Aust. J. Agric. Res., 1976, 27, 309-21 Bos indicus and Bos taurus Crossbred Dairy Cattle in Australia. IV* Progeny Testing and Expected Rate of Genetic Improvement I. R. Franklin,* R. H. HaymanB and R. W. HewetsonC A Division of Animal Genetics, CSIRO, P.O. Box 90, Epping, N.S.W. 2121. Division of Animal Genetics, CSIRO, F. D. McMaster Field Station, CSIRO Mail Bag, Liverpool, N.S.W. 2170. Division of Animal Genetics, CSIRO, c/- Agricultural Research Centre, New South Wales Department of Agriculture, Wollongbar, N.S.W. 2480. Abstract A dairy improvement program designed to develop a breed of cattle adapted to tropical environments is described. Each year young crossbred (Bos indicus x Bos taurus) bulls are screened for heat tolerance and tick resistance, and then progeny-tested in the herds of cooperating dairy farmers. Estimates of phenotypic and genetic means, variances and correlations are presented for production of milk and milk components, and the rate of genetic improvement is discussed. In particular the heritability of milk yield in the crossbred progeny is 0.27, and the theoretical rate of improvement is 2.6% per year. Introduction The first paper in this series (Hayman 1972) described milk production among the filial generations of Red Sindhi x Jersey, and Sahiwal x Jersey crossbred dairy cattle. This work established that production in some crossbred individuals was sufficiently high to compete with existing Bos taurus breeds, and that selection proved to be effective in eliminating the tendency found in Bos indicus cattle to fail to let down milk after removal of their offspring. These two findings were crucial, since without the potential for high production under Australian conditions of management a dairy breed derived from this intercrossed population would have little chance of commercial success. In 1962 when enough prospective sires became available, a progeny-testing program was implemented in an environment where heat, humidity, and inadequate winter feed were normal conditions. This paper describes the development of the program, and gives estimates of the heritability and selection differentials in the daughters of progeny test sires. From these data the expected response to selection is calculated. Experimental Procedure The region chosen for progeny testing is in the vicinity of the Wollongbar Agricultural Research Centre in northern New South Wales. This area has a tropical pattern of hot humid summers and dry winters, although the climate is not extreme (Table 1). The area is climatically suited to the tick species Boophilus microplus and Haemaphysalis bispinosa, but Boophilus is controlled on cattle by a regulatory dipping program. With the cooperation of the New South Wales Department of Agriculture * Part 111, Aust. J. Agric. Res., 1974, 25, 1023.
I. R. Franklin et a[. a group of dairy farmers were found who were willing to participate in a progenytesting program with crossbred bulls. Initially, eight herds were chosen, with a total breeding population of c. 600 cows. At a later date one cooperating farmer retired from dairying and three more were added, and the total population was increased to c. 750 cows. All cows and heifers are artificially inseminated and herdrecorded by CSIRO staff, and this, together with subsidies to the cooperating farmers, has allowed tight control over the breeding program. The initial intention was to progeny-test six sires per year on the basis of 30 daughters per sire, but during the first few years an average of only five sires were used. Sires for progeny testing were chosen from the crossbred herd maintained at the F. D. McMaster Field Station near Sydney. In the second generation eight sires were being used each year, and these are bred predominantly in cooperating farmers' herds. Table 1. 28-year means for average daily maximum and minimum temperatures, average index of mean relative humidity, and average monthly rainfall at Lismore, N.S.W. Wollongbar is c. 20 km from Lismore. Climatic records, Australian Bureau of Meteorology, 1969 Total Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. year Au. daily max. temperature ('C) 30.3 29.3 27.8 26.0 22.5 20.3 20.0 21.8 24.3 26.9 28.4 29.8 25.6 Av, daily min. tentperature ('C) 18.6 18.3 16.8 13.6 10.3 7.6 7.0 7.0 9.7 13.3 15.7 17.6 12.9 Au. index of relative humidity (% saturation) 74 74 79 77 75 73 70 69 72 71 70 72 73 Av. monthly rainfall (mm) 156 142 176 144 130 87 85 52 54 68 100 111 1302 Since 1964, preliminary screening of young bulls in a climate room (Allen and Donegan 1974) has been combined with the progeny test, and in 1968 a test for tick