Vol. 6 (1997): 283-294. Effects of composite casein and P-lactoglobulin genotypes on renneting properties and composition of bovine milk by assuming an animal model Tiina Ikonen, Matti Ojala Department ofAnimal Science, PO Box 28, FIN-00014 University ofHelsinki, Finland, e-mail: liina. ikonen@helsinki.fi Eeva-Liisa Syväoja Valio Ltd., Research and Development Centre, PO Box 390, FIN-00101 Helsinki, Finland The effects of K-p-casein genotypes and P-lactoglobulin genotypes on the renneting properties and composition of milk were estimated for 174 and 155 milk samples of 59 Finnish Ayrshire and 55 Finnish Friesian cows, respectively. As well as the random additive genetic and permanent environ- mental effects of a cow, the model included the fixed effects for parity, lactation stage, season, K-p- -casein genotypes and P-lactoglobulin genotypes. Favourable renneting properties were associated with K-P-casein genotypes ABA,A, and AAA,A 2 in the Finnish Ayrshire, and with ABA 28, AAA,A,, AAA2 A 3, ABA,A, and ABA 2A 2 in the Finnish Friesian. The favourable effect of these gen- otypes on curd firming time and on firmness of the curd was partly due to their association with a high K-casein concentration in the milk. The effect of the K-casein E allele on renneting properties was unfavourable compared with that of the K-casein B allele, and possibly with that of the A allele. The P-lactoglobulin genotypes had no effect on renneting properties but they had a clear effect on the protein composition ofmilk. The P-lactoglobulin AA genotype was associated with a high whey protein % and P-lactoglobulin concentration and the BB genotype with a high casein % and casein number. Key words: coagulation properties, milk protein polymorphism ntroduction Several studies have discussed the environmen- tal and genetic factors that influence milk ren- neting properties. A number of workers have re- ported the favourable effect of the K-casein B allele on renneting properties (e.g., Schaar 1984, Aaltonen and Antila 1987, Pagnacco and Caroli 1987). There is therefore interest in using K-ca- sein genotypes as a selection criterion when breeding for more favourable renneting proper- ties and, thereby, better use of milk in cheese production. Conflicting results have been ob- tained for the effects of genotypes of ots| - and (3- caseins and (3-lactoglobulin on renneting prop- erties. The origin, size and structure of the data, milk protein genotype frequencies, statistical © Agricultural and Food Science inFinland Manuscript received February 1997 283 AGRICULTURAL AND FOOD SCIENCE IN FINLAND https://www.c-info.fi/info/?token=qTY7tKX2eubDyKlO.o-Nk_2tyf7AyWDltgw1Usg.ILnVkP8kir7eHgCPw4YFRA3jI37nfjvZM1lqoV1RFD7pQmn4kk0NHylHrd4Hs54PbqIaaaz-mla3Em4NAKlNNcZK3sl3y0NrmindjazG5TBFgxXxfZvldkyDzPmcnNCS85n-0eewpIv9CQ4yI1zXtw7HpNEJ5AENnB48MM6V2aZB9ak3iWYEuZ1Pfi25gl8nOc8ryfBKvq6qWW0Rg2qmGbBEO8Ry1HDO-jJ6p21008Smq9m836Q4TiRXSV6HFP0aO20bRJuwt-3chsZjR9x1jYwzLLk3G4SYHP1MWVVHAehZ5R61VutbhcE methods and models used or linkage disequilib- rium between the casein loci may explain some of the discrepancies. The effects of milk protein genotypes on ren- neting properties have been estimated by a least squares method (e.g., Tervala et al. 1985, Pag- nacco and Caroli 1987,Davoli et al. 1990, Mache- boeuf et al. 1993). When used for estimating single-gene effects on quantitative traits, this meth- od ignores some or all of the polygenic effects (Kennedy et al. 1992). Because of probable con- founding between single-gene effects and poly- genic effects, it is thus possible to find an ex- cess of spurious significant effects of the single genes. Kennedy et al. (1992) showed that the use of mixed model procedures under an animal model treating single-gene effects as fixed ef- fects can provide unbiased estimates of single- gene effects and exact tests of associated hypoth- eses for pedigreed populations. Various models have been applied for esti- mating milk protein genotype effects. Afew stud- ies have estimated the effect of genotypes of one protein at a time (Schaar 1984,Schaar et al. 1985, Davoli et al. 1990, Machboeuf et al. 1993). Others