Factors influencing average milk yield of herds at two levels of production.
An experiment was started in 1950 designed to find out what proportion of the difference in milk yield between herds is genetic in origin. Sixty-five pairs of twins were split at birth equally between farms having high-yielding herds and farms having low-yielding herds. A previous paper on this experiment reported on the growth of the twins, the present paper on milk yield and on management practices in the herds. Twenty-four pairs of twins survived to complete one lactation and these provide an estimate of 24%±18% for the genetic part of the difference between the herds. Aspects of rearing of the twins and of management of the herds were examined. Twins in the high-yielding herds were served and calved earlier than those in the low-yielding group but they had a longer interval between service and calving. Differences between average heifer yields and average cow yields were from one-and-a-half to two times greater in the high- than in the low-yielding herd group. There were no obvious quantitative differences in feeding practice between the two groups although differences in body size suggest that twins in the high group were done better. For the age at first service there was a positive rank correlation of 0·30 between body size differences and subsequent differences in first lactation milk yield. There was no single factor of management or environment which appeared to be the cause of the yield difference between the herds.
Effect of Genetic Differences Among Herds on Accuracy of Selection and Expected Genetic Change
Abstract Improperly accounting for genetic differences among herds affects the accuracy of estimates of breeding values. The loss is expected genetic change resulting from this lower accuracy was quantified. Expected genetic change was calculated by sequential selection theory and was expressed in terms of accuracy of estimates of sire and cow breeding values. Simple linear models were used to explain milk production. Selection index theory was used to calculate accuracy as the expected correlation among indices and breeding values. The expected genetic change was calculated and plotted for variations in accuracy of estimates of breeding values. Expected genetic change increased from .4 to .6 kg and 1.1 to 1.2 kg of milk per cow per year for each .01 increase in accuracy of estimates of breeding values of cows and sires, respectively. Evaluation of the potential for genetic differences among herds indicated that the maximum improvement in the expected genetic progress would be around 10 kg of milk per cow per year.
45. The Ayrshire Breed of Cattle: A Genetic Study
1. A study has been made of the degree of inbreeding in the Ayrshire breed of cattle. 2. The coefficient of inbreeding for the whole breed, as calculated by Wright's “Approximate (or Short) Method,” showed a progressive increase from nil in 1877 to a mean value of 5·3 in 1927. 3. No difference was found in the coefficient of inbreeding as between registered bulls and cows. 4. A large proportion of the inbreeding was traced to two foundation sires, Burnhouses (8) and Hover-a-Blink of Drumjoan (892). 5. Using Wright's “Long Method,” it was found that high milk-yielding cows showed a markedly lower coefficient of inbreeding than the breed average. On the other hand, when the coefficient of inbreeding of high milk-yielding cows (over 1000 gallons) was compared with that of cows giving average yields (under 800 gallons) it appeared that inbreeding itself had no detrimental effect on milk yield. 6. In 1923, 39 per cent, of the inbreeding ties of high milk-yielding cows were traced to the Drumjoan herd, as against the next highest herd which had only 6·37 per cent, of ties. Of the total coefficient of inbreeding the Drumjoan herd was responsible for 18·36 per cent., whereas the next highest herd was only responsible for 8·35 per cent. The 1928 results also demonstrated the marked influence of the Drumjoan herd. Practically 50 percent, of these ties were traced to animals related to the foundation sire, Hover-a-Blink of Drumjoan. These results demonstrate the profound influence which one herd, and indeed one bull, may exert in the genetic construction of a recently established breed. 7. It is pointed out that Wright's “Long Method,” when applied to breeds of relatively recent origin, gives far more reliable results than his “Short Method.”