- Email: [email protected]
Variation in Modified California Mastitis Test1 Scores in Dairy Cattle2
Variation in Modified California Mastitis Test Scores in Dairy Cattle R. C. W. DANIEL 3, D. A. BARNUM, and J. C. RENNIE University of Guelph, Guelph, Ontario, Canada Abstract
Statistical analysis of coded modified C.M.T. scores, recorded over a 3-yr period from cows in 40 herds in two areas of Ontario, showed that an average of 22% of the within-herd variation in the scores was associated with variation in age of cow, stage of lactation, month of the year, and monthly production level. Age of cow was responsible for the greater portion of this and accounted for an average of 18% of the variation left after that due to the three other variables had been removed. The remaining three variables accounted for an average of 5.6% of the variation remaining after that due to age had been removed. The coded modified C.1VI.T. scores were adjusted for effects of age of cow, month of year, stage of lactation, and monthly milk production within herds, and the adjusted values were subjected to a component analysis of variance within each of the two areas. Herd components of variance were 39 and 30%, years within herds 0%, cows within herd years 43 and 48%, and monthly tests wltl~in cows 15 and 21% for esch of the two areas, respectively. The California mastitis test (C.M.T.) was developed by Schalm and Noorlander in 1957 (12). Since that time, the test has found wides~)read use as an indicator of subclinical mastitis in dairy cattle in North America and elsewhere. The test appears to be a relatively efficient indicator of large changes in the leucocyte content of milk, when applied to fresh milk or to milk which has been collected withRecei,,ed for publication April 4, 1966.
hi 24 to 36 hr and refrigerated in the interval (2, 12). Although far from being ideal, due to the limitation of a range of five values, the test could be used as an index of the degree of subclinical mastitis of the cow, when applied to fresh bucket-milk samples. By using such an index, it is possible to relate the variation in levels of subclinical mastitis to physiological and enviromnental factors. Age of cow has been reported to be positively correlated with values for indicators of udder irritation (1, 8, 11, 14). These values can also be affected by stage of lactation (1, 4, 5) and season of the year (6, 8, 10). Level of milk production may be related to susceptibility to mastitis (7). The purpose of the investigations was to associate the variation in levels of subclinical mastitis, as measured by a modified C.M.T, with age of cow, stage of lactation, month of year, production level, other cow factors and herd factors. A knowledge of the relative contribution of each factor to the total variation in levels of subclinical mastitis could be useful in designing and evaluating control methods. Experimental Procedure
Monthly milk samples were collected from each cow in 40 herds in two areas of Ontario over a period of approximately 3 yr. There were five herds of each of the dairy breeds, Holstein, Ayrshire, Guernsey, and Jersey, from each area. Samples were the same as those used for official milk-recording purposes in the herds involved in tile study and consisted of mixed aliquots from an evening and morning milking. The evening aliquot was refrigerated overnight in an insulated icebox and the mixed sample later transported to the laboratory in the same container. At the laboratory, samples were heated to 43 C in a constant-temperature water bath and immediately cooled to 20 C, to allow dispersion of the milk fat. Each sample was then mixed thoroughly and a 15-ml aliquot taken for a modified C.M.T. This test was performed on each sample within 8 hr of the morning milking. The technique and scoring of the test was similar to that described elsewhere for the C.M.T. (12). The reagent
1 The proprietary C.M.T. reagent was not used; the term modified C.M.T. is, therefore, used in this paper. The actual reagent is described. "Based in part on a thesis submitted by the senior author to the Faculty of Graduate Studies, University of Guelph, in partial fulfilhnent of the requirements for the M.Sc. degree. Present address--Department of Veterinary Clinical Sciences, Massey University of Manawatu, Palmers~on North, New Zealand. 1226
