32 
Journal of Agricultural Research 
Vol. VIII, No. a 
age for all ewes, 46 in number, gives practically similar results for three 
successive lactation periods. 
Fat is still quantitatively the most variable factor of the solids in 
milk from breeds of animals in which selection has been practiced for 
generations to increase the fat content. Such selection has so far failed 
to stabilize the percentage of fat in milk partly because no definite 
limits have been set to the standard. 
. If selection without a definite limited standard as an objective results 
in instability, a possible inference would be that the milk of animals in 
which no selection is made in either the quantitative or qualitative 
factors would become more stable in this respect. The literature cited 
on this point is corroborated by the data shown in this paper that this 
is not the case. In our mutton breeds of sheep, in which no endeavor 
has been made to modify the fat content or increase the milk yield by 
selection, there exists apparently an equally unstable variation as regards 
the percentage of fat in milk. 
In breeds of animals that are kept for dairy purposes this variability 
is the factor of basic importance. In breeds of sheep that are not kept 
for dairy purposes and where the entire yield is suckled by their young, 
a variation in fat content of milk can assume importance only inasmuch 
as it may be a limiting factor in the rapid growth of suckling lambs. This 
question is partly answered in Table III, which shows increases in the 
weights of lambs from milk varying in fat from 2 to 10 per cent. The 
increase shown from the milk richest in fat is no greater than that from 
the milk which has the lowest fat content. In fact, the summary shows 
the highest gains (37 pounds) from 2 to 3 per cent milk, and the lowest 
gains (18 pounds) from milk testing 10 per cent or over. The limiting 
factor in this case was apparently the quantity of milk, as the high 
gains were made from good to high milk yield and the low gains from 
poor to good milk yield. On the elimination of these extreme cases a 
considerable fluctuation in weight increase is found that bears no defi¬ 
nite relationship to the percentage of fat contained in the milk within 
the limits shown here. 
Table HI .—Average increase of lambs in weight at 8 weeks on varying quantities of 
dams’ milk varying in fat 
Num¬ 
ber of 
ewes. 
Milk yield (estimated). 
Aver¬ 
age fat 
test. 
Weight increase. 
Fat content..per cent.. 
2 to 3 
3 to 4 
4 to 5 
5 to 6 
6 to 7 
7 to 8 
8 to 9 
9 to 10 
10 or 
over. 
Aver¬ 
age. 
13 
78 
35 
12 
138 
High. 
Good. 
Fair. 
Poor. 
Average. 
Per ct. 
4. 82 
6. is 
6.05 
6.03 
Lbs. 
42. 0 
32- 5 
Lbs. 
35 -o 
3 i*o 
25. 0 
15.0 
Lbs. 
29. 0 
36. 0 
22. s 
21. O 
Lbs. 
38. 2 
31- 0 
27-5 
19. 0 
Lbs. 
34 -o 
32. 0 
26. s 
26. 0 
Lbs. 
42. 0 
33 -o 
24. 0 
19. 0 
Lbs. 
29. 0 
310 
26. 0 
22.0 
Lbs. 
25. 0 
23. 0 
Lbs. 
25.0 
22.0 
9. 0 
Lbs. 
34 -o 
29-3 
24.6 
19.0 
37 -o 
26. 5 
27 . I 
30. 0 
29. 6 
29- 5 
27.0 
24.0 
18.7 
