on the financia l structure of the enterprise, p red ic tab ility of the seasons and attitude to uncertainty.

Most o f the costs of production are associated d irec t ly or in d irec tly with the provision o f feed. A c t iv it ie s include the clearing of land, sowing of improved pastures, application o f fe r t i l i z e r , fencing and cap ita l costs o f land. Because of the seasonal and annual fluctuations in feed supply and the im practica lity o f constantly varying stocking rates, the e ff ic ien cy of u t iliza t io n o f feed in the tropics is low. Stocking rates are largely determined by the long term carrying capacity of the land with some minor adjustments in the short term. Therefore the system is less fin e ly controlled than in temperate areas and the need fo r improved e ffic ien cy o f feed u t iliza t io n is reduced.

The major components o f beef production are market liveweight, carcass y ie ld and qu ality , reproductive rate and survival. In addition, the ease of handling ca tt le in extensive systems is important (Elder e t a l . , 1980). Phenotypic and genetic variation in these components can be partitioned into two components (Frisch and Vercoe, 1984). F irs tly , there is variation in the physiological and biochemical processes d irec tly associated with the expression o f the component t r a it . The second component relates to adaptation to the environmental stresses (nutritional le ve l, heat, parasites and diseases). In simple terms, varia tion in adaptation largely a ffec t the supply o f nutrients to those processes d ire c tly involved in the expression o f the components and therefore determines what proportion of potential or capacity is expressed. In stressfu l trop ica l environments, temperate breeds rea lize a smaller proportion o f their potentia l production than Zebu breeds because they are less well adapted. Although the concept o f potentia l and adaptation is a useful background to a discussion of breeding objectives, i t is not of use in practice since objectives must re la te to measurable tra its . Adaptation can be defined and measured, but i t is impractical to measure potentia l. Nevertheless, there are long-term implications i f potentia l and adaptation are genetica lly antagonistic within a genotype, as Frisch and Vercoe (1984) suggest may be the case. Such a situation is considered la ter in the context o f genotype- environment in teractions. In the present discussion o f breeding objectives attention is focussed on performance per se as w ell as on adaptation and temperament. Survival is la rge ly a function of adaptational factors and so w il l be considered under that heading.

Breeding objectives are implemented in a breeding program through the choice of appropriate selection tra its . To be useful, selection tra its must be correlated to the ob jective , heritab le, cost e ff ic ie n t and id ea lly , favourably correlated genetica lly with other tra its . Simplicity is extremely important in the tropics where herd s ize is usually large and decisions must often be made on the same day as measurement because o f the high cost o f mustering. Labour is the greatest cost in implementing breeding programs. Consequently i t is highly desirable fo r measurements to f i t into normal commercial management practices. This often means that genetic gains w ill be lower than theoretica l expectations. However implementation over larger populations w i l l o ffs e t the sh ortfa ll.

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Growth

O bjective. In recent years, the usefulness of growth rate to market weight as a breeding ob jective has been questioned (Barlow, 1978; Dickerson, 1978, 1982a). Barlow (1984) even suggested that there was no sound basis for advocating selection for growth rate in maternal breeds and that a moratorium be held until a case is established. Concern is based on the results o f a number o f studies in a range o f species which indicate that whilst increased growth resu lts in higher gross e ffic ien cy (gain/feed intake) in growing stock, the higher maintenance costs o f heavier breeding females leads to no advantage or even lower e ffic ien cy at the herd leve l (Dickerson, 1982a; Barlow, 1984). Certainly growth rate has been advocated widely as a breeding ob jective because i t is easy to measure, results are read ily v is ib le and most producers have subscribed to the adage that bigger is better. To date there is no information on correlated changes in weights at la ter ages to selection for early growth in the trop ics. However, i t is d i f f ic u lt to see how a genetic increase in mature size is avoidable. Nevertheless there are a number o f reasons why increased growth rate is desirable in trop ica l environments, despite potentia l increases in mature s ize.

