The usefulness to the animal producer of research findings on the genetics of lactation.

Abstract

THE USEFULNESS TO THE ANIMAL PRODUCER OF RESEARCH FINDINGS ON THE GENETICS OF LACTATION GOLDA L. MUNRO* I. INTRODUCTION Lactation, a characteristic of mammals, involves the secretion The length of lactation, the of milk, a product unique in nature. yield, and the composition of the milk secreted vary greatly between species. Since most work on the genetics of lactation has been concentrated on the bovine species (in particular the dairy cow), discussion in this paper will be limited to that species. To the animal producer, genetic improvement is a matter of selecting animals from which to breed replacement stock,. His concern is to improve the genetic makeup of the herd so as to provide maximum economic return. There is little point in striving for an estimated theoretical maximum gain in production if the product itself is of little economic value; Currently in Australia, the producer is paid for the milk and fat he supplies. Recommendations have been made that the protein produced be paid for on a M basis with the fat, On the world market, the fat, the protein and the meat produced by the animal have each meant at certain times greatest financial returns to the farmer. The genetics of bovine lactation can be considered from three levels - the breed level, the quantitative genetics level within breeds, and the individual gene level. Selection may occur at each level with varying degrees of success depending on the reliability of the information on which the decision to select was based. The important factors in genetic improvement per generation are the variability in the population, heritability, and intensity of selection, The importance of performance (herd) recording schemes in the estimation of the parameters, and in providing information on which to base selection, cannot be overemphasized. While these schemes in the past have concentrated on the measurement of milk and fat production only, necessitating special programmes for the measurement of other components such as protein and lactose, they still provide the geneticist with the best available information on the dairy cattle population. To the animal breeder, they provide the only reliable means of selecting superior genetic material. II. ( a> BASES FOR SELECTION Selection on the basis of breed The different breeds of dairy cattle have been selected for the main attribute for which each breed excels. The Friesian breed, for example,' produces the greatest yields of milk, while the other common Australian breed, the Jersey, has greatest percentages of components. *Northfield Research Centre, Department of Agriculture and Fisheries, Box 1671, G.P.O. Adelaide. South Australia. 5001, Greatest variability in yields is also found in the Friesian (or Holstein) breeds, with more variability in percentages in the Jersey and Guernsey breeds (Wilcox, Gaunt, and Farthing 1971). TABLE I Changes in average composition occurring throughout the lactation aDpear to be different in Holsteins from those occurring in Differences in fatty Jerseys and Guernseys (Erb and Ashworth 1963). acid composition of milk from the Holstein breed compared with the Channel Island breedswere reported by Stull and Brown (1964) and higher p-lactoglobulin percentages were reported in Jerseys compared with Friesians by McLean, Bailey and Munro (1974). Crossbreeding as a means of genetic improvement relies on the selection of breeds with emphasis on different traits which Frhen combined will provide hybrid superiority. A mixture of breeds offers . the greatest variability for selection, since average within breed ' differences are not substantially greater than between breed differences (Touchberry 1974). The scope to significantly alter the ratio of the components of milk (e.g. the fat to protein ratio) may well come from crossbreeding (Barker 1968). 'New dairy cattle breeds may be developed from crossbreeding existing breeds. In the United Kingdom, a new red and white breed based on Dairy Shorthorn is being developed to compete with the Friesian (Bowman and .Hocking 1974). The Australian Milking Zebu developed in . Northern New South Wales from crosses of Jerseys (Bos taurus) with Sahiwals or Red Sindhis (Bos indicus) has been selxeforyield and resistance to heat(Hayman$rBy this cross, it is hoped to develop a high yielding breed suitable for tropical areas, (b) Selection on the basis of Quantitative inheritance within a breed Quantitative genetics has so far played the greatest role for the animal producer in genetic improvement. Within a breed, the dairy 1farmer selects the sire and dam from which to breed replacement stocks for his herd, Intensity of selection on the female side is hampered by insufficient replacement stock in a herd. With artificial breeding, 66 . - a bull may sire 100,000 progeny, thus selection of the male has greatest Unfortunately, lactation is only influence on genetic improvement. The main role of the quantitative geneticist expressed in the female. in dairy cattle breeding has been to define and refine the best methods for estimating a bull's breeding value. Predictions of breeding value , genetic changes and responses to selection require knowledge of heritabilities, genetic .correlations Yields of and population variation of the traits under selection. milk and components, and percentages of components have significant Reliable Australian heritabilities and are genetically correlated. , estimates of these parameters do not exist, and a recent study in the United States of America on 25,000 records provides the best estimates available at present [Wilcox, Gaunt and Farthing 1971). TABLE II Heritabilities and genetic correlations of yields and percentages* Length of lactation has a heritability not significantly different from zero, but is positively genetically correlated with