Autor: Durán Castro, Carlos Vicente.
Tipo: Master of Science.
Universidad: North Carolina State University.
Presidente de tesis: Dillard, Emmett U.
Relationships among previous days dry, days open, age of cow, and 305-day milk production were evaluated for 10,782 lactation records during the period 1950-1973 in the Lucerna Herd of cattle of Bugalagrande, Colombia, The data were analyzed in three groups; 2,248 first, 8,534 second and later, and 10,782 total lactations. Results from the maximum likelihood method of analysis are presented, The percentage of total variation in 305-day milk yield attributed to the previous dry period was 2.14. Days open explained .5%, 4,5%, and 3.5% of the variation in 305-day milk yield for first, second and later, and all lactations, respectively, Age at the beginning of lactation contributed 12.6% of the variation in milk production. Age adjustment factors for mi1k yield were developed. Estimates of heritability and genetic correlations for these traits were obtained by within-year regression of daughter on dam. Heritability of milk yield were .29, .27, .18, and .25 for first, second, third and the average of three consecutive lactations, respectively. They ranged from, .06 to .25 for lactation length, were in the range of -.06 to .17 for days open, and for dry period values were between .18 to .30. The repeatability values for milk yield, lactation length and days open were .39, .23, and .10 respectively. Genetic correlations between mi1k yield and lactation length ranged from .55 to .84 and for lactation length and days open from .35 to .65. Genetic correlations between milk yield and days dry, lactation length and days dry, and days open and days dry were quite variable. Genetic correlations between milk yield and days open were high for first 1aotation and near zero for third and the average of three consecutive lactations. The phenotypic correlations between lactation length and milk production were near to .60, between days open and milk production were in the range of .05 to .18, between days dry and milk production were negative and ranged from -.08 to -.13. The phenotypic correlations between days open and lactation length were in the range of .39 to .56. Those between lactation length and days dry were small and negative in sign while those between days open and days dry were near zero. Yearly environmental and genetic changes from 1948 to 1973 were estimated using a maximum likelihood method outlined by Henderson et al. (1959). The genetic change in milk production and lactation length occurring in this herd showed four trends differing in magnitude and direction from 1948 through 1970. Estimated changes in real producing ability were obtained by subtraction of the regression coefficients for environmental change from the regression coefficient9 for phenotypic change as outlined by Burnside et al. (1968). The annual estimated trend in real producing ability was 33.16 kgs of M.E. milk or 1.59% of the mean in milk yield.
A study was made of 10,782 lactation records during the period 1950-1973 in the Hacienda Lucerna, Ltd. of Bugalagrande, Colombia. This study was divided in three major parts. The first was to determine the relationships between previous days dry, days open, age of cow, and 305-day milk production. The second was to estimate the heritability and repeatability of 305-day milk production, days open, days dry and lactation length, and to determine the genetic and phenotypic correlations among these traits in the Lucerna herd. In the third part of the study environmental and gene tic changes occurring in this herd over a 24 year period were estimated, and the selection differentials of cows to produce bulls were determined for the period after the herd was closed to outside breeding.
The data consisted of 2,248 first and 8,534 second or later lactations. They were analyzed in three groupings: first, second or later, and all lactations for the first part of the study, and all lactations were considered together for the second and third parts of the study. Multiple regression, least squares and maximum likelihood procedures were compared in the selection of the appropriate method for adjusting 305-day milk yield for age of cow, days open, and previous days dry effects. Results from the maximum likelihood method form the basis of the discussion and results. Length of the previous dry period accounted for 2.14% of the variation in 305-day milk production for second and later lactations. The regressions were non-significant, indicating that adjustment of milk records for days dry was not recommended. The regression equations showed that as the length of the dry period increased from O to 40 days milk production increase only 20.0 kg (44 pounds) in 305-day records.
The effects of days open on milk production accounted for .53, 4.5 and 3.50% of variance for milk production in first, second and later, and all lactations, respectively. Thus, differences in production that can be attributed to days open should be taken into account when production records are compared on second and later lactations.
Age at the beginning of lactation contributed about 12.66% of the variation in milk production. Multiplicative age-adjustment factors were derived for use in this herd. The rate of increase in yield from young age to maturity in the Lucerna herd appears to be higher than reported elsewhere for cattle in temperate climates, and the greater part of the increase seems to occur from the first to the second lactation. This difference may be associated with the low energy intake from pasture alone, since heifers in this herd are reared without grain supplement.
Heritabilities, repeatabilities, genetic correlations, and phenotypic correlations of milk production and associated traits were estimated in the second part of the study. The heritability estimates for these traits were obtained by within-year regression of daughter on dam. The heritability values for milk yield were .29, .27, .18 and .5 for first, second, third and the average of three consecutive lactations, respectively. Heritability ranged from .06 to .25 for lactation length were in the range of -.06 to .17 for days open, and for dry period values were between .18 to .30. Estimated heritabilities for milk yield and associated traits were in agreement with the majority of studies reported for temperate and tropical countries.
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