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Title: Studies in herd analysis
Author: Mahadevan, Pararajasingham
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1950
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1. A statistical study of the causes of variation in milk yield, persistency and butterfat percentage has been made on the basis of 5000 lactation records from 12 leading herds of Ayrshire cattle in south-west Scotland, for the period 1930-1959. 2. The unit of measurement of milk production employed was the yield during the first 180 days of the lactation period. This eliminated the effect of variations in length of current calving interval on milk yield. Persistency was measured as the ratio of A-B to B where A was the milk yield during the first 180 days and B was the initial milk yield during the first 10 weeks of lactation. For the investigations on butterfat percentage, the average butterfat percent- age of the milk secreted during the total lactation period was utilised, since calving interval was found to have little or no influence on it. 3. A study of the accuracy of milk yield deter- minations made under the official Scottish milk record-ing system was carried out by using data from the Kirk-hill herd belonging to the Hannah Institute. This herd was recorded on the official system since its inception, and daily records were also kept. It was found that although the yields by the official recording system were, on the average, about 5% higher than the corresponding yields obtained by daily recording, they are sufficiently accurate to be used for genetical studies 4. Preliminary analyses of the data from the 12 herds investigated, showed an average 180 day milk yield of 6600 lb, for the total population. The mean age at first calving was 2 years and 9 months, and the calving intervals were about 30 days longer for the first lactation than for subsequent ones. Length of dry periods, however, varied very little from one lactation to another. 5. From a study of the non-genetic causes of variation in milk yield, it was concluded that the interrelations among the non-genetic variables should be considered when devising suitable corrections for their influence on milk yield. Corrections were found to be necessary for four factors; month of calving, lactaticin number, age at calving and length of preceding calving interval. 6. The effect of month of calving on milk yield varied significantly between herds, and it was shown that correction factors for month of calving should be calculated on a within-herds basis. The average diff erence in 180 day milk yield between the summer and winter calvers of all herds was about 10 per cent, in favour of winter calvers. 7. A study of the variation of milk yield with age showed that the milk yield of a cow is influenced both by the number of her previous lactations and also by her age at calving. The types of corrections for age employed by previous investigators have been discussed, and it was shown that percentage corrections are the most satisfactory. 8. The variation of milk yield with length of preceding calving interval showed a pronounced positive trend. From an economic point of view, however, the optimum length of calving interval was found to be about 400 days for the first lactation, and about a year for subsequent ones. Corrections for length of preceding calving interval, like those for age, were most satisfactory when they were proportionate and not additive. 9. The effect of corrections for non-genetic influences on the variance in milk yields has been discussed. It was estimated that a reduction of 27. 1% of the total variance was accomplished through these corrections.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available