resistance (Hewetson 1968) was added to the selection procedure. Since 1968 between 30 and 40 bull calves have been screened each year, first for heat tolerance and then for tick resistance, leaving six to eight young bulls for the progeny test. The cooperating farmers are subsidized to rear most of their heifers, and c. 30% of all milking cows are in their first lactation. Statistical Estimates Estimation of Annual Production Production of milk and milk components is estimated on the basis of a 300 day lactation period, but lactations shorter than 300 days have not been corrected to a standard length in calculating the mean performance of heifers in the progeny test. One of the major problems with Zebu cattle is the poor persistence of lactation (Branton et al. 1966; Hayman 1972), and it has been considered essential to exert strong selection on this trait. While it would have been more efficient to treat poor
Crossbred Dairy Cattle. IV persistence as a trait separate from production, the genetic correlations and heritabilities necessary to construct an index were not available at the beginning of the program. The inclusion of short lactations ensures strong selection for length of lactation, but presents some statistical problems with non-normality in the distribution of total yield. Determinations of total milk, percentage fat and percentage protein are obtained for each animal every 28 days, and these data points are used to fit a cubic equation for each animal. The area under the curve between the calving date and 300 days (or the day at which the animal is noted to be dry if less than 300 days) is the estimated total yield. Separate curves are calculated for milk, fat and protein yield. Correction for Calving Date The only correction routinely applied to these data is for calving date. The calving season in the experimental area is concentrated in the months June-November, and there is a pronounced effect of calving date on production, particularly amongst mature animals. A linear regression of production on calving date is estimated for each herd, and production corrected accordingly. Estimation of the Contemporary Comparison Contemporary comparisons among bulls are calculated each year from production in daughters' first lactation only. Subsequent lactations are not used for bull selection. Only those heifers calving at 22-27 months of age, and between the months of June and November, are used in the progeny test calculations. Daughters are classified by sire and by herd, and estimates of sire and herd effects are obtained by analysis of variance. The estimated sire effects, expressed as a deviation from the mean, are then corrected by the expected regression on future daughters. That is, $nh2 contemporary comparison = sire effect x 1 + (12 - l)$h2 ' where h2 is the heritability and n is the number of daughters. Results Genetic and Demographic Changes in Cooperating Farmers' Herds The original herds of the eight cooperating farmers were predominantly Jersey (72 %), but other breeds were represented in significant proportions, namely, Guernsey (13 %), Australian Illawarra Shorthorn (AH) (12 %), Friesian (3 %) and a few Ayrshire (0.6 %). The three new herds introduced later consisted of roughly equal proportions of Jersey and Guernsey. As a result of repeated crossing of half- and quarter-bred Zebu bulls in the progeny test herds the proportion of Bos indicus genome has increased to about 25 % and is expected to approach three-eighths by the end of the second generation. During the first 7 years of progeny testing (i.e. the first generation of progeny test sires) 37 bulls were used, 21 being derived from Sahiwal x Jersey crosses and 16 from Red Sindhi x Jersey crosses. Hence the Sahiwal breed was represented to a greater extent than the Red Sindhi, and culling within herds and the selection of
I. R. Franklin et al. primarily Sahiwal x Jersey bulls in the progeny test has increased the proportion of Sahiwal even further. Table 2 shows the distribution of Bos indicus ancestry among the heifers milked from 1965 to 1971. The mean level of Zebu was 24 %: 15 % from the Sahiwal and 9 % from the Red Sindhi. Table 2. Percentage Bos indicus among heifers milked, 1965-1971 Number of Percentage Percentage Total Year heifers Sahiwal Red Sindhi Bos indicus Mean As a result of the requirement that each farmer rear 30% of his herd each year for the purpose of progeny testing, there has been a considerable change in the age structure of each herd, and the mean age at calving has dropped considerably in all herds. Table 3 shows the distribution of age of calving in 1962 (at the start