have included the genotypes of some or all major milk proteins in a model simultaneously (Feagan et al. 1972, Pagnacco and Caroli 1987, Tervala et al. 1985, Oloffs et al. 1992). Only a few authors have estimated the effects of com- posite genotypes of some or all major milk pro- teins (El-Negoumy 1972, Pagnacco and Caroli 1987). Because the casein loci are tightly linked (e.g., Grosclaude et al. 1973, Threadgill and Womack 1990), the genotype effect of a casein locus may not be independent of the genotype effect of another locus. It seems therefore rea- sonable to estimate casein genotype effects si- multaneously by using combined genotypes (Ojala et al. 1997). In this study we estimated the effects ofcom- posite K-P-casein genotypes and p-lactoglobu- lin genotypes on the renneting properties and composition of bovine milk by assuming an an- imal model. We also studied the associations of renneting properties with the composition of milk. Material and methods Milk samples A total of 59 Finnish Ayrshire (FAy) cows from Helsinki University’s experimental herd Viikki and 55 Finnish Friesian (FFr) cows from the ex- perimental herd Suitia were genotyped for (X.,-, P- and K-caseins and P-lactoglobulin by isoelec- tric focusing in polyacrylamide gels (Erhardt 1989). The FAy cows were born between 1980 and 1989, and the FFr cows between 1982 and 1989. The effects of milk protein genotypes on the renneting properties and composition of milk were estimatedby sampling the cows three times during lactation: 1, 3 and 5 months after calv- ing. The cows calved from July 1990 to June 1991, and the sampling period lasted from the end of September 1990 to the end of October 1991. When the cows were housed indoors, the average proportion of concentrates in the feed, as determined on energy bases, was 37% for the FAy and 42% for the FFr. In 1991, the FAy cows were on pasture from the end of May to the end of September, and the FFr cows from mid-June to the end of September. Because the cows were at different stages of lactation during sampling, the number of sam- ples per cow varied from two to three, but for most cows it was three. The total number of milk samples was 174for theFAy and 155 for the FFr. The milk samples (evening + morning milkings) were analysed for the following characteristics: daily milk yield, renneting properties, gross com- position and protein composition. Milk renneting properties The renneting properties of individual milk sam- ples (10 ml) determined by a Formagraph (Foss Electric, Hillerpd, DK-3400, Denmark) at 32 °C for 30 min with 0.20 ml rennet (Renco) liquid diluted in 0.07 M sodium acetate buffer (1:100) were: renneting time (R), curd firming time (K ) 284 Ikonen, T. et al. Renneting properties and composition ofbovine milk AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 6 (1997): 283-294. and firmness of the curd (E J0). R was the time from the addition of rennet to milk to the begin- ning of coagulation. K 2O was the time from the beginning of coagulation to the moment the width of the curve was 20 mm. E, O was the width of the curve 30 min after the addition of rennet. Because milk samples were allowed to coagu- late for only 30 min, renneting or curd firming times, or both, were not achieved for some sam- ples owing to poor coagulation. Because the sam- ples that did not coagulate in 30 min (nine sam- ples from six FAy cows) were divided more or less equally among the milk protein genotypes, these samples were omitted from the statistical analyses of renneting properties. Composition of milk The fat and protein percentages were determined with a Milko-Scan 605 (Foss Electric) and the somatic cell count was made with a Fossomatic cell counter. Because the frequency distribution for the somatic cell count was far from normal in both breeds, the somatic cell counts were log- arithmically transformed. pH was also measured. The protein composition values determined were: casein and whey protein percentages, non- protein nitrogen (mg/g), casein number and the concentrations ofa a-, P- and K-caseins, a-sl ’ s 2 “ ’ lactalbumin and P-lactoglobulin (g/1) in milk. The