THE SELECTION PROBLEM IN ANIMAL BREEDING 503 
given period is a logarithmic function of her age. His investigations 
further show that the curve of milk production with respect to age 1s 
of the general form 
Y=a+b6X + cX?+4 dlogX. 
In this equation, Y denotes the amount of milk produced in a given 
time, X the age of the cow, and a, b, c, and d are constants the value of 
which must be calculated for different breeds. Stated in general terms, 
therefore, milk production increases with age until at about 5 years 
a maximum is reached, after which it decreases with advancing age. 
With reference to milk production within a given lactation period, it is 
of course a matter of common experience that milk production decreases 
gradually during the lactation period. These two factors have been 
taken into account by Pearl in preparing a table of comparative effi- 
ciency percentages for dairy cows. In this table the maximum efficiency 
is set at 100 per cent. and the comparative excellence at any given age 
or stage of lactation is given a value in per cent. of this maximum value. 
Pearl has calculated such a table for 24 months of which 10 months 
only are given in Table LXVI. This table provides a very satis- 
factory method of comparing a set of individual records from cows of 
different ages and in different stages of lactation. 
TaBLe LXVI.—Erriciency PERCENTAGES FoR MILK PropucTION IN Datry CATTLE 
(Data of Pearl) 
Months since freshening 
Age of cow in years and months 
an 
Peeeieee, Ss seen | eis or|iera 
| Per | Per} Per | Per | Per | Per | Per} Per] Per | Per 
cent.| cent.| cent.| cent.| cent.) cent.| cent.] cent.| cent.| cent. 
1:6 to 1:11 58 | 54 | 51 | 47 | 44 | 41 | 37 | 34 | 30 | 27 
2:0 to 2:5 73 | 69 | 64 | 60 | 56 | 52 | 48 | 43 | 39 | 35 
2:6 to 2:11 82 | 77 | 72 | 67 | 62 | 57 | 52 | 47 | 41 | 36 
3:0 to 3:5 89 | 83 | 77 | 71 | 66 | 60 | 54 | 48 | 43 | 37 
3:6 to 3:11 93 | 87 | 81 | 75 | 69 | 62 | 56 | 50 | 44 | 38 
4:0 to 4:11 97 | 91 | 84 | 78 | 71 | 65 | 58 | 52 | 45 | 39 
5:0 to 5:11 100 | 93 | 86 | 79 | 72 | 66 | 59 | 53 | 46 | 39 
6:0 to 6:11 100 | 93 | 86 | 79 | 72 | 65 | 58 | 52 | 45 | 38 
7:0 to 7:11 99 | 92 | 85 | 78 | 71 | 64 | 57 | 51 | 44 | 37 
8:00to 8:11 97 | 90 | 84 | 77 | 70 | 63 | 56 | 50 | 48 | 37 
9:00to 9:11 | 94 | 88 | 82 | 75 | 69 | 62 | 55 | 49 | 43 | 36 
10:00 to 10: 11 91 | 85 | 79 | 73 | 67 | 61 | 54 | 48 | 42 | 36 
11:00 to 11:11 88 2|76 | 71 | 65 | 59 | 53 | 47 | 41 | 35 
12:00 to 12:11 85 | 80 | 74 | 68 | 63 | 57 | 52 | 46 | 40 | 35 
13: 00 to 13: 11 82 | 76 | 71 | 66 | 61 | 55 | 50 | 45 | 39 | 34 
Digitized by Microsoft® 