of the program) and in 1973. The 1962 figures represent calvings among B. taurus cattle, and the 1973 data are for crossbred animals only. A few purebred European cattle still exist in the herds, particularly among the recent cooperators, but these now represent a negligible fraction of the population. This change in age structure is an important factor when considering the production figures presented below. Table 3. Age at calving, 1962 and 1973 Number of cows in each age group Years of age: Mean Percentage Year 2 3 4 5 6 7 8 9 10 11 >12 Total age heifers Emergence of the Australian Milking Zebu Breed It is clear from the above that the mixture of Bos taurus and Bos indicus breeds, together with the selection program for production, tick resistance and heat tolerance, has resulted in a population of animals which cannot be classified into any of the existing breeds. Accordingly, the cooperating farmers formed a breed society in 1970, and named the population of dairy cattle the Australian Milking Zebu (AMZ). Production Table 4 shows the total milk, total fat and percentage fat among purebred European and AMZ COWS over the period 1962-1973. Also shown is the percentage of AMZ cattle
Crossbred Dairy Cattle. N in the total herd. While there is a trend upward in production over the 10 year period, any genetic change that may have taken place is totally confounded with systematic changes in management. In addition, the trends are confused by marked seasonal effects and by the changes in age structure shown above. We note, however, Table 4. Average production of milk and fat in original cooperating herds, 1962-1973 No. of Percentage Average yield (kg) Fat Year cows of AMZ Milk Fat (%) that the change-over from B. taurus to crossbred cattle was not accompanied by a dramatic fall in production (Fig. 1). Indeed production from AMZ COWS appears comparable with that of the pre-existing cattle, and the fat percentage has increased appreciably. When we look at the production figures of AMZ cattle only, it appears that the improvement noted in Fig. 1 is directly attributable either to the substitution of AMZ for European cows or to the change in age structure. Fig. 2 shows the mean production Fig. 1. Milk production in original cooperating farmers' herds. of AMZ COWS over the years 1965-1973 for each age group. Here the upward trend has almost completely disappeared, and this argues against any systematic improvement in management. There is a very large age effect, and the improvement noted earlier is evidently due to the increasing proportion of older AMZ cattle in the herds.
I. R. Franklin et al. Thus, either older AMZ COWS are producing better than their predecessors or the increase in the proportion of heifers (which in itself will tend to decrease mean production) allows more intensive culling within herds, which more than offsets the inherent low production of heifers. No substantial genetic improvement is expected until 1972, when the first daughters of selected bulls enter the milking herds. Fig. 2. Milk production by age in AMZ cattle. A direct comparison of production in AMZ and European cattle will have to await the results of experimental trials in suitable tropical environments. We can, however, predict the expected rate of genetic improvement in the nucleus population. This is discussed later. Table 5. Means and variances amongst heifers Coeff. of Trait Mean Variance SD variation Milk (kg) 1228 175700 419 34 % Fat (kg) 58.7 466 21.6 37% Protein (kg) 43.7 238 15.4 35% % fat 4.70 0.292 0.54 11% % protein 3.51 0.045 0.21 6% Estimates of Phenotypic and Genetic Parameters Means, variances and correlations have been estimated from the first 7 years' data, collected between 1966 and 1972. Means and phenotypic variances, for heifers only, are shown in Table 5. Herd effects have been removed by analysis of variance. Estimates of genetic parameters were obtained from the regression of offspring on female parents by using: (a) production figures in contemporary years, i.e, regression of heifer production on mature cow production;