casein and whey protein percentages and non-protein nitrogen were determined according to International Dairy Federation (IDF) stand- ards (1979 and 1986). Casein number was the proportion of casein in total protein. Concentra- tionsof individual caseins in milk were obtained by multiplying proportions of individual caseins in total casein by casein content. The proportions of individual caseins were determined by fast protein liquid chromatography (FPLC) (Pharma- cia Biotech, Uppsala, Sweden) as described by Syväoja (1992). Individual whey proteins were fractionated by FPLC gel filtration on a Super- dex 75 HR 10/30 column (Pharmacia Biotech) as described by Syväoja and Korhonen (1994). Statistical analyses The effects of parity, lactation stage, season, k- (3-casein (k-P-CN) genotypes and P-lactoglobu- lin (P-LG) genotypes on the renneting proper- ties and composition of milk were estimated us- ing an animal model. Owing to the difference in k-P-CN genotypes formed in the FAy and FFr, the records from the two breeds, and thus from the two herds, were analysed separately. The following linear model was assumed; Model 1: y ijkimno =P + Parity, + Istage. + seasonk + casge, + lactge + a + p + e.... ,m n r n ijkimno where y ijkjmno = °th observation of a milk renneting trait or a milk composition variable of the n, cowth (i = general mean parity, = fixed effect of the ith parity class Istage. = fixed effect of the j th lactation stage class season, = fixed effect of the k, season classk th casge, = fixed effect of the l(h k-P-CN genotype class lactge m = fixed effect of the mih p-LG genotype class a n = random additive genetic effect of the n th cow, (0, Aa 2 a ) pn = random permanent environmental ef- fect of the n, cow, (0, lo2 ) th ’ v ’ pe 7 e.... = random residual effect, N(0, lo2 ) ijkimno x e 7 Parity was grouped into three classes: first, second and third to ninth lactation; lactation stage into three classes: 1, 3 and 5 months after calving; and season into four classes: Sep to Nov, Dec to Feb, Mar to May and Jun to Aug. The classification ofk-P-CN and P-LG genotypes is presented in Table 1. Because the FAy was mon- omorphic for a s| -casein and there were a few cows with the a ,-casein C allele in the FFr, asl ’ si casein genotypes were not considered in the for- mation of composite genotypes. The 59 FAy cows with records were daugh- ters of25 sires and the 55 FFr cows daughters of 32 sires. The number of daughters per sire ranged 285 AGRICULTURAL AND FOOD SCIENCE IN FINLAND from one to six in the FAy and from one to four in the FFr. Ten FAy sires and 16 FFr sires had only one daughter each. The pedigrees of the cows with records were known for at least two generations, and the total number of animals in the statistical analyses was 352 for the FAy and 568 for the FFr. In subsequent analyses, the associations of renneting properties with the composition vari- ables of milk were estimated using Model 2, in which one milk composition variable at a time was included as a covariate in Model 1. Other- wise Model 2 worked like Model 1. Variance components for the random effects (o2 , o 2 and o 2) in Models 1 and 2 were esti-v a’ pe e' mated from the data sets with the REML VCE package (Groeneveld 1993). The effects of pari- ty, lactation stage, season, K-p-CN genotypes and P-LG genotypes on various characteristics were tested with the PEST program of Groeneveld (1990). The hypothesis tested was K’b=o, in which K’b contained the maximum number of independent estimable contrasts between classes ofa fixed factor in the model. The statistical sig- nificance ofregression coefficients (Model 2) was obtained by calculating F values using the differ- ence between o2 £ from Model 1 without a covari- ate and G 2 from Model 2 with a covariate. How- E ever, no consideration was made about the effect due to the number of independent tests generated by several traits within two differentpopulations. Results Frequencies of K-p-CN and P-LG genotypes The expected number of all possible combina- tions of K- and P-casein genotypes was 15 in the FAy and 18 in the FFr. Owing to the small size Table