VARIATION
IN MODIFIED
used was ]0% TeepoP in water containing 0.01% brom-cresol purple. The modified C.M.T. scores were coded 1, 2, 3, 4, and 5 negative, trace 1+, 2+, and 3~-, respectively, to facilitate statistical analysis. Coded modified C.M.T. values, age of cow, stage of lactation, date, monthly milk production,~ and breed of cow were recorded within herds for each monthly test on each cow between 5 and 300 days of lactation. A linear, stepwise multiple regression program (3) carried out on a 1620 I.B.M. computer, was used to analyze the data. Variables entered the regression according to their goodness of fit by the least-squares method. Coded modified C.M.T. score was the dependent variable, whereas the independent variables were age of cow, month of year, (month) :, days in milk, (days in milk) ~, monthly milk production, and the cross-product term for month × days in milk. This program yielded the partial regression coefficients, the pure constants, and the multiple coefficients of dete~nination. These values were obtained for each of the 40 herds. Partial coefficients of determination were calculated for each independent val~iablc which yielded a significant (P < 0.]) ~ partial regression coefficient. Adjusted monthly modified C.M.T. values were then calculated, using the sig~Teepol no. 610--Sodium salt of secondary a]kyl sulphate (G. H. Wood & Co. Ltd., Toronto). 5Total monthly production in 36 herds and 24-hr production × days in the month in four herds. 6The 90% level of probability for the significance of the partial regression coefficients was chosen because of the large amount of variation remaining in most of these herds after the multiple regression analysis had been run. This would increase the error of the b values and perhaps obscure true relationships. The greater risk of a Type I I error was accepted.
C.M.T. SCOI%ES
1227
nificant (P < 0.1) partial regression coefficients of the final multiple regression formula. A component analysis of variance was carried out on these adjusted values within each area, using a nested samples anlysis of variance program which also adjusted for unequal sample size. The components were herds, years within herds, cows calving within herd years, and error (months within cows). Results and Discussion
The percentage of variation in modified C.M.T. scores accounted for, in the present study, by variation in age of cow, stage of lactation, month of year, and monthly milk production varied considerably from herd to herd. The range was 8 to 50% and the average 22%. The average partial coefficient of determination for age of cow was 0.18, showing that this variable was responsible for the greater part of the total variation accounted for. Average partial coefficients of determination for monthly milk production and for the sum of the other independent variables were 0.029 and 0.026, respectively. Means and ranges of the significant partial regression coefficients are shown in Table 1. The average value of 0.182 for age indicates that modified C.M.T. scores increased by approximately this much as age of cow increased by I yr. Marshall and Edmondson (8) reported an annual increase of 0.228 per cow when they coded the C.M.T. values 0, 1, 2, 3, and 4 for negative, trace, 1+, 2+, and 3+, respectively. Some of the differences between herds in the age partial regression coefficients, and in the amount of variation accounted for, could be due to interaction with other variables, such as the herd levels of udder infection. For example, Spencer and K r a f t (13) compared two herds with a Streptococcus agalactiae infection incidence of 21% with
TABLE 1 Means and ranges of significant" wlthln-herd partial regression coefficients of age, month, days in milk (I).I.M.), and monthly milk production on modified C.M.T. scores Independent variable Age Month (Month) ~ D.I.M. (D.I.M.) ~ Milk produe~on Month X D.I.M. No. of tests per herd
Mean
Range 0.339 -0.215 ---0.0035 ---0.00220
0.182 0.130 --0.0108 --0.00477
(40) b (15) (17) (12)
0.042 --0.053 --0.0170 --0.00863
0.0000115 --0.000715 0.00022
(11) (32) (14)