F irs t ly , as outlined ea r lie r , the supply o f feed in the tropics varies from super-abundance to a restric tion in qu a lita tive rather than quantitative terms. In most years, surplus feed is burnt and therefore feed has a lower re la t iv e cost than in temperate situations. Secondly, maintenance costs o f breeding stock are reduced in trop ica l environments by the negative environmental e ffec ts on mature size (S e ife r t and Rudder, 1976). The phenotypic e ffe c ts of genetic increases in mature s ize and correlated changes in tra its such as reproduction and survival are however unknown and need to be elucidated. Thirdly, at least in northern Australia, there is a substantial premium for large carcasses because many o f the processing and marketing costs are on a per head basis. This applies equally to fa t bullocks and cu ll cows. Thus, increases in growth rate, even i f accompanied by increases in mature s ize , w i l l be advantageous.

C learly, there is a need to model the types o f extensive production systems found in the tropics to try and quantitate the arguments presented above. Herd models, such as those developed at Texas A S M University have been useful in matching genotypes to production systems and to examine the e ffe c ts o f management changes on herd productivity (ILCA, 1978). Dickerson (1982b) outlined a method to account for d irec t and correlated genetic changes in such models, but at present estimates of the required phenotypic and genetic parameters are generally not availab le. Nevertheless, i t is concluded that for the forseeable future, increased growth rate is a va lid breeding ob jective for the trop ics.

Selection . Measures o f growth are simple to obtain up to breeding age. The h e r ita b il it ie s of both pre-weaning and post-weaning growth rate are moderate to high (Table 1). At 18-24 months of age, dam age corrections and thus mothering are not required (S e ife rt, 1975). S e ife r t e t a l. (unpublished) found the genetic correla tion between pre-and post-weaning growth to be low (+ 0.19), indicating that selection for growth to weaning is only moderately e ffe c t iv e at increasing post-weaning growth. Maternal performance to weaning is highly heritab le which is in agreement with the high repea tab ility o f weaning weight (S e ife r t e t a l . , 1982). I t has been suggested that high milk production is correlated with lower cow f e r t i l i t y (Barlow, 1978), On the other hand, dairy breeds exh ib it high leve ls of each t ra it . Preliminary results of studies in

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temperate areas show l i t t l e change in reproduction fo llow ing selection for growth (Baker and Morris, 1984). However, under some management systems e .g . where yearling mating is used, delayed age of puberty may be s ign ifican t.

Estimates o f the genetic correlation between growth to 24 months and b irth weight have ranged from 0.59 (S e ife r t, 1975) to 0.23 (S e ife r t e t a l . , unpublished), which are sim ilar to those reported from temperate environments (Koch et a l ., 1982). With the exception o f one set o f lin es in the U.S.A., increases in b irth weight accompanying e ffe c t iv e selection fo r growth rate in temperate areas have not been associated with increased dystocia (Baker and Morris, 1984). Furthermore, natural selection against dystocia is high in extensive management systems.

In temperate climates, selection for growth decreases fat content at a given weight, i.e. at a lower proportion of mature weight (Koch et al.,1982) . Tropical studies have not been carried out but adverse changes are unlikely and w i l l be overridden by environmental factors.

Reproduction

O bjective. Higher reproductive rate in males and/or females leads to higher turnoff. A ltern a tive ly the same turnoff can be maintained with fewer breeders. In addition, higher reproductive rate permits greater selection in tensity because o f the surplus of breeding stock. There is greater scope for improving female f e r t i l i t y than male reproductive performance since multiple s ire mating as used in commercial herds reduces the impact o f individual bu ll f e r t i l i t y .

In intensive production systems, reproductive rate is one of the key factors in p ro f ita b il ity . However, the re la tive importance is presently low in extensive trop ica l systems in Australia for a number o f reasons. F irs t ly the age o f turnoff is high, commonly ranging from three to f iv e years, depending on the environment. The la ter the age of turnoff, the less important reproduction is as an economic t r a it . Taylor et a l . (1980) concluded that i f reproductive rate of the herd exceeds the minimum leve l required to maintain the herd in equilibrium, f e r t i l i t y is o f less economic importance than growth. Secondly, in extended dry periods, and especia lly in droughts, the probability of lactating cows dying is higher than fo r non-lactating cows. Cows which calve regu larly remain in re la t iv e ly poor condition, and ra re ly have the opportunity to gain weight. During the dry season, highly f e r t i l e cows are 'h igh -r isk '. Non-lactating cows function as insurance for herd, and therefore economic surviva l. Thirdly, the non-pregnant heifer is a marketable commodity. There is only a small price d iffe re n t ia l for sex, and heifers fin ish at a younger age and weight.