Crossbred Dairy Cattle. IV (b) production of both offspring and dam in the first lactation, additional environmental variation being introduced because of seasonal effects. The heritability estimates obtained are shown in Table 6. The heritability estimates for milk, fat and protein yield clearly do not differ significantly from each other, and are in good agreement with estimates obtained from other breeds. A value of h2 = 0.27 has been used in subsequent calculations. Similarly the estimated heritabilities for percentage fat and percentage protein agree with previously reported figures. Table 6. Heritability estimates for milk, fat and protein Standard errors of estimates are shown in parenthesis Trait Contemporary lactation First lactation Milk yield 0.268 (0,068) 0.250 (0.091) Fat 0.271 (0.070) 0.292 (0.100) Protein 0.269 (0,104) - A % fat 0.526 (0.090) 0.331 (0.114) % protein 0.548 (0.126) - A A Protein determinations were not made during 1965-1968, and there were insufficient data for estimation. The estimated genetic correlations between total milk and percentage fat, and between total milk and percentage protein are very close to zero. The genetic correlation between percentage fat and percentage protein was 0.209 with a standard error of 0.193. This estimate was based on 256 parent-offspring pairs, and clearly more data are necessary if good estimates of genetic correlations are to be obtained. The phenotypic correlations between these three traits among heifers, and among their dams, are shown in Table 7. Since the heifers are all crossbred animals and their dams are primarily purebred European, the two populations are genetically different. It is perhaps not surprising therefore that the phenotypic correlations are different in the two groups. Table 7. Phenotypic correlations for milk, percentage fat and percentage protein Heifers Dams Milk-% fat +0.158-0.018 Milk-% protein i- 0.147-0.087 % fat-% protein +0.518 +0.523 Rate of Genetic Improvement Milk production The theory of response to selection in a progeny test program was presented in detail by Robertson and Rendel (1950). The selection process can be divided into four pathways, namely, Bulls to breed Bulls, Bulls to breed Cows, Cows to breed Bulls and Cows to breed Cows. The selection pressure in each of these pathways is evaluated separately, and the total genetic gain is obtained by dividing the total
I. R. Franklin et al. genetic improvement (XI) by the sum of the generation lengths (EL) in each pathway. i.e. genetic improvement = 1 I L. I For mass selection, where individuals with the highest phenotype are chosen to be the parents of the next generation, we have where i is the standardized selection differential, h2 is the heritability, and o, is the phenotypic standard deviation. If selection is by means of a progeny test, the appropriate expression is where n is the number of daughters tested per sire. We will now consider each pathway in detail. Bulls to Breed Bulls During the first generation of the progeny test, approximately 50 bulls were tested each year, with approximately 30 daughters for each sire. If only one of these bulls is chosen, the appropriate selection differential (i) is 1.163. With h2 = 0.27 and n = 30, therefore I(BB)= 1.163 x 0.827 x JO.27 x o, = 0 5000,. The phenotypic standard deviations for milk and fat yield are 420 kg and 21 a6 kg respectively (Table 5), hence the expected improvement through selection among sires is 210 kg (milk) or 10.8 kg (fat) per generation. During the first 7 years of the program bulls were chosen on the fat yield of their daughters, since the demand in northern New South Wales was primarily for manufacturing rather than market milk. Recently the demand has changed, and selection is now directed at milk yield. It is of little consequence whether fat or milk yield is used as the selection criterion, as they have the same heritability, and the phenotypic and genetic correlations are very high (v, = 0.95, r, = 0-87). Table 8 shows the results of the progeny test during the first 7 years of the program, representing the first generation of bulls in the progeny test. In 1968 and 1969 the bulls chosen were not those with the highest mean values, and this was due to difficulties in statistical analysis at that time. Also note that in three of the seven years two bulls rather than one was chosen, and this was either to avoid inbreeding or to keep the level of Zebu ancestry as high as possible. The estimated relative breeding value (contemporary comparison) of each selected bull is shown in the last column of the table. The average contemporary comparison of the bulls is 147 kg, but if we average only the top bulls in each year the mean is