0.000006 -0.000018 --0.000207 --0.001250 --0.00015 -0.00071 345 -- 932
604
a Forty, 11, 14, 10, 8, 30, and 13 of the values for age, month, (month) ~, D.I.M., (D.I.M.) ~, production, and month × D.I.M., respectively, were significant at 1) < 0.05. Six, 4, 3, 5, 2, and 4 of the values for month, (month) 2, D.I.M., (D.I.M.) 2, production, and month × D.I.M. were significant at P < 0.1 > 0.05, respectively. b Figures in parentheses indicate the number of herds with significant (]? < 0.1) values. J. DAIRYSCIENCEVOn. 49. NO. 10
R. C. W. D A N I E L
1228
ET AL
TABLE 2 Analyses of variance of adjusted modified C.M.T. scores
Area I
Source
D~ Herds 19 Years within herds 57 Cows within herd years ~ 2 , 0 6 1 Error (monthly tests within cows) 10,868
SS 2,670.749 62.872 3,181.699
MS 140.566b 1.103 1.544b
1,077.222
0.099
II
Component % 0.215 38.97 --0.002 .00 0.237 43.04 0.099
Herds 19 1,533.846 80.729~ 0.137 Years within herds 51 76.912 1.508 -0.001 Cows within herd years ~ 1 , 6 3 2 2,550.595 1.563 b 0.216 Error (monthly tests within cows 9,909 953.207 0.096 0.096 " Cows within herd years ~ cows calving in a particular herd during one year. b Significant (P ~ 0.01). K = 648.935 (Area I) and 579.014 (Area II) --correction factor for unequal sample Mean = 1.953 (Area I). Mean ----1.694 (Area II). ten herds in which the incidence was 71%. In the low-incidence herds, levels of infection in first-, second-, third-, and fourth-lactation cows were 5.3, 15, 25, and 35%, respectively, whereas in the high-incidence herds the corresponding levels were 61, 68, 68, and 73%. The two anMyses of variance on the a& justed C.M.T. values, shown in Table 2, yielded variance components of 39 and 30% for herds, 0% for years within herds, 43 and 48% for cows within herd years, and 18 and 21% for monthly tests within cows. Factors causing herd differences could include levels of udder infection, milking management practices, milking machine factors, and mean age of the herd. Cow differences might be explained by genetic differences or by factors affecting the individual cow in the herd environment, e.g., the position of her stall or her place in the milking order. The components suggest that the proportion of variation in C.M.T. values attributable to herd factors is at an approximate ratio of 35:45 to the propm~ion associated with individual cow factors in these herds. Assuming that bacteria are the major factors involved in the etiology of mastitis, it seems unlikely that the risk of exposure to infection will ever be completely removed from the herd environment, although Neave, Dodd, and Kingwill (9) have shown that one control method, hygiene at milking, will reduce the opportunities for bacteria to infect udders. Rendcl and Sundberg (11), however, reported that although a control program in 45 herds based on better milking methods and improved hygiene was followed by a marked decline in the incidence of mastitis (based on leucocyte counts greater than 500,000/ml), no less than 30% of the cows in their third to fifth lactation developed
17.97 30.44 .00 48.10 21.45 size.
mastitis when all their lactations fell within the control program. They suggested that one should not be too optimistic about decreasing the incidence of mastitis in cows at their most productive age below 25%, by the aid of improved milking management and hygiene measures alone. The proportion of variation in adjusted modified C.M.T. values accounted for by herd and cow factors in the present study indicate that attention should also be paid to the factors causing between-cow variation within herds. Inheritance of susceptibility to mastitis is possibly one of the most important factors, but others may also be involved and would need to be identified. Selection for resistance to mastitis and control of any other factors responsible for betweencow variation in adjusted C.M.T. values might assist in reducing the remaining 30% incidence of mastitis in cows at their most productive age [reported by Rendel and Sundberg (11)].
Acknowledgments The authors gratefully acknowledge the assistance of Dr. M. A. McGregor with the statistical aspects; G. C. Smith with the I.B.M. computing, and Mrs. Luella Smith with the California mastitis tests.