Because the reproductive rate is usually low in the tropics (Entwistle,1983) , there is clearly scope for improvement. However, the harsher the environment, the more fertility needs to be regarded as having an optimum, less than the maximum value, under current market conditions. The future trend in markets will be towards younger, heavier animals. The importance of fertility as a breeding objective will correspondingly increase as will the need for environmental improvements such as nutritional supplements and improved tropical pastures. In addition, the exploitation of heterosis for fertility using systematic crossbreeding programs may become more feasible under extensive grazing conditions as stock control improves and more suitable breeds become available. Although such systems have produced substantial gains over

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stra ight breeding in temperate areas (Gregory and Cundiff, 1980), the benefits and p ra c t ic a lit ie s have yet to be demonstrated in extensive production systems in the tropics.

Selection. Calving rate in adapted ca tt le is moderately heritable (Table 1). The estimates are higher than those reported fo r Bos taurus genotypes in temperate areas (MacNeil e t a l . , 1984). Culling o f in fe r t i le and su b fertile cows is e f fe c t iv e in making substantial phenotypic gains in herd f e r t i l i t y (S e ife r t et a l . , 1980). To date, no re lia b le measure o f f e r t i l i t y prior to mating exists fo r heifers. However c r it ic a l weights to achieve given pregnancy rates can be defined (Rudder et a l ., 1985). Selection o f the heaviest heifers w i l l phenotypically improve f e r t i l i t y . No estimates o f the genetic correlation between cow f e r t i l i t y and growth are availab le.

The major cause o f low reproductive rate is lactational anoestrus, being greatest in f i r s t ca lf cows (Entwistle, 1983). Although hormonal measurements fo r ovarian a c t iv ity are available, from a p ractica l point o f view, i t is best measured by conception rate during a restric ted mating period. Culling cows on this basis w i l l improve herd reproductive rate (S e ife r t e t a l . , 1980).

In a recent review, Entwistle (1983) concluded that no accurate measure of male f e r t i l i t y currently ex ists. Semen evaluation can detect c l in ic a lly abnormal bulls but w ill not always id en tify lowly f e r t i le bu lls . Further, serving capacity tests developed with temperate breeds have so far shown poor correlations with paddock f e r t i l i t y (Christensen et a l ., 1982). Scrotal measurements are correlated to sperm production in Bos indicus genotypes (Wildeus et a l . , 1985) and to female f e r t i l i t y in Bos taurus breeds (Toelle and Robison, 1985), but the genetic relationship with either male or female f e r t i l i t y in Bos indicus genotypes is unknown. Recently, Post and Reich (1982) reported that plasma testosterone or lu te in iz in g hormone leve ls fo llow ing an in jection o f gonadotrophin releasing hormone were highly correlated with bull f e r t i l i t y . Preliminary results from follow-up studies showed a phenotypic correlation o f only 0.35 (Hetzel, e t a l ., unpublished), although the small size o f mating groups restricted the variation in f e r t i l i t y in this study. Further investigations to evaluate poten tia lly useful f e r t i l i t y markers in bulls are needed since the scope for selection pressure is considerably greater in males than in females.

Carcass Quality

Objective. Genetic d ifferences between trop ica l genotypes in carcass tra its such as dressing percentage, fa t d istribu tion , fat/muscle ra tio have been reported (see Seebeck, 1984) but tend to be small. Whilst within breed genetic variation in trop ica l breeds has not been investigated, studies in temperate breeds (Swiger e t a l., 1965) indicate that i t may be considerable. However, e f f ic ie n t use o f carcass tra its as breeding ob jectives is restric ted by the lack of simple accurate liv e animal measurement techniques. At present, i t seems that environmental methods e .g . a lterin g the age o f turnoff, processing procedures and the improvement of quality by increasing growth rate (Seebeck,1984) are the most e ffe c t iv e means o f a lterin g carcass composition to meet sp ec ific markets. A lternatively, where terminal crossbreeding is feas ib le , between breed variation in carcass characteristics can be u tilis ed .