yield (Barker and Robertson 1969). An estimate of .30 for heritability of feed efficiency was reported by Lamb (1972), and level of production and gross feed efficiency were highly correlated. High estimates of the heritability of the proportion of fatty acids were obtained by Edwards, King`and Yousef (1973), but since these estimates were subject to large variation, they should not be considered reliable, Norman and Van Vleck (1972) stated that bulls that sire daughters with high yields tend to sire daughters with weaker udder attachments. The repeatability of a trait is estimated from the correlation between different records forthesame animal - in this case the correlation between different lactations for the same cow. It has value as a measure of the degree of permanence of trait in the cow. and can be used to help decide which cows are likely to be the best producers in the next lactation (Touchberry 1974). Repeatabilities of 67 yields of milk and components are about 0.5 and of, percentages of These values components 0.6-0.8 (Wilcox, Gaunt and Farthing 1971). reflect the fact that almost half the variation among lactation yields in Thus there is the same herd is among lactations of the same cow. considerable error associated with the prediction of what a given cow will product in the next lactation. Selection for one trait will give greatest gains in that trait with The estimated genetic less significant responses in associated traits. change for a trait under selection may be given by Since protein yield is less heritable than fat yield, direct selection for protein yield will make slower progress than will direct selection for fat yield. Selection for milk yield in Friesians wiil be Jerseys, however, more effective than selection for yield in Jerseys. will respond better to selection for percentages of components. The response to selection for a trait other than the one under direct selection may be given by Selection for milk yield alone would give'an increase in yields On the basis of the of components, and decrease in percentages. correlated responses to selection for Holsteins given by Wilcox, Gaunt and Farthing (1971), selection for milk yield would give 67% of the response in fat yield as would direct selection for fat, and 96% of the response in protein yield as would direct selection for protein. Indices may be constructed to allow for selection for several traits at one time, with relative economic values assigned as weights for parameters. One such index constructed by Hanna and Cunningham (1974) suggests that under a pricing scheme of 1:l fat:protein, selection of sires on the basis of progeny records of fat and protein yields would be 22% more valuable than selection on the basis of milk yield alone. However, the more traits are selected for, the less is the response in any one trait. Legates (1964) showed that as the number of traits selected for increased from one to three, mean superiority of milk yield of daughters over herd mates' yields. fell from 364 kilograms to 209 kilograms. 68 A bull for use in artificial breeding is generally selected on the basis of the production of his daughters as compared with that of their contemporaries. Recent overseas work has been concerned with improving the accuracy of estimates of a bull's breeding value. One such improvement has been allowance for genetic merit of contemporaries, A bull undergoing progeny testing makes no effective genetic contribution to the population for ten years, but provides a small but constant contribution thereafter (Hinks 1971). Selection of a sire on the basis of his half-sisters' production may have some merit in shortening this interval (Owen l975), Progeny testing requires the retention of a large number of bulls pending results of progeny production, This can become a financial burden to an artificial breeding organization. Collection of large doses of,semen followed by slaughter of young bulls prior to the results of progeny production, and more extensive use of semen after selection may help to alleviate this problem. (c). Selection on the basis of individual genes Identification of an individual gene controlling a trait enables selection to be made with more certainty. The major milk proteins are controlled by simply inherited genes detectable by starch gel electrophoresis. Different variants for a-casein, p-casein and K-casein; and the whey protein p-lactoglobulin have been found in all major European breeds. A variant of a-la&albumin has been discovered in the Zebu strains of cattle. Frequencies of the variants can differ between breeds (Table III). TABLE III Gene Frequencies of major milk. protein variants in two breeds from an Ade.laide Hi-Us herd. re.cording association 69 Whether selection on the basis of a milk protein polymorphism will influence yields of milk, fat or total protein has yet to be determined. Sherbon, Ledford and Regenstein (1967) found a lower fat content in fl-lactoglobulin type A cows but this was not substantiated by other There has been workers (Brum et al 1968, Hoogendoorn et al 1969). determined, ho=vG, significant influ=crof the variants of @-lactoglobulin on the percentage of fl-In.cto,globulin produced (McLean, Bailey and Munro, 1974). Relationship of polymorphisms of other milk proteins and the production of these proteins are still uncertain but there appears to be scope for selection on this basis. The milk protein variants have been found to influence the The B variants of@processing properties of the milk produced. and K- casein produce milk with firmer curds and shorter rennet clotting times i.e. milk more suitable for making cheese. (Feagan et al .Milk of the 6lactoglobulin A variant produces a more v- 1972). heat stable milk powder, due to the higher 'level ofB=lactoglobulin. To an industry based economically on milk powder and cheese manufacture, the gene distribution of the suppliers of a