Crossbred Dairy Cattle. IV 188 kg, which is quite close to the theoretical gain of 210 kg. The agreement is good despite the fact that the wrong bull was chosen in two of the seven years. The testing of five bulls per year is less than optimal for this breeding program (see Robertson 1957), and currently eight bulls are progeny-tested each year. By 1976-77 10 bulls, each with 20 daughters, will be used. The contribution of the Bull to breed Bull pathway will then be Cows to Breed Bulls As indicated earlier, bulls used in the first generation of the progeny test were obtained primarily from the McMaster Field Station, whereas those used in the second generation have been bred almost exclusively in cooperating farmers' herds. The dams of the first generation bulls were drawn from a highly selected group of F, and F, crossbred animals, the details of which can be found in Hayman (1972). Table 8. Progeny test results for the first generation of sires Total No. of No. of No. of Contemporary Year no. of sires sires daughters comparison (kg) proven daughters tested selecteda tested Milk Fat 1965 144 4 2 (i) 47 +294 +13.5 (ii) 40 $107 + 4.0 1966 149 6 1 25 +333 +17.6 1967 148 6 2 (i) 31 +200 + 6.7 (ii) 20-126 - 1.3 1969 127 6 2 (i) 14 +I52 + 6.5 (ii) 25 + 84 + 0.8 1970 176 5 1 27 +229 +12.4 1971 179 5 1 40 +74 +7.3 A All bulls selected were half-breed Sahiwal x Jersey, except in 1967 when (i) was one-quarter Sahiwal and (ii) was one-half Sindhi x Jersey, and in 1969 where the selected sire was a one-quarter Sahiwal, three-quarters Jersey from a cooperating herd. In the second generation 90 cows were chosen from the total breeding population by selecting the top 12 % of cows in each herd. These dams are approximately evenly distributed in age from 3 to 7 years, and are mated with the most recently selected sire to breed a further generation of young bulls. The appropriate selection differential is 1.638, and the generation length of this pathway is approximately 5 years. Hence, I(CB) = 1.638 x 0.27 x a, = 0.442~~. Bulls to Breed Cows All but 12 % of the population are mated to young bulls, hence the only contribution to progress from this pathway comes from the matings of selected bulls to 'elite' cows. These matings account for 12 % of replacement heifers each year.
I. R. Franklin et al. and Therefore, I(BC) = (0.615 x 0.12)~~~ = 0.0740, L(BC) = (2 x 0.88 x 4 x 0.12) = 2.4 years. Cows to Breed Cows This pathway is essentially under the control of the individual farmer, and genetic progress depends primarily on his efficiency at culling for production. In most dairy breeding programs this culling, theoretically at least, accounts for a small fraction of the total rate of improvement. For example, Rendel and Robertson (1950) showed that only limited improvement was possible through farmer culling, and Skjervold (1967) in an investigation of optimum breeding structure in a progeny test program calculated that the dam-daughter pathway accounts for only 6% of the annual improvement. A theoretical analysis of the expected improvement through this pathway is aot simple, since it depends on the age structure in the herd, loss of individuals through causes not related to production, and the individual farmers' judgment. A detailed study is beyond the scope of this paper, but we can get some idea of the improvement by examining the difference in production between those cows saved and those discarded within each age group. The first line in Table 9 shows the difference in Table 9. Difference in production between selected and unselected cows, averaged over years 1969-1972 Lactation Selected Total Ratio A Milk A fat A protein A % fat A % protein First 407 662 0.62 176 8.9 6.4 0.05 0.01 Seconci 291 453 0.64 169 9.0 6.3 0.05 0.04 first lactation production between those heifers which were milked the following year, and the mean production for all heifers. The figures are averaged over all herds and over the 4-year period 1969-1972. The second line shows the same calculation for the second lactation. The first conclusion that may be drawn from Table 9 is that culling within herds is quite effective. The theoretical difference between the top 62% of heifers and the mean for all heifers is 257 kg, and the observed difference of 176 kg compares favourably with this. The effective selection on first lactation heifers corresponds to a culling of about 25% of animals for production. Culling on the second lactation is also quite effective. The difference of 169 kg corresponds to about 145 kg at the first lactation, compared with a theoretical difference of 245 kg. Assuming then that all selection is concentrated in these first two lactations, and that 30% of all producers are first lactation, 19 % second lactation and 51 % third or greater, we have a weighted selection differential of (0-3 x 0) + (0.19 x 176) + (0.51 x 321) = 197 kg, which is 0.47 phenotypic standard deviations. Therefore the expected genetic progress is The average age of cows at calving is c. 4-4 years (Table 3). We can now add each of these pathways to obtain the overall rate of genetic gain.