References (1) Braund, D. G., and Schultz, L. tt. 1963. Physiological and Environmental Factors Affecting the California Mastitis Test Under Field Conditions. J. Dairy Sei., 46: 197. (2) Daniel, R. C. W., Smith, G. C., and Barnum, D. A. 1966. The Relationship of California Mastitis Test Scores with Leucocyte Counts on Bucket Milk Samples. Canadian Vet. J., 7: 80. (3) Efroymson, M. A. 1960. Multiple Regression Analysis. In Mathematical Methods for J, DAIRY SCIENCE VOL. 49, NO. 10
V A R I A T I O N I N M O D I F I E D C.M.T. S C O R E S
(4) (5)
(6)
(7)
(8)
(9)
Digital Computers. p. 191. Ralston, A., and Will, It. S., eds. 2rid printing, 1962. John Wiley and Sons, Inc., New York and London. Frank, N. A., and Pounden, W. D. 1961. Prevalence of Bovine Mastltis. J. Am. Vet. Med. Assoc., 138:184. Macleod, P., and Anderson, •. O. 1952. A Study of Cell Counts of Milk from Healthy Cows. Storrs Agr. Expt. Sta., Bull. 290. Macleod, P., Anderson, E. O., and Plastridge, W. N. 1954. Ceil Counts of P l a t f o r m Samples of Herd Milk. J. Dairy Sci., 37: 919. Macleod, D. I-I., and Wilson, S. M. 1951. Milk Yield in Relation to Infection with S~reptococcus agalactiae. J. Dairy Research, 18:235. Marshall, R. T., and ]~dmondson, J. E. 1962. Value of California Mastitis Test Records to the Practitioner. J. Am. Vet. Med. Assoc., 140:45. Neave, F. K., Dodd, F. H., and Kingwill,
J . DAIRY SCIENCE ~OL. 49, NO. 10
(10)
(11)
(12)
(13)
(14)
]229
R. G. 1964. Control of Mastitis by Hygiene. Tij dschr. Diergeneesk., 89:31 (Supp]. no. 2). Oliver, J., Dodd, F. H., Neave, F. K., and Bailey, G. L. 1956. Variations in the Incidence of Udder Infection and Mastitis with Stage of Lactation, Age and Season of the Year. 5. Dairy Research, 23:181. Rendel, J., and Sundberg, T. 1962. Factors Influencing the Type and Incidence of Mastitis in Swedish Dairy Cattle. Acta Vet. Scand., 3:13. Schahn, O. W., and Noorlander, D. O. 1957. Experiments and Observations Leading to the Development of the California Mastitis Test. J. Am. Vet. Med. Assoc., 130:199. Spencer, G. R., and Kraft, M. E. 1949. Pathogensis of Bovine Mastitis. The Relationship of Age to Streptococcal Infections. Am. J. Vet. Research, 10: 115. Van Rensburg, S. W. J. 1947. The Secretion of Abnormal Milk by Quarters Free from Known Pathogens. Onderstepoort J. Vet. Sci., 22 : 91.
Statistical analysis of coded modified C.M.T. scores, recorded over a 3-yr period from cows in 40 herds in two areas of Ontario, showed that an average of 22% of the within-herd variation in the scores was associated with variation in age of cow, stage of lactation, month of the year, and monthly production level. Age of cow was responsible for the greater portion of this and accounted for an average of 18% of the variation left after that due to the three other variables had been removed. The remaining three variables accounted for an average of 5.6% of the variation remaining after that due to age had been removed. The coded modified C.1VI.T. scores were adjusted for effects of age of cow, month of year, stage of lactation, and monthly milk production within herds, and the adjusted values were subjected to a component analysis of variance within each of the two areas. Herd components of variance were 39 and 30%, years within herds 0%, cows within herd years 43 and 48%, and monthly tests wltl~in cows 15 and 21% for esch of the two areas, respectively. The California mastitis test (C.M.T.) was developed by Schalm and Noorlander in 1957 (12). Since that time, the test has found wides~)read use as an indicator of subclinical mastitis in dairy cattle in North America and elsewhere. The test appears to be a relatively efficient indicator of large changes in the leucocyte content of milk, when applied to fresh milk or to milk which has been collected withRecei,,ed for publication April 4, 1966.