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Adaptation

O bjective. In evolutionary terms, adaptation can be defined as the f i t o f an animal to i t s environment. However, where domesticated animals are farmed fo r meat productivity as well as survival is important. An a lternative d efin ition fo r adaptation could be related to the leve l o f expression o f genetic potentia l fo r productive processes. Adaptation is c lea rly not an a l l or none t ra it . Although Frisch and Vercoe (1984) found the Brahman to be more resistant to a l l the major environmental stresses in northern Australia than an an unadapted Bos taurus genotype, i t is more l ik e ly that breeds w il l have adapted to the sp ec ific environments in which they have evolved e .g . the adaptation o f West African breeds to trypanosomiasis. In general, trop ica l beef breeds have not been extensively characterized for their adaptation to d iffe ren t environmental components. This type o f information w il l allow better u t iliza t io n of spec ific characteri s t ie s .

C learly a certain leve l o f adaptation is required fo r survival and production in trop ica l environments. Rising labour and treatment costs make i t important to have easy-care animals. In many trop ica l areas, this has been achieved by crossing adapted genotypes, e .g . Brahman, Africander with Bos taurus breeds. However the question arises as to how to fin e tune the le v e l o f adaptation, or whether in fa c t this is necessary, i . e . whether selection fo r production tra its such as growth, w ill bring adaptation to the required le v e l.

Frisch (1981) reported that response to selection for growth in Hereford- Shorthorn ca tt le in a trop ica l environment had arisen la rge ly due to increased adaptability in terms of resistance to heat stress and parasites. However the starting le v e l o f adaptation was low in this genotype. What w il l occur in moderate or well-adapted genotypes? Such a study is currently underway at Rockhampton. I t may be that although selection for growth d irects s ign ifican t selection pressure on adaptive tra its , further improvement in some components is desirable. For example, since improved growth is associated with increased feed intake, more e f f ic ie n t systems for the d issipation o f d igestive and metabolic heat loads w il l be required, i . e . further selection pressure on heat tolerance by d irec t selection may be desirable. I f se lection for growth increases metabolic ra te, maintenance requirement w ill be higher. This w i l l be a disadvantage during periods of poor nutrition. Therefore i t may be advisable to se lec t animals capable o f lowering metabolic rate under these conditions (Frisch and Vercoe, 1984).

Selection for production w ill act on genes for adaptation as well as those genes d ire c t ly involved in the productive processes. Frisch and Vercoe (1984) have postulated that these two processes may be negatively correlated since breeds o f high potentia l and high adaptability have not been id en tified and selection in an unadapted genotype appeared to resu lt in an increase in adaptation but a decrease in potential (Frisch, 1981). However there is no evidence that high adaptation and high potential are physio logica lly incompatible so that i t only remains to define such a breeding program to achieve this ob jective . Additional selection pressure fo r certain components o f adaptation may be required in some environments.

Selection . In general, the h e r ita b ility and, where repeated measures are made, repea tab ility are moderate to high for adaptive tra its (Table 2 ). Permanent environmental e ffe c ts are therefore re la t iv e ly unimportant. The estimates cited are from Bos indicus cross populations. Most o f the tra its investigated thus

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fa r are not as simple or cheap to measure as growth rate. Other adaptive tra its correlated to production have been proposed but due to practica l d i f f ic u lt ie s in measurement most have not been evaluated. Immunological measures o f parasite resistance have been investigated. Products o f genes in the bovine histocompatability complex may be useful as markers fo r tick resistance (Stear e t a l ., 1984).

Where economically viable vaccines, chemical treatments or nutritional treatments ex is t, i t w il l not be appropriate to se lec t d ire c t ly fo r adaptation. Heat is one environmental stress for which non-genetic control is unlikely. However, the genetic option is becoming more a ttractive fo r parasite control as the demand for easy-care ca ttle increases.

The few genetic correlations between adaptive tra its and production which have been reported are favourable. The correlation between recta l temperature and cow f e r t i l i t y was found to be -0.76 (Turner, 1982). Correlations between recta l temperature and growth to 18 months o f age were zero in one group o f heifers and -0.86 in another (Turner, 1984). In these cases around half of the variation in the adaptive tra its and growth or f e r t i l i t y is common. However the remaining half is independent and may account fo r important additional variation which could be u tilized by d irect se lection . Better estimates of the genetic correlations are required before the re la t iv e benefits o f d iffe ren t selection programs using combinations of t ra its in selection indices can be evaluated.