Crossbred Dairy Cattle. IV That is, = 31.4 kg per year This corresponds to a rate of improvement of 2.6% per year, which is greater than might have been predicted for a progeny test program based upon a breeding population of 650 cows. For example, Skjervold (1967) calculated a maximum rate of 1.7% per year for a population of 2000 cattle. The difference is largely due to the high coefficient of variation (34 %) for milk production, and this is characteristic of crossbred B. taurus by B. indicus cattle (Branton et al. 1966). Almost all of the improvement is expected through the sire pathways, and hence we have to await the production of the daughters of second and third generation bulls to see if these predictions are realized. Table 10. Means and variances for tick counts, 1973-1975 Year: 1973 1974 1975 Average Number of cattle tested 16 20 20 Mean no. of adult ticks 862 520 255 532 Mean In (ti~ks)~ 6.44 6.172 5.330 6.057 Variance In (ticks) 0.8155 0.1908 0.5468 0.4952 Standard deviation 0.9030 0.4368 0.7395 0.7037 A Following Wharton et al. (1970) we have used a log transformation on the tick numbers. Tick resistance Tick resistance is assessed each year by three successive infestations with 40,000 tick larvae of the young bulls prior to progeny testing. The number of ticks on one side of each animal is counted, and the scores on the third infestation are used to rank the bulls (see Wharton and Utech 1970). The eight animals with the lowest tick counts enter the progeny test. Table 10 shows the means and variances for tick counts in the years 1973-1975. Prior to 1973, bulls were infested and counted in stalls, rather than in the field, and because this regime resulted in much higher mean values for the number of adult ticks these data have not been included in the table. There are considerable differences from year to year which can be attributed to seasonal variations in test conditions, and to genetic causes, since each group of animals differ in the mean proportion of Zebu ancestry and are the sons of different selected sires. The final column of Table 10 shows the averages over the three years. The expected rate of improvement is i h2 o,, as discussed in the previous section. Selecting the top eight of 20 animals tested we have i = 0.928. The heritability has been estimated as c. 0.4 (Wharton et al. 1970; Hewetson 1972). Then ih2op = 0.928 x 0.40 x 0.704 = 0.26. Dividing by 2, since selection is among males only, we have Expected genetic change = 0-13 per generation = 0.019 per year.
I. R. Franklin et al. - In terms of the original variate, tick numbers, we have the approximate relationship Ax l Alogx = 532 x 0.019 = 9.9 ticks per year. This represents an annual improvement of about 2% per year in tick resistance. Heat tolerance A climate room test for heat tolerance is used in the selection of young bulls, and the procedure is described in detail in a recent paper by Allen and Donegan (1974). Their data show that the measurements have a significant repeatability, and there is some suggestion that an index of heat tolerance, used for ranking bulls, is positively associated with high milk production. However, not enough data have yet been collected to obtain satisfactory estimates of the heritabilities of each of the components of heat tolerance, nor to adequately measure the association between heat tolerance and milk production in a hot environment. Accordingly we are currently measuring the performance in the climate room of c. 80 progeny test heifers each year; the subsequent records of these animals, and heat tolerance tests of their daughters, should provide us with the necessary heritabilities and correlations. Discussion The data and estimates presented in this paper must be considered as a preliminary anaiysis of the genetic and phenotypic parameters of the AMZ population. Most of the heifers in the first generation are the daughters of purebred European dams and crossbred sires, and this means, for example, that the heritabilities and genetic correlations were obtained by regression of daughters from one population on dams from another. We would not expect the heritability estimates to be greatly affected, but the genetic correlations may be more seriously in error. Similarly, the progeny test in the first few years was based almost entirely on