I. Number of cows and observations for the composite K-p-casein genotypes and (3-lactoglobulin genotypes. Finnish Ayrshire Finnish Friesian No. of Percentage No. of cows of cows obs. No. of Percentage No. of cows of cows obs.of cows obs. K-(3-casein AAA,A, 2 7 3.0 6 aaa,a2 AAAA AAA'„ ABA,A, ABA,A, ABAA ABA>‘ AEA^A, AEA,A, AEAA BEAA EEA,A, P-lg 12,0 21 10 18.0 29 14 24.0 40 21 38.0 60 2 4.0 4 2 8 2 3.0 6 1 2.0 3 14.0 24 8 4 15.0 23 3.0 6 7.0 I I 2 4 4.0 5 3 5.0 9 7.0 II 13 22.0 38 3 5.0 9 2 2 4 3.0 6 63.0 7.0 12 AA AB BB 5 9.0 15 7 13.0 20 28 47.0 82 34 62.0 97 26 44.0 77 14 25.0 38 'AAAm = AAA,A, + AAA,A, p-lg = p-lactoglobulin 286 Ikonen, T. et al. Renneting properties and composition ofbovine milk AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 6 (1997): 283-294. of the data sets and linkage disequilibrium in the casein loci, the observed number ofcombinations was II in the FAy and 9 in the FFr (Table 1). The most common k-(3-CN genotypes were AAA2 A 2 and in the FAy, and AAA 2A, and AAA, A, in the FFr. Consequently, 46% of the FAy cows and 56% of the FFr cows had one of the two most common tc-p-CN genotypes. The rarest tc-p-CN genotypes were carried by only one or two cows. The P-LG AB and BB geno- types were almost equally frequent in the FAy whereas AB was most frequent in the FFr. The P-LG AA genotype was rather rare in both breeds. Means and Variation Renneting properties The average renneting and curd firming times were longer and the firmness of the curd was poorer for milk of the FAy than for milk of the FFr (Table 2). There was considerable variation in renneting properties in both breeds. The co- efficients of variation for renneting and curd firming times would have been even larger had the poorly coagulating milk samples reached their extremely long renneting or curd firming times, or both. Table 2. Milk renneting traits, daily milk yield, and gross and protein composition of milk. Finnish Ayrshire Finnish Friesian X s.d. cv x s.d. cv Milk renneting R, min 12.44.9 40 11.33.6 33 K,0, min 8.13.3 41 7.43.5 47 E w, mm 25.812.5 48 31.210.1 33 e;o, mm 27.211.2 41 Milk yield and composition Daily milk yield, kg 26.2 5.7 21 26.0 6.0 23 Fat % 4.51 0.66 15 4.27 0.69 16 Protein % 3.20 0.29 9 3.14 0.25 8 pH 6.76 0.07 I 6.77 0.08 I Somatic cell count (In) 4.59 1,63 36 5.67 1.35 24 Protein composition Casein % 2.49 0.25 10 2.44 0.22 9 Whey protein % 0.53 0.08 15 0.54 0.07 13 Non-protein nitrogen, mg/g 0.29 0.06 21 0.26 0.03 12 Casein number 78 2.49 3 78 2.41 3 a ,-casein, g/1 9.47 0.98 10 9.32 0.89 10 cx' 2-casein, g/l 3,19 0.67 21 2.97 0.45 15 (3-casein, g/1 9.40 0.99 11 9.12 0,99 II K-casein, g/l 2.87 0.41 14 2.98 0.45 15 a-lactalbumin, g/1 0.96 0.13 14 0.96 0.14 15 P-lactoglobulin, g/l 3.28 0.61 19 3.31 0.54 16 Finnish Ayrshire; total 59 cows and 174 observations, for R and E| o ; 58 cows and 165 observations and for K 2n ; 53 cows and 134 observations. Finnish Friesian; total 55 cows and 155 observations, for K 2O; 55 cows and 142 observations, x = mean, s.d. = standard deviation, cv = coefficient of variation E| o = samples with renneting time, used in statistical analyses. 287 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Gross and protein composition of milk Even though therenneting properties were some- what weaker in milk of the FAy, the fat, protein and casein contents and the concentrations of otvl -, a- and p-caseins were higher than in milk of the FFr (Table 2). The somatic cell count and concentration of K-casein were higher in milk of the FFr than in that of the FAy. There were no major differences in concentrations of a-lactal- bumin and P-lactoglobulin between the FAy and FFr. Estimates of genetic variation The moderately high heritability estimates for milk renneting properties in both breeds suggest- ed that additive genetic effects made an impor- tant contribution to variation in these character- istics (Table 3). When K-p-CN genotypes and P- LG genotypes were excluded from Model 1, the heritability estimates increased by 3-16 percent- age units. A moderate proportion of the additive genetic variation in renneting properties was therefore due to milk