Temperament

O bjective. Under extensive management where ca ttle are rarely handled, temperament is a major problem for producers (Elder et a l . , 1980). While Zebu ca ttle are extremely docile and tractable under intensive management, they are often d i f f ic u l t to manage under extensive conditions. Although d irec t associations between behaviour and production are l ik e ly to be small, in tractab le animals add to production costs. Therefore temperament under extensive management should be considered as a breeding ob jective . A potentia l problem in improvement by breeding arises where breeding herds are managed more in tensively than commercial herds. Behaviour in the two systems may be under d iffe ren t genetic control or behaviours in one system may not be exhibited in the other. I t is d i f f ic u lt to determine the re la t iv e importance o f temperament as a breeding objective because o f the lack o f quantitative information on its e f fe c t on production costs, but i t is one o f the major disadvantages of Zebu breeds c ited by producers.

Selection . Fordyce at a l . (1982) have quantified some o f the temperament tra its both in crush and yard situations. Of those investigated 'f l ig h t distance' appears to be the most heritable (Table 1 ). Its relationship to range behaviour and production tra its appears to be favourable (Fordyce, 1984). Estimates of repeatab ility of th is t r a it are around 0.75 (Fordyce,1984), indicating a large permanent environmental component and therefore potentia l benefits from management stra teg ies early in l i f e . Investigation into more e f f ic ie n t , precise and accurate measures o f temperament is needed.

GENOTYPE - ENVIRONMENT INTERACTIONS

Where diverse genotypes such as trop ica l and temperate breeds have beenevaluated under widely d iffe r in g environments, genotype-environment interactions have been large (S e ife r t, 1971b; Frisch and Vercoe, 1984).

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However, whilst within breed genotype-environment interactions have been reported in temperate areas e .g . (Burns et a l . , 1979), there have been no studies in trop ica l environments with adapted genotypes. So the question “ is genetic improvement or superiority in one environment transferred to other environments?" remains unanswered. Where the stud breeding sector, for which the environmental le v e l (e .g . nutrition, parasite control) is generally high, s e lls bulls to commercial producers, the question re la tes to the significance o f s ire x environment interactions.

The orig in of between breed interactions appears to be la rge ly due to the a b il ity o f the genotype to cope with environmental stresses such as heat and parasites. Frisch (1981) reported that response to selection la rge ly for growth in an unadapted genotype in a trop ical environment was due to an increase in adaptation at the expense of growth poten tia l, implying a negative genetic correla tion between the two components. I f such a situation applied in adapted genotypes, selection in a harsh environment would resu lt in genotypes which performed re la t iv e ly poorly in good environments where adaptation is r e la t iv e ly unimportant and where potential was the lim itin g factor. The converse would equally apply. Further information on th is question is required, and studies are in progress at Rockhampton. There are no good reasons to expect that adaptive tra its are negatively correlated genetica lly to productive t ra its . Even i f the correlations were close to zero, selection in a poor environment would not depress performance in a good environment. I t is most l ik e ly that selection for growth in stressfu l environments w il l be e ffe c t iv e in more benign environments, but that selection in benign environments without attention to adaptive tra its may not be suitable for more stressfu l trop ica l environments.

IMPACT OF NEW TECHNOLOGIES

The development o f new technologies is occurring at an ever increasing rate. I t is unlikely that these technologies w i l l grea tly change breeding objectives but they w i l l a lte r breeding methods. For example, developments in meat processing may reduce the need to consider meat quality in breeding programs. New and improved pasture va rie ties w i l l improve the nutritional le v e l, obviating the need to consider adaptation to low nutrition in many situations. The benefits o f cheaper vaccines through the use o f recombinant DNA technology may lessen the need for host resistance to parasites and disease. One potentia l danger is that such vaccines are used only in the more intensive stud breeding operations, further widening the environmental d ifference with commercial producers.

In the medium term i t is more l ik e ly that new technologies w ill improve the e ff ic ien cy o f breeding programs and the rate at which ob jectives are met. Techniques such as embryo and egg transfer and embryo s p lit t in g are now ava ilab le and w i l l have some application in the extensive livestock industry. Other developments such as computerized marketing and meat c la ss ifica tion schemes, e lec tron ic id en tifica tion o f stock, together with automation of weighing and drafting w ill greatly fa c i l i ta te genetic progress.