hi 24 to 36 hr and refrigerated in the interval (2, 12). Although far from being ideal, due to the limitation of a range of five values, the test could be used as an index of the degree of subclinical mastitis of the cow, when applied to fresh bucket-milk samples. By using such an index, it is possible to relate the variation in levels of subclinical mastitis to physiological and enviromnental factors. Age of cow has been reported to be positively correlated with values for indicators of udder irritation (1, 8, 11, 14). These values can also be affected by stage of lactation (1, 4, 5) and season of the year (6, 8, 10). Level of milk production may be related to susceptibility to mastitis (7). The purpose of the investigations was to associate the variation in levels of subclinical mastitis, as measured by a modified C.M.T, with age of cow, stage of lactation, month of year, production level, other cow factors and herd factors. A knowledge of the relative contribution of each factor to the total variation in levels of subclinical mastitis could be useful in designing and evaluating control methods. Experimental Procedure
Monthly milk samples were collected from each cow in 40 herds in two areas of Ontario over a period of approximately 3 yr. There were five herds of each of the dairy breeds, Holstein, Ayrshire, Guernsey, and Jersey, from each area. Samples were the same as those used for official milk-recording purposes in the herds involved in tile study and consisted of mixed aliquots from an evening and morning milking. The evening aliquot was refrigerated overnight in an insulated icebox and the mixed sample later transported to the laboratory in the same container. At the laboratory, samples were heated to 43 C in a constant-temperature water bath and immediately cooled to 20 C, to allow dispersion of the milk fat. Each sample was then mixed thoroughly and a 15-ml aliquot taken for a modified C.M.T. This test was performed on each sample within 8 hr of the morning milking. The technique and scoring of the test was similar to that described elsewhere for the C.M.T. (12). The reagent
1 The proprietary C.M.T. reagent was not used; the term modified C.M.T. is, therefore, used in this paper. The actual reagent is described. "Based in part on a thesis submitted by the senior author to the Faculty of Graduate Studies, University of Guelph, in partial fulfilhnent of the requirements for the M.Sc. degree. Present address--Department of Veterinary Clinical Sciences, Massey University of Manawatu, Palmers~on North, New Zealand. 1226
VARIATION
IN MODIFIED
used was ]0% TeepoP in water containing 0.01% brom-cresol purple. The modified C.M.T. scores were coded 1, 2, 3, 4, and 5 negative, trace 1+, 2+, and 3~-, respectively, to facilitate statistical analysis. Coded modified C.M.T. values, age of cow, stage of lactation, date, monthly milk production,~ and breed of cow were recorded within herds for each monthly test on each cow between 5 and 300 days of lactation. A linear, stepwise multiple regression program (3) carried out on a 1620 I.B.M. computer, was used to analyze the data. Variables entered the regression according to their goodness of fit by the least-squares method. Coded modified C.M.T. score was the dependent variable, whereas the independent variables were age of cow, month of year, (month) :, days in milk, (days in milk) ~, monthly milk production, and the cross-product term for month × days in milk. This program yielded the partial regression coefficients, the pure constants, and the multiple coefficients of dete~nination. These values were obtained for each of the 40 herds. Partial coefficients of determination were calculated for each independent val~iablc which yielded a significant (P < 0.]) ~ partial regression coefficient. Adjusted monthly modified C.M.T. values were then calculated, using the sig~Teepol no. 610--Sodium salt of secondary a]kyl sulphate (G. H. Wood & Co. Ltd., Toronto). 5Total monthly production in 36 herds and 24-hr production × days in the month in four herds. 6The 90% level of probability for the significance of the partial regression coefficients was chosen because of the large amount of variation remaining in most of these herds after the multiple regression analysis had been run. This would increase the error of the b values and perhaps obscure true relationships. The greater risk of a Type I I error was accepted.
