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.
The significance of grading-up in the Ayrshire cattle population of Great Britain
The grading-up system, in the Ayrshire Cattle breed, whereby the descendants of non-pedigree cows enter the registered (Herd Book) population, is studied principally from three aspects: (1) its numerical importance in the population; (2) the breeding structure of herds practising grading-up, and (3) the possible genetic effect of grading-up on milk and fat yield. Cows at various stages of grading-up are noted in an Appendix to the Herd Book. The numerical importance of the grading-up system has varied with the rate of expansion of the breed as a whole—being greatest during the most rapid expansion—but, at all times, Appendix cows have had more registered female offspring than have pedigree cows. The volume of grading-up was regulated by the number of cows in Appendix B. Some 6% of the pedigree females registered in the last 30 years have Appendix A mothers and a further 5% or so owe their existence to grading-up during the last 30 years. On average, the proportion of herds registering bulls is not greatly different among the herds which grade-up and those which do not. Similarly, the ratio of males to pedigree females registered was about the same in both types of herd. None-theless, there appears to be considerable selection against Appendix animals in the ancestry of bulls compared with cows, and none of the large bullbreeding studs appear to pursue a grading-up policy. A greater proportion of Ayrshire herds were grading-up in Scotland than in England. Entry into Appendix B of the Herd Book is confined to cows reaching a qualifying yield. The daughters (A) of these B cows mated to unselected pedigree bulls were found to yield significantly more in their first lactation (36 gal. milk and 14 lb. butterfat) than contemporary pedigree Ayrshire heifers in the same Scottish milk recorded herds in 1955. Part, if not all, of this superiority is thought to be genetic.
Water Quality Aspects of Irrigation and Drainage: Past History and Future Challenges for Civil Engineers
This paper presents a brief history of irrigation and drainage related to ASCE activities on its Jubilee. This paper discusses legislation and policies that affect irrigation and drainage practices, water quality constituents of increasing concern in irrigation and drainage practices, and presents a prognosis on the future of declining freshwater resources available for irrigated agriculture and growing water quality problems in irrigation and drainage. Civil engineers in ASCE’s Irrigation and Drainage Division have compiled an 80-year history of highly meritorious service and accomplishments. In the next millennium, civil engineers will face a formidable challenge in managing and protecting the precious freshwater resources in the U.S.