factory may be of importance. Use of the milk protein polymorphisms in conjunction with blood polymorphisms in determining genetic distances between breeds may enable wider genetic variation for selection (Kidd 1974). Studies on the polymorphism frequencies of the South African South Devon breed and the German Gelbvieh breed proved the similarity of the breeds(Kidd et al 1974). To the breeders of South Africa this meant greater sczefor selection in their populatt:on, whilst still retaining the purity of their breed. Polymorphisms in enzymes in the milk may provide future information on the control of lactation. Xanthine oxidage, an enzyme which controls the last stages of purine catabolism, has been .found to have two alleles X0-A and X0-B with high and low activity respectively (Zikakis and Treece 1971). This enzyme has been implicated in development of undesirable oxidized flavours in market milk, and selection on the basis of the polymorphism may be of value. III . DISCUSSION The theoretical rate of genetic improvement in mean milk yield for a breeding population of 10,000 cows is 2.1% per annum (Syrstad 1972). Current Australian estimates based on the recorded Friesian md Jersey populations of N.S.W. do not differ significantly from zero (Hammond 1974 personal communication). Part of the reason for this low level of genet.i.c improvement is due to lack of participation in herd recording schemes and inadequate use of artificial insemination. Only 27% of the dairy cattle population in Australia is under herd recording, and only 3% are artificially bred. This, along with lack of co-ordination of information between states and organisations in the proving of bulls and the collection of information has hampered the adequate use of the breeding population for selection of superior stock. Moves now afoot for a national herd improvement scheme co-ordtnating herd recording and artificial breeding services on a national level must be supported. 70 In using selection, the animal producer must have a view to the. More emphasis is being placed by the economic trends in his industry. In'addition, consumer on the quality of the product supplied. efficiency of production will assume far greater importance in the Performance recording schemes will need to be more flexible future. and cater for information on feeding and breeding, and measurement of milk, fat, protein, lactose, and possibly even the protein fractions. As the capacity to change ratios of components (e.g. protein to fat ratio) appears limited within existing breeds, traditional ideas of Crossbreeding and development of breeds may need to be abandoned. Selection on the basis of the simply new breeds may be required. inherited milk protein polymorphisms appears to have a role in Individual genetic variants improving the quality of the product. of enzymes and othkr.milk and blood parameters may hold a key to an effective way of selecting for increased production. One area in which research findings appear to be lacking is in knowledge of genotype-environment interactions in dairy cattle. Branton et al (1974) found no genotype-climatic interactions over a mInteractions of genotype with temperature range of -1OC to 3OOC. level of feed intake, feed conversion efficiency and other aspects of nutrition need to be examined in more detail. The Friesian breed and selection for milk yield currently still appear to be the most economic programme for the animal producer to follow. However, care must be taken that significant downturns in Lower percentages of components can cause other traits do not occur. Selection factory problems in reaching requirements of composition. for milk yield should not be practised to the complete exclusion of Loss of genetic variability which may result from all other traits. such unidirectional selection would have serious repercussions if later trends dictate more emphasis on components. IV. BARKER, BARKER, BOWMAN, 1.: BRANTEN, REFERENCES J.S.F. (1968). The Australian Journal of Dairy Technology 2372. J.S.F. and ROBERTS-ON, Alan (1969). Animal Production 8:2217 J.C., and HOCKING, P.M. (1974). Livestock Production STience 401. C., RIOS, G., EVANS, D.L., FARTHING, B.R., and KOONCE, K.L. Journal of Dairy Science 57: 833. BRUM, E.W., RAUSCH, W.H., HINES, H.C. a; LUDWICK, TM. (1968). Journal of Dairy Science 51: 1031. EDWARDS, R.A., KING, J.W.B. anTYOUSEF, I.M..(1973). Animal Production 16: 307 ERB, R.E., ASHWOE, W.S. (1963). Journal of Dairy Science 46: 1228 FEAGAN, J.T.F., BAILEY, L.F., HEHIR, A.F. MC LEAN, D.M. and ELLIS, N.J.S. (1972). The Australian Journal of Dairy Technolou 27: 129. HANNArM.V., and CUNNINGHAM, E.P. (1974). Irish' Journal of Agriculture Research 13: 239. HAYMAN, R.H. (1974). World Animal Review 11: 31. C (1974) l 77 Animal Production. 13: 209, HINKS, C.J.M. (1971). HOOGENDOORN, M.P., MOXLEY, J.E.,HAWES, R.O.and MAC RAE, H.F. (1969). Canadian Journal of Animal Science 49: 331. KIDD, K.K. (1974). ?st World Congress orGenetics applied to livestock Production, ~321, KIDD, K.K., OSTERHOFF, D., ERHAND, L., AND STONE, W.H. (1974). Animal Blood Groups and biochemical Genetics 5': 21. LAMB, R.C. (1972). Utah Science. December 1972. = LEGATES, J.E. (1964). Journal of Dairy Science 47: 446. MC LEAN, D.M., BAILEY, L.F. and m0, Golda L. Fy74). XIX InterJournal of Dairy Science OWEN,j.B. (1975). Animal Production 20: I. SHERBON, J.W., LEDFORD, R.A., and REGEETEIN, J.M. (1967) - Journal of Dairy Science 50: 951. STULL, J.W. and BROWN,?.H. (1964). Journal of .Dairy Science 47: 1412, SYRSTAD, 0. (1972). World Review of Animal Production 8: 59. = TOUCHBERRY, R.W. (1974). in 'Lactation - A Comprehen&e Treatise' Vol. III ed. B.L. Larson and V.R. Smith. (Academic Press Inc., New York). WILCOX, C.J., GAUNT, S.N., and FARTHING, B.R. (1971). Southern Co-operative Series Bulletin No. 155. ZIKAKIS, J.P., and TREECE, J.M. (1971). Journal of Dairy Science 54: 648 0 72