the performance of daughters of Bos taurus dams. Since the bulls differed in the proportion of Bos indicus, the possibility of heterosis due to the breed cross may have biased the results. Note, however, that the bulls used in the program were drawn from F, and F, populations, so heterotic and more general specific combining ability effects would not be as serious as if purebred B. indicus or F, bulls were used. In the second generation such potential biases do not exist. The parent population consists of a relatively uniform crossbred population, and the sires are dravn from the same population. Hence, we can be much more confident that the calculated contemporary comparisons accurately reflect differences in breeding value among the tested sires. Estimation of the actual rate of genetic improvement will have to await the production figures from second generation bulls. It was indicated earlier that the production estimates shown in Table 4 and Figs. 1 and 2 represent the progeny of first generation sires, which are expected to be genetically similar. However, because of the firm control exercised by CSIRO over the entire breeding program, there is no reason to suppose that the realized response to selection will fall far short of that predicted. The major difficulty associated with the program is ensuring the continuity of the breeding population, as individual farmers retire from dairying through old age and
Crossbred Dairy Cattle. IV other causes. Already there has been a demand for these cattle from some countries in south-east Asia, and this has inflated their market value over other European breeds. Hence farmers are tempted to sell their cattle overseas rather than to cooperating farmers entering the scheme. Finally, it should be emphasized that the low production figures reported in this paper reflects the level of management in the northern New South Wales area, and the inclusion of all records, regardless of length of lactation, in the estimates. These figures cannot be taken as an indication of production under improved husbandry. Acknowledgments We wish to thank Mr B. J. Thompson and Mr R. H. Clarke for their excellent technical assistance, Mr A. Packham for his supervision of many aspects of the program, and above all, Dr J. M. Rendel for his guidance. References Allen, T. E., and Donegan, S. M. (1974). Bos indicus and Bos taurus crossbred dairy cattle in Australia. 111. A climate room test of heat tolerance used in the selection of young sires for progeny testing. Aust. J. Agric. Res. 25, 1023-35. Branton, C., McDowell, R. E., and Brown, M. A. (1966). Zebu-European crossbreeding as a basis of dairy cattle improvement in the U.S.A. U.S. Dep. Agric. Sth. Co-op. Ser. Bull. No. 114. Hayman, R. H. (1972). Bos indicw and Bos taurus crossbred dairy cattle in Australia. I. Crossbreeding with selection among filial generations. Aust. J. Agric. Res. 23, 519-32. Hewetson, R. W. (1968). Resistance of cattle to cattle tick, Boophilus microplus. 11. The inheritance of resistance to experimental infestations. Aust. J. Agric. Res. 19, 497-505. Hewetson, R. W. (1972). The inheritance of resistance to cattle tick. Aust. Vet. J. 48, 299-303. Rendel, J. M., and Robertson, A. (1950). Estimation of genetic gain in milk yield by selection in a closed herd of dairy cattle. J. Genet. 50, 1-8. Robertson, A. (1957). Optimum group size in progeny testing and family selection. Biometries 13, 442-50. Robertson, A., and Rendel, J. M. (1950). The use of progeny testing with artificial insemination in dairy cattle. J. Genet. 50, 21-31. Skjervold, H. (1967). Selection schemes in relation to artificial selection. Proc. Int. Congr. Anim. Prod., Edinburgh, vol. 27, pp. 250-61. Wharton, R. H., and Utech, K. B. W. (1970). The relation between engorgement and dropping of Boophilus microplus (Canestrini) (Ixodidae) to the assessment of tick numbers on cattle. J. Aust. Entomol. Soc. 9, 171-82. Wharton, R. H., Utech, K. B. W., and Turner, H. G. (1970). Resistance to the cattle tick, Boophilus microplus in a herd of Australian Illawarra Shorthorn cattle: its assessment and heritability. Aust. J. Agric. Res. 21, 163-81. Manuscript received 11 July 1975 Corrigendum Volume 26, Number 6 Page 1066, subheads to Table 1. For [3H] isomer, [I4C] isomer, read [3H] leucine, [14C] leucine respectively.