142 A MANUAL OF MENDELISM 



Briefly stated, the deviations due to these causes are, 

 approximately, as follows : 



(a) A cow does not come to her full yield till she has 

 had four or five calves, that is, when she is about 

 six years old. If the full normal yield be put at 100, 

 then, for cows having their first calf when from 30 to 

 33 months old, the previous yields may be set down 

 as : after the first calf 66, second 80, third 90, fourth 97. 

 For cows having their first calves at an earlier age, 

 the figures for the earlier lactations must be lowered 

 slightly. From observations on 110 cows in Lord 

 Rayleigh's herds, Mr. W. Gavin, of Trinity College, 

 Cambridge, got the following figures : first calf 60*1, 

 second 80, third 90, fourth 95-6, fifth 98-7, sixth 100. 



(b) A cow which calves in winter gives from 10 to 

 20 per cent, more milk than one which calves in summer. 

 The extremes lie between October and February on 

 the one hand and May and August on the other, 



(c) The normal time between calves is twelve months. 

 For one month extra between the last and the next calf, 

 the yield for the lactation is increased from about 

 8 to 12 per cent., for two months extra by about 14 

 to 20 per cent. For a month short of the normal twelve 

 between calves, the decrease is very variable, but may 

 be put roughly at from 10 to 20 per cent. 



(d) The yield of a cow in low condition before calving 

 may be decreased thereby from 10 to 20 per cent, in 

 the succeeding lactation period. 



(e) The effects of poor feeding, discomfort, and ill- 

 health cannot be stated, because of their great varia- 

 tion, and all data so affected must be neglected. 



As with poultry, however, an observation was made 

 by which a cow's yield can be predicted early in the 