C.M.T. SCOI%ES
1227
nificant (P < 0.1) partial regression coefficients of the final multiple regression formula. A component analysis of variance was carried out on these adjusted values within each area, using a nested samples anlysis of variance program which also adjusted for unequal sample size. The components were herds, years within herds, cows calving within herd years, and error (months within cows). Results and Discussion
The percentage of variation in modified C.M.T. scores accounted for, in the present study, by variation in age of cow, stage of lactation, month of year, and monthly milk production varied considerably from herd to herd. The range was 8 to 50% and the average 22%. The average partial coefficient of determination for age of cow was 0.18, showing that this variable was responsible for the greater part of the total variation accounted for. Average partial coefficients of determination for monthly milk production and for the sum of the other independent variables were 0.029 and 0.026, respectively. Means and ranges of the significant partial regression coefficients are shown in Table 1. The average value of 0.182 for age indicates that modified C.M.T. scores increased by approximately this much as age of cow increased by I yr. Marshall and Edmondson (8) reported an annual increase of 0.228 per cow when they coded the C.M.T. values 0, 1, 2, 3, and 4 for negative, trace, 1+, 2+, and 3+, respectively. Some of the differences between herds in the age partial regression coefficients, and in the amount of variation accounted for, could be due to interaction with other variables, such as the herd levels of udder infection. For example, Spencer and K r a f t (13) compared two herds with a Streptococcus agalactiae infection incidence of 21% with
TABLE 1 Means and ranges of significant" wlthln-herd partial regression coefficients of age, month, days in milk (I).I.M.), and monthly milk production on modified C.M.T. scores Independent variable Age Month (Month) ~ D.I.M. (D.I.M.) ~ Milk produe~on Month X D.I.M. No. of tests per herd
Mean
Range 0.339 -0.215 ---0.0035 ---0.00220
0.182 0.130 --0.0108 --0.00477
(40) b (15) (17) (12)
0.042 --0.053 --0.0170 --0.00863
0.0000115 --0.000715 0.00022
(11) (32) (14)
0.000006 -0.000018 --0.000207 --0.001250 --0.00015 -0.00071 345 -- 932
604
a Forty, 11, 14, 10, 8, 30, and 13 of the values for age, month, (month) ~, D.I.M., (D.I.M.) ~, production, and month × D.I.M., respectively, were significant at 1) < 0.05. Six, 4, 3, 5, 2, and 4 of the values for month, (month) 2, D.I.M., (D.I.M.) 2, production, and month × D.I.M. were significant at P < 0.1 > 0.05, respectively. b Figures in parentheses indicate the number of herds with significant (]? < 0.1) values. J. DAIRYSCIENCEVOn. 49. NO. 10
R. C. W. D A N I E L
1228
ET AL
TABLE 2 Analyses of variance of adjusted modified C.M.T. scores
Area I
Source
D~ Herds 19 Years within herds 57 Cows within herd years ~ 2 , 0 6 1 Error (monthly tests within cows) 10,868
SS 2,670.749 62.872 3,181.699
MS 140.566b 1.103 1.544b
1,077.222
0.099
II
Component % 0.215 38.97 --0.002 .00 0.237 43.04 0.099
Herds 19 1,533.846 80.729~ 0.137 Years within herds 51 76.912 1.508 -0.001 Cows within herd years ~ 1 , 6 3 2 2,550.595 1.563 b 0.216 Error (monthly tests within cows 9,909 953.207 0.096 0.096 " Cows within herd years ~ cows calving in a particular herd during one year. b Significant (P ~ 0.01). K = 648.935 (Area I) and 579.014 (Area II) --correction factor for unequal sample Mean = 1.953 (Area I). Mean ----1.694 (Area II). ten herds in which the incidence was 71%. In the low-incidence herds, levels of infection in first-, second-, third-, and fourth-lactation cows were 5.3, 15, 25, and 35%, respectively, whereas in the high-incidence herds the corresponding levels were 61, 68, 68, and 73%. The two anMyses of variance on the a& justed C.M.T. values, shown in Table 2, yielded variance components of 39 and 30% for herds, 0% for years within herds, 43 and 48% for cows within herd years, and 18 and 21% for monthly tests within cows. Factors causing herd differences could include levels of udder infection, milking management practices, milking machine factors, and mean age of the herd. Cow differences might be explained by genetic differences or by factors affecting the individual cow in the herd environment, e.g., the position of her stall or her place in the milking order. The components suggest that the proportion of variation in C.M.T. values attributable to herd factors is at an approximate ratio of 35:45 to the propm~ion associated with individual cow factors in these herds. Assuming that bacteria are the major factors involved in the etiology of mastitis, it seems unlikely that the risk of exposure to infection will ever be completely removed from the herd environment, although Neave, Dodd, and Kingwill (9) have shown that one control method, hygiene at milking, will reduce the opportunities for bacteria to infect udders. Rendcl and Sundberg (11), however, reported that although a control program in 45 herds based on better milking methods and improved hygiene was followed by a marked decline in the incidence of mastitis (based on leucocyte counts greater than 500,000/ml), no less than 30% of the cows in their third to fifth lactation developed
17.97 30.44 .00 48.10 21.45 size.