The greatest promise undoubtedly l ie s in recent developments in molecular b io logy. The a b il ity to now characterize individual animals at the DNA leve l is a major advance. Genetic markers id en tified by gene probes or as re s tr ic t ion fragment length polymorphisms (So ller and Beckman, 1982) w i l l soon be available fo r disease and parasite resistance as w ell as production tra its . Further i t is now possible to contemplate the genetic engineering of

livestock, although assessing the possible impact is limited by our ignorance of the genetics of most production traits. Even when transgenic livestock become a reality, the task of the animal breeder will remain to match genotype and environment to achieve maximum productivity. In this respect, it will be even more important to define precisely what the breeding objectives should be for specific production systems.

REFERENCES

BAKER, R.L. and MORRIS, C.A. 1984. A review o f correlated responses to weight selection in beef ca ttle under d iffe ren t management and clim atic conditions. In 'Proc. 2nd Wld. Congr. on Sheep and Beef Cattle Breed.' Pretoria, Eds. J.H. Hofmeyer and E.H.H. Meyer, pp 236-251.

BARLOW, R. 1978. B io logical ramifications of selection fo r preweaning growth in c a tt le . A review. Anim. Breed. Abstr. 46, 469-494.

BARLOW, R. 1984. Selection for growth and s ize in ruminants. Is i t time for a moratorium? In 'Proc. 2nd Wld. Congr. on Sheep and Beef Cattle Breed.' Pretoria, Eds. J.H. Hofmeyer and E.H.H. Meyer, pp 421-432.

BARLOW, R. and PIPER, L.R.A. 1985. Genetic analyses o f nematode egg counts in Hereford and crossbred Hereford ca ttle in the subtropics o f New South Wales. L ivest. Prod. S c i. 12, 79-84.

BURNS, W.C., ROGER, M. , BUTTS, W.T., PAHNISH, O.F. and BLACKWELL, R.L. 1979. Genotype by environment interaction in Hereford ca ttle : I I . Birth and weaning tra its . J. Anim. S c i. 49, 403-409.

CHRISTENSEN, H.R., SEIFERT, G.W. and POST, T.B. 1982. The relationship between a serving capacity test and f e r t i l i t y o f beef bu lls. Aust. vet. J. 58, 241- 244.

DA SILVA, R.G. 1973. Improving tropical beef ca tt le by simultaneous selection fo r weight and heat tolerance. H e r ita b ilit ie s and correlations o f the t ra its . J. Anim. S c i. 37, 637-642.

DICKERSON, G.E. 1978. Animal size and e ff ic ien cy : basic concepts. Anim.Prod. 27, 367-379.

DICKERSON, G.E. 1982a. Effects of genetic changes in components o f growth in b io log ica l and economic e ffic ien cy o f meat production. Proc. 2nd Wld. Congr. Genet. Appl. L ives t. Prod., Madrid, Ed. C.L. deCuenca, Vol. 5, 252-267.

DICKERSON, G.E. 1982b. Princip les in establish ing breeding ob jectives in livestock . In ' Proc. Wld. Congr. Sheep and Beef Cattle Breed.' Palmerston North, Eds. R.A. Barton and W.C. Smith, Vol. 1, 9-22.

ELDER, J.K ., WATERS, K.S., DUNWELL, G.H., EMERSON, F .R ., KEARNAN, J .F ., MORRIS, R.S. and KNOTT, S.G. 1980. A survey concerning ca tt le tick control in Queensland - 2. Managerial aspects which in d irec tly a ffe c t tick control. Aust. vet. J . 56, 205-211.

ENTWISTLE, K.W. 1983. Factors influencing reproduction in beef c a tt le in Australia. A.M.R.C. Review 43, 1-30.

254

FORDYCE, G. 1984. The temperament of Bos indicus cross c a tt le , M.Sc. Thesis, James Cook University o f North Queensland, Townsville.

FORDYCE, G ., GODDARD, M.E. and SEIFERT, G.W. 1982. The measurement of temperament in ca tt le and the e ffe c t o f experience and genotype. Proc. Aust. Soc. Anim. Prod. 14, 329-332.

FRISCH, J.E. 1981. Changes occurring in ca ttle as a consequence of selection fo r growth rate in a stressfu l environment. J. agric. Sci. Camb. 96, 23-28.