protein genotypes. The magnitude of heritability estimates for rennet- ing properties was about the same as that for protein and casein contents and concentrations of P- and K-caseins in both breeds, and for fat content, and concentrations of a - and a -ca-si s 2 seins and p-LG in the FAy (Table 3). Because the data sets were small, the standard errors of the heritability estimates were high for some traits, but reasonable for renneting properties. Effects of K-p-CN genotypes Of the several traits studied, k-b-CN genotypes had a statistically significant effect on firmness Table 3. Heritability (h2 ) and repeatability (r) estimates forrenneting properties, daily milk yield, and gross and protein composition characteristics of milk from the Finnish Ayrshire and Finnish Friesian. Finnish Ayrshire Finnish Friesian h2 ±s.e.' r h2 ±s.e. r Milk renneting R, min 0.62 ±0.14 0.66 0.3510.21 0.58 K2O, min 0.5410.13 0.63 0.6610.10 0.71 Ew, mm 0.4110.19 0.640.57 10,06 0.57 Milk yield and composition Daily milk yield, kg 0.1210.11 0.47 0.0610.20 0.60 Fat % 0.3710.07 0.57 0.1410.15 0.30 Protein % 0.3410.23 0.59 0.1910.18 0.57 pH 0.0810.19 0.46 0.0510.23 0.43 Somatic cell count (In) 0.1810.09 0.57 0.3810.10 0.41 Protein composition Casein % 0.5010.07 0.52 0.2010.25 0.56 Whey protein % 0.01 10.04 0.37 0.1510.18 0.47 Non-protein nitrogen, mg/g 0.00 1 0.00 0.42 0.02 1 0.06 0.17 Casein number 0.2010.10 0.31 0.3010.35 0.33 a ,-casein, g/l 0.5210.07 0.55 0.0610.13 0.46 a,-casein, g/1 0.31 10.22 0.46 0.0010.00 0.40 (i-casein, g/l 0.4010.07 0.41 0.3310.24 0.51 K-casein, g/l 0.2710.18 0.43 0.4210.19 0.61 a-lactalbumin, g/l 0.0010.00 0.26 0.2710.08 0.27 P-lactoglobulin, g/l 0.3510.29 0.48 0.2110.14 0.54 's.e. = standard error of heritability estimate 288 Ikonen, T. et al. Renneting properties and composition ofbovine milk AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 6(1997): 283-294. of the curd and concentrations of a s- and k-ca- seins in the FAy, and on curd firming time, firm- ness of the curd and k-casein concentration in the FFr. In the FAy, the AAA,A 2 genotypes had a favourable effect on firmness of the curd and k-casein concentration, and the and AAA ( A 2 genotypes on a s- casein concentration (Table 4). In the FFr, the k- b-CN genotypes associated with the most favour- able renneting properties and the highest k-ca- sein concentration were ABA 28, AAA,A,, ABA,A 2 and ABA 2 A2 (Table 5). Effects of p-LG genotypes The P-LG genotypes had no statistically signif- icant effect on renneting properties in either breed but they had a strong effect on the protein composition of milk in both breeds (Table 6). Casein content and casein number were highest for the P-LG BB genotype, and whey protein and P-lactoglobulin concentrations for the AA gen- otype. Associations between renneting properties and composition of milk An increase in the pH of milk had an unfavoura- ble effect on each renneting characteristic in both breeds (Table 7). Some of the milk samples from the FFr had a very high somatic cell count. The somatic cell count did not, however, have a sta- tistically significant effect on renneting proper- ties in either breed. High protein and casein con- Table 4. Estimates of K-fS-casein genotype effects (with standard errors below the estimates) on firmness of the curd, and ots- and (i-casein (cn) concentrations of milk from the Finnish Ayrshire. AA AA AA AB AB AB AE AE AE BE EE A,A 2 A 2A 2 A |A ) A,A2 A 2A2 A,A 2 A 2A2 A |A ] AjA j n=6 n=2l n=4o n=6 n=24 n=6 n=9 n=3B n=6 n=6 n=l2 F-test E 3O, mm 1 -0.2 10.4 0 16.0 16.1 2.8 -4.94.1 -3.62.8 0.0 P=0.005 (8.8) (6.1) (3.5) (7.0) (6.7) (5.1) -0.36 -0.86 -0.31 -0.07 -0.78 -0.98 P0.10) after the K-casein concentration had been included in Model 2. Consequently, the favourable effect of certain k-(3-CN genotypes on curd firming time and firmness of the curd was partly due to the high K-casein concentra- tion associated with these genotypes. Discussion Genetic variation of characteristics The small data sets in this study were not suita- ble for estimating variance components and her- itability values for the traits studied. Errors in the heritability