mastitis when all their lactations fell within the control program. They suggested that one should not be too optimistic about decreasing the incidence of mastitis in cows at their most productive age below 25%, by the aid of improved milking management and hygiene measures alone. The proportion of variation in adjusted modified C.M.T. values accounted for by herd and cow factors in the present study indicate that attention should also be paid to the factors causing between-cow variation within herds. Inheritance of susceptibility to mastitis is possibly one of the most important factors, but others may also be involved and would need to be identified. Selection for resistance to mastitis and control of any other factors responsible for betweencow variation in adjusted C.M.T. values might assist in reducing the remaining 30% incidence of mastitis in cows at their most productive age [reported by Rendel and Sundberg (11)].
Acknowledgments The authors gratefully acknowledge the assistance of Dr. M. A. McGregor with the statistical aspects; G. C. Smith with the I.B.M. computing, and Mrs. Luella Smith with the California mastitis tests.
References (1) Braund, D. G., and Schultz, L. tt. 1963. Physiological and Environmental Factors Affecting the California Mastitis Test Under Field Conditions. J. Dairy Sei., 46: 197. (2) Daniel, R. C. W., Smith, G. C., and Barnum, D. A. 1966. The Relationship of California Mastitis Test Scores with Leucocyte Counts on Bucket Milk Samples. Canadian Vet. J., 7: 80. (3) Efroymson, M. A. 1960. Multiple Regression Analysis. In Mathematical Methods for J, DAIRY SCIENCE VOL. 49, NO. 10
V A R I A T I O N I N M O D I F I E D C.M.T. S C O R E S
(4) (5)
(6)
(7)
(8)
(9)
Digital Computers. p. 191. Ralston, A., and Will, It. S., eds. 2rid printing, 1962. John Wiley and Sons, Inc., New York and London. Frank, N. A., and Pounden, W. D. 1961. Prevalence of Bovine Mastltis. J. Am. Vet. Med. Assoc., 138:184. Macleod, P., and Anderson, •. O. 1952. A Study of Cell Counts of Milk from Healthy Cows. Storrs Agr. Expt. Sta., Bull. 290. Macleod, P., Anderson, E. O., and Plastridge, W. N. 1954. Ceil Counts of P l a t f o r m Samples of Herd Milk. J. Dairy Sci., 37: 919. Macleod, D. I-I., and Wilson, S. M. 1951. Milk Yield in Relation to Infection with S~reptococcus agalactiae. J. Dairy Research, 18:235. Marshall, R. T., and ]~dmondson, J. E. 1962. Value of California Mastitis Test Records to the Practitioner. J. Am. Vet. Med. Assoc., 140:45. Neave, F. K., Dodd, F. H., and Kingwill,
J . DAIRY SCIENCE ~OL. 49, NO. 10
(10)
(11)
(12)
(13)
(14)
]229
R. G. 1964. Control of Mastitis by Hygiene. Tij dschr. Diergeneesk., 89:31 (Supp]. no. 2). Oliver, J., Dodd, F. H., Neave, F. K., and Bailey, G. L. 1956. Variations in the Incidence of Udder Infection and Mastitis with Stage of Lactation, Age and Season of the Year. 5. Dairy Research, 23:181. Rendel, J., and Sundberg, T. 1962. Factors Influencing the Type and Incidence of Mastitis in Swedish Dairy Cattle. Acta Vet. Scand., 3:13. Schahn, O. W., and Noorlander, D. O. 1957. Experiments and Observations Leading to the Development of the California Mastitis Test. J. Am. Vet. Med. Assoc., 130:199. Spencer, G. R., and Kraft, M. E. 1949. Pathogensis of Bovine Mastitis. The Relationship of Age to Streptococcal Infections. Am. J. Vet. Research, 10: 115. Van Rensburg, S. W. J. 1947. The Secretion of Abnormal Milk by Quarters Free from Known Pathogens. Onderstepoort J. Vet. Sci., 22 : 91.