FRISCH, J.E. and VERCOE, J.E. 1984. An analysis of growth of d iffe ren t ca ttle genotypes reared in d iffe ren t environments. J. agric. S c i. Camb. 103, 137-153.

GREGORY, K.E. and CUNDIFF, L.V. 1980. Crossbreeding in beef ca tt le : evaluation o f systems. J, Anim. Sci. 51, 1224-1242.

ILCA 1978. Mathematical modelling o f livestock production systems : application o f the Texas A. & M. University beef ca tt le production model to Botswana. International Livestock Centre for A frica, Addis Ababa, Ethiopia.

KOCH, R.M., GREGORY, K.E. and CUNDIFF, L.V. 1982. C r it ic a l analysis of selection methods and experiments in beef ca ttle and consequences upon selection programs applied. Proc. 2nd Wld. Congr. Genet. Appl. L ivest. Prod., Madrid, Ed. C.L. deCuenca, Vol. 6, 514-526.

MacNEIL, M.D., CUNDIFF, L.V ., DINKEL, C.A. and KOCH, R.M. 1984. Genetic correlations among sex lim ited tra its in beef ca tt le . J. Anim. Sci. 58, 1171— 1180.

MORRIS, C.A. 1979. Goals fo r improving net income from beef ca ttle by selection . A.M.R.C. Review. 38, 1-23.

POST, T.B. and REICH, M.M. 1982. Relationship between f e r t i l i t y and the testosterone response to GnRH or HCG in bulls. Proc. 2nd Asian-Australian Animal Science Congress, Manilla, p 21 (abstract).

RUDDER, T.H., SEIFERT, G.W. and BURROW, H.M. 1985. Environmental and genotype e ffe c ts on f e r t i l i t y in a commercial beef herd in central Queensland. Aust. J. exp. Agric. 25, 489-496.

SEEBECK, R.M. 1973. Sources o f variation in the f e r t i l i t y o f a herd o f Zebu, B ritish and Zebu x B ritish ca tt le in northern Australia. J. agric. S c i., Camb. 81, 253-262.

SEEBECK, R.M. 1984. Body composition of British and Zebu x B ritish ca ttle in northern Australia. 1. Breed and growth rate e ffec ts on y ie ld of carcass,dressed carcass composition and o ffa l composition. Anim. Prod. 39, 177-193.

SEIFERT, G.W. 1971a. Variations between and within breeds o f ca ttle in resistance to f ie ld in festations of the ca tt le tick (Boophilus microplus). Aust. J. agric. Res. 22, 159-168.

SEIFERT, G.W. 1971b. Ecto- and endoparasitic e ffec ts on the growth rates ofZebu crossbred and British ca ttle in the f ie ld . Aust. J. agric. Res. 22, 839- 850. ----------------------------- ---

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SEIFERT, G.W. 1975. Effectiveness of se lection fo r growth rate in Zebu x B ritish crossbred ca ttle . I I . Post-weaning growth and genetic estimates. Aust. J. agric. Res. 26, 1093-1108.

SEIFERT, G.W. and RUDDER, T.H. 1976. The genetic implications o f selecting fo r large s ize . In 'Princip les of Cattle Production. Eds. H. Swan and W.H. Broster, Butterworth and Co. Ltd., London, pp 373-386.

SEIFERT, G.W., BEAN, K.G. and CHRISTENSEN, H.R. 1980. Calving performance o f rec ip roca lly mated Africander and Brahman crossbred ca ttle at Belmont. Proc. Aust. Soc. Anim. Prod. 13, 62-64.

SEIFERT, G.W., RUDDER, T.H., and BEAN, K.G. 1982. Repeatability o f weaning weights o f trop ica l beef ca tt le breeds. Proc. Aust. Assoc. Anim. Breed, and Genet. _3, 90-92.

SOLLER, M. and BECKMANN, J.S. 1983. Genetic polymorphism in va rie ta l id en tifica tion and genetic improvement. Theor, Appl. Genet. 67, 25-33.

STEAR, M.J., NEWMAN, M.J., NICHOLAS, F.W., BROWN, S.C. and HOLROYD, R.G. 1984. Tick resistance and the major histocompatability system. Aust. J. Exp. B io l. Med. Sc. 62, 47-52.