values assumed can change the significance levels, and possibly lead to bias in estimates of the effects under study (Kennedy et al. 1992). We, however, used variance compo- nents estimated from the data, there being no estimates in the literature of the variance com- ponents deduced using a repeatability model. The heritability estimates for renneting properties we obtained were about twice as high as those re- ported by Lindström et al. (1984) for milk ren- neting time and by Tervala et al. (1985) for each milk renneting trait. However, in Tervala et al. (1985), the standard errors of heritability esti- mates were very high. In both previous studies, the cows were sampled only once. Effects of milk protein genotypes k-P-CN genotypes with theK-casein B allele had a favourable effect on firmness of the curd in both breeds. There was, however, some varia- tion between the effects ofK-casein AB, AA and AE genotypes depending on the effect of (3-ca- sein genotypes or alleles in genotype combina- tions. In theFAy the (3-casein A,A 2 genotype, and in the FFr the (3-casein A, and B alleles also had a favourable effect on firmness of the curd. There was a difference between the effects of theK-casein A and E alleles on renneting prop- erties. The K-casein AA genotype had a favour- able effect on renneting properties when in com- bination with the p-casein A( A 2 genotype in the FAy and with A,A 3 and A 2A } genotypes in the FFr. The effect of the K-casein E allele was, in contrast, rather unfavourable in each k-P-CN genotype. In the FAy, the K-casein E allele was rather common (30%), whereas in the FFr it was rare (6%). It is possible that the differences in K-casein E allele frequency and the K-casein concentration in milk (Table 2) between the FAy and FFr were partly responsible for the differ- ences in renneting properties between the breeds. A higher frequency of the K-casein E allele in the FAy than in the FFr was also observed by Ahlfors (unpublished) in data on about 800 FAy and 100 FFr cows. The favourable effect of the K-casein B al- lele on milk renneting properties has been re- ported in several other studies (e.g., El-Negou- my 1972, Schaar 1984, Schaar et al. 1985, Pag- nacco and Caroli 1987, Davoli et al. 1990, Oloffs et al. 1992, Van den Berg et al. 1992, Mache- boeuf et al. 1993,Walsh et al. 1995). Nothing has previously been known, however, of the effect of the K-casein E allele on renneting properties. A favourable effect of the P-casein B allele on renneting properties similar to that we observed in the FFr was reported by Feagan et al. (1972). According to Marziali and Ng-Kwai-Hang (1986), P-casein genotypes had no statistically significant effect on renneting properties. A statistically non-significant effect of p-LG genotypes on renneting properties such as ob- served in this study was also reported by Feagan et al. (1972) and Pagnacco and Caroli (1987). According to van den Berg et al. (1992), the p- LG AA genotype was associated with the short- est renneting and curd firming times.The favour- 291 AGRICULTURAL AND FOOD SCIENCE IN FINLAND able effect of the (i-LG BB genotype on casein concentration and casein number, and that of the AA genotype on whey protein and p-lactoglob- ulin concentrations were also reported by McLean et al. (1984) and Schaar et al. (1985). As well as renneting properties, the milk sam- ples were analysed for several gross and protein composition characteristics to establish wheth- er the variation in renneting properties due to milk protein genotypes could be explained by differences in gross or protein composition char- acteristics between the genotypes. Of the sever- al characteristics, the high K-casein concentra- tion in milk explained part of the favourable ef- fect of certain k-P-CN genotypes on the rennet- ing properties in both breeds. We estimated the direct effects of milk pro- tein genotypes on renneting properties by assum- ing an animal model. We did this because the results for the K-casein genotype effects on ren- neting properties are consistent suggesting that the K-casein locus itself affects the renneting properties. There