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Scientific Proceedings^ Royal Dublin Society. 







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A. 



their ages passed seven years, and lie re- 

 marked that their j'ields " cannot be strictly 

 comparable with the others, since as soon 

 as one ceases to deal with the same number 

 of cows throughout, the influence of selec- 

 tion must come in. It is probable that only 

 the best or the healthiest of the 336 cows 

 would tend to remain in the herd after 

 their fifth calf." 



A scale to show how milk yield varies 

 with length of lactation was even more 

 necessary, for though the cow's capacity can 

 be told from her yield at the flush, the yield 

 for the lactation period is what is nearly 

 always published ; and as the intervals be- 

 tween successive calvings are not always 

 the same, a scale had to be found which 

 would indicate how much should be added 

 to short time and how much subtracted 

 from long time yields to bring them to the 

 normal. A normal yield is one in which 

 the calf which induced it is followed by 

 another at about twelve months. 



In the absence of disturbing causes, the 

 cow's yield rises quickly for a week or two, 

 remains near the maximum for two or three 

 more, and then slowly but gradually falls. 

 If the cow is not in calf again, the yield 

 may continue, falling slowly all the time, 

 for fifteen or eighteen months ; if she is in 

 calf, it begins, at a certain time, to fall 

 more quickly, and ends sooner. This cer- 

 tain time Mr. Gavin found to be when the 

 cow had been in calf about sixteen weeks. 

 Till about this time the cow's yield is not 

 affected by her being in calf. 'Thus the 

 average yield of a number of cows till they 

 are about sixteen weeks in calf gives their 

 average yield, over the same time, as if 

 they were not in calf. The thick line in 

 the following diagram, which is constructed 

 from the average of Mr. Gavin's figures,' 

 shows the rise and fall in yield for cows 

 which are not in calf. The line is con- 

 tinued at both ends by dotted lines : at the 

 beginning, because some of the cows were 

 suckling calves during the two first weeks 

 and the recorded yields are not the true 

 yields ; at the end, because Mr. Gavin's 

 figures stop at the thirty-sixth week. 



> " Journal of Agricultural Science," vol. v, p. 314. 