SWIGER, L .A ., GREGORY, K .E ., SUMPTION, L .J ., BREIDENSTEIN, B.C. and ARTHAUD, V.H. 1965. Selection indexes for e ff ic ien cy o f beef production. J. Anim, S c i. 24, 418-424.

TAYLOR, W.J., RUDDER, T.H. and NEAL, J.A. 1980. Influence o f rates of reproduction, survival and growth on beef herd productivity in a sub-tropical environment. Proc. Aust. Soc. Anim. Prod. 13, 365-368.

TOELLE, V.D. and ROBISON, O.W. 1985. Estimates o f genetic correlations between testicu lar measurements and female reproductive tra its in ca tt le . J, Anim. S c i. 60, 89-100.

TURNER, H.G. 1982. Genetic variation o f rec ta l temperature in cows and its relationship to f e r t i l i t y . Anim, Prod. 35, 401-412.

TURNER, H.G. 1984. Variation in recta l temperature o f c a tt le in a trop ica l environment and it s relation to growth rate. Anim. Prod. 38, 417-427.

WILDEUS, S ., HOLROYD, R.G. and ENTWISTLE, K.W. 1985. Patterns o f puberal development in Sahiwal and Brahman cross bu lls in trop ica l Australia. I . Growth and semen characteristics. Theriogenology. 22, 361-373.

WHARTON, R.H., UTECH, K.B.W. and TURNER, H.G. 1970. Resistance to the ca ttle tick (Boophilus microplus) in a herd of Australian Illawarra Shorthorn ca ttle : i t s assessment and h er ita b ility . Aust, J. agric . Res. 21, 163-181.

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TABLE 1. SUMMARY OF SELECTION TRAITS FOR GROWTH, REPRODUCTIVE RATE AND TEKPERAMBTT IN ADAPTED GENOTYPES IN THE TROPICS

Breeding

Objective

Selection Ease of

Trait MeasurementCost Heritability Reference

Growth Pre-weaning High Low 0.66 Seifert (1975)0.21 Seifert et al. (unpub 1.

Weaning weight High Low 0.64 Seifert (1975)0.23 Seifert et al. (unpub1

Maternal Pre-weaning growth rate

High Low 0.57 Seifert et al. (unpub1

Two year old weight High Low 0.52 Seifert (1975)0.34 Seifert et al. (unpub1.

Reproductive Calving rate High Medium 0.39 Deese & Koger (1967)Rate 0.25 Seebeck (1973)

0.44 Turner (1982)

Temperament Flight distance Low Low 0.13 Fordyce (1985)

258

Breeding

Objective

TABLE 2. SUMMARY OF SELECT I OH TRAITS FOR ADAPTATION IN ADAPTED GHIOTYPES IN THE TROPICS

S e l e c t i o n

T r a i t

E a s e o f

'M e asu rem e n tC o s t R e p e a t a b i l i t y H e r i t a b i l i t y R e f e r e n c e

HeatTolerance

Rectaltemperature

High Medium 0.27 - 0.33

0.25 - 0.31

0.330.25

Turner (1984) Turner (1982) Hetzel (unpubl.)

Rise inrectaltemperature

High Medium - 0.44 da Silva (1973)

TickResistance

Number of engorgedticks (Boophilus microplus)

Medium Medium 0.42 - 0.63 0.07 - 0.58

0.47

0.82 Seifert (1971a)a Seifert U971b)a

0.21 - 0.39 Seifert (unpubl.)15 0.40 Wharton et al. (1970)

Gastro­intestinalhelminthresistance

Faecal egg count

Medium Medium 0.27 - 0.68 0.12 - 0.24 0.31

Seifert (1971b)0.16 - 0.23 Seifert (unpubl.) 0.04 Barlow & Piper

(1985)

Species egg count - Haemonchus

Low High 0.600.220.30

0.330.29

Seifert (1971b) Seifert (unpubl.) Barlow & Piper

(1985)

Resistance to buffalo fly

Visual fly counts (Haematobia irritans exigua)

Medium Low 0.25 - 0.71 “ Bean et al.(pers. commO

Resistance to cancer eye

Eyelid pigmentation Medium Low 0.64 0.66 French (1959)

a Field tick counts onlyb Counts from artificial tick infestations