| Popis: |
Summary Three facts have been given which support the belief that persistency of lactation in dairy cattle is an inherited characteristic: 1) the variation in persistency values is much greater among non-related animals than among closely related ones, 2) most animals tend to keep their characteristic lactation curves throughout their productive period, 3) the influence of the sire or dam is easily detected in their progeny. The first lactation usually gives a fair indication of future performance. The persistency values for the second lactation are usually about 10 per cent lower than the first (as measured by the formula P = 4 / 9 X 2 X 1 + 1 / 3 X 3 X 2 + 2 / 9 X 4 X 3 \) and subsequent lactations are very similar to the second. Total production and persistency are closely correlated; however, some variation in production must be allowed before attempting to predict production from persistency values alone. The association of total production and persistency is probably of a curvilinear nature rather than linear. The combining of data from several different breeds for the purpose of studying inherited factors in dairy cattle is usually an unreliable procedure. The variation in persistency, however, seems to be the least affected by combining data from several breeds. It is postulated that a major portion of the variation in persistency is probably the result of the inheritance of factors or genes which govern the development and rate of function of various endocrine glands, and the interaction and interdependence of such glands, or the inherited or acquired ability of various tissues to respond to various glandular secretions. The mode of inheritance is undoubtedly complex. |