are no previous reports of an animal model being used for estimating the ef- fects of milk protein genotypes on renneting properties. It is, however, possible that there are other quantitative trait loci near the K-casein lo- cus that have a considerable effect on renneting properties. Thus, it would be interesting to esti- mate associations between milk protein geno- types and renneting properties within sires. We could not do so here due to the restricted size of the data sets. Conclusions The K-P-CN genotypes AAA,A 2 in the FAy and genotypes ABA 28, AAA,A,, AAA 2 A3 , ABA t A2 , ABA 2 A 2 in the FFr were associated with favourable renneting prop- erties, partly due to their association with the high K-casein concentration in the milk. The ef- fect of the K-casein E allele on renneting prop- erties was unfavourable as compared with that of the K-casein B allele, and possibly also with that of the K-casein A allele. Results for the ef- fect of the K-casein E allele on renneting prop- erties need to be confirmed with a larger data set. The P-LG genotypes had no effect on ren- neting properties but they had a strong effect on the protein composition of milk. Acknowledgements. The authors thank Professor Dr. Georg Erhardt from Justus-Liebig-University, Gießen, Germany for help with the lEF method. The Finnish Animal Breeding Association is thanked for labora- tory facilities. References Aaltonen, M.L. & Antila, V. 1987. 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Netherlands Milk and Dairy Journal 46: 145-168. 293 AGRICULTURAL AND FOOD SCIENCE IN FINLAND SELOSTUS Kaseiinien yhdistelmägenotyyppien ja p-laktoglobuliinin genotyyppien vaikutus maidon juoksettumisominaisuuksiin ja koostumukseen Tiina Ikonen, Matti Ojala ja Eeva-Liisa Syväoja Helsingin yliopisto ja Valio Oyj Tutkimuksen tarkoituksena oli selvittää k- ja P-ka- seiinien yhdistelmägenotyyppien ja (3-laktoglobulii- nin genotyyppien vaikutusta maidon juoksettumis- ominaisuuksiin sekä maidon yleis- ja valkuaisaine- koostumukseen. Tutkimuksen aineisto koostui Helsin- gin yliopiston Viikin opetus-ja tutkimustilan 59 ayr- shirelehmän 174 maitonäytteestä ja Suitian opetus- ja tutkimustilan 55 friisiläislehmän 155 maitonäyt- teestä. Maitonäytteitä kerättiin lehmiltä kolme kertaa lypsykauden aikana; kuukauden, kolmen kuukauden ja viiden kuukauden kuluttua poikimisesta, k- ja (3- kaseiinien yhdistelmägenotyyppien ja (3-laktoglobu- liinin genotyyppien vaikutusten selvittämiseen käy- tettiin eläinmallia, jossakiinteinä tekijöinä olivat poi- kimakerta, lypsykauden vaihe, vuodenaika, k- ja (3- kaseiinien yhdistelmägenotyypit ja P-laktoglobuliinin genotyypit. Satunnaisina tekijöinä mallissa olivat eläimen additiivinen geneettinen vaikutus, eläimen pysyvä ympäristö ja jäännöstekijä. Kaseiinien kytkeytyneisyyden vuoksi havaintojen lukumäärä eri k- ja P-kaseiinien yhdistelmägenotyy- peissä vaihteli selkeästi. Ayrshirelehmillä maidon juoksettumisominaisuudet olivat parhaimmat K-ja P- kaseiinien yhdistelmägenotyypeillä ABAjA, ja AAA,A2 ja friisiläislehmillä yhdistelmägenotyy- peillä ABA ; B, AAA |A 3, AAA,A,, ABA( A 2 ja ABA2 A2 . K-kaseiinin E-alleeli vaikutti epäedullisesti maidon juoksettumisominaisuuksiin B-alleeliin ja luultavasti myös A-alleeliin verrattuna. Juoksettu- misominaisuuksiltaan parhaimmilla yhdistelmägeno- tyypeillä oli yhteys maidon korkeaan K-kaseiinipitoi- suuteen kummallakin rodulla. Maidon K-kaseiinipi- toisuus vaikutti edullisesti maidon kiinteytymisaikaan sekä juoksettuman kiinteyteen kummallakin rodulla. Osa yhdistelmägenotyyppien vaikutuksesta maidon juoksettumisominaisuuksiin johtui siten niiden vaiku- tuksesta maidon kaseiinikoostumukseen. P-laktoglo- buliinin genotyypeillä ei ollut vaikutusta maidon juoksettumisominaisuuksiin. P- laktoglobuliinin genotyypit vaikuttivat kuitenkin maidon valkuaisaine- koostumukseen. P-laktoglobuliinin AA genotyyppi oli yhteydessä maidon korkeaan heraproteiini- ja P-lak- toglobuliinipitoisuuteen ja BB genotyyppi korkeaan kaseiinipitoisuuteen sekä kaseiinilukuun. 294 Ikonen, T. et al. Renneting properties and composition ofbovine milk AGRICULTURAL AND FOOD SCIENCE IN FINLAND