47. The Errors Involved in Certain Methods of Estimating the Lactation Yield of Milk and Butterfat

Abstract

An analysis of the data collected at the Agricultural Research Institute for Northern Ireland, Hillsborough, County Down, was undertaken to find if the system of milk recording practised in Northern Ireland is satisfactory.The standard deviations of the errors in the calculated milk yields, on a weekly and 2-weekly basis, of 24 cows were found. It is shown that weekly weighings give results which approximate closely to the true milk yield.By basing the calculated butterfat yields on tests made at intervals of from 1 to 8 weeks, the standard deviations of the errors in the calculated butterfat percentages for each interval, as well as the maxmium errors, were found. It is shown that as the interval between the tests is lengthened, the errors increase. With tests made at intervals of 6 weeks, most of the errors in the calculated butterfat percentages will be less than 8-5 per cent., but in some cases the calculated butterfat percentage may differ from the true butterfat percentage by over 12 per cent.In order to ensure that the errors in calculated butterfat percentages do not exceed 10 per cent., the interval between butterfat tests should not exceed 1 month.