Dec. 15, 1924 
597 
Feed Cost of Milk Production 
The next question is as to the rela¬ 
tion between fat percentage and milk 
yield. Gaines and Davidson (3) have 
shown that in so far as yield is affected 
by percentage fat content, the milk 
yield is inversely proportional to the 
energy value of the milk solids per 
unit milk and may be expressed as: 
2 0++ + * n a * s a constant, 
affected in value only by factors other 
than fat percentage. The mainte¬ 
nance requirements per pound of milk, 
Nm , are therefore 
K m (2.66 + 0 
a 
and 
Nm — Km ( 2.66 + 0 
where K M is a constant so f ar 
as affected by fat percentage. 
Since size is independent of fat per¬ 
centage, it follows that growth is like¬ 
wise independent of fat percentage; 
hence the inclusion of growth require¬ 
ments in the maintenance require¬ 
ments involves no error from the pres¬ 
ent standpoint. The same reasoning 
may be applied To foetal growth. Ob¬ 
viously, however, when we include 
different breeds there may be some 
correlation (presumably negative) be¬ 
tween fat percentage and size. If the 
lower testing cow is a larger cow this 
will imply increased maintenance and 
increased yield, both of which are 
factors in the maintenance require¬ 
ments per pound of milk. The point 
is further considered below. 
THE MILK PRODUCTION 
REQUIREMENTS 
The total nutrients, N T , required 
for milk production are the sum of the 
lactation and maintenance require¬ 
ments. That is, Nt—K l (2.66 + 0 + 
Km (2.66 + 0 =K t (2.66 + 0* 
As an intrabreed relation, then, the 
nutrients required for milk production, 
so far as affected by fat percentage , are 
K T} (2.66 + 0 where Kt is a constant 
( K l +K m )* (The value of K T is, of 
course, greatly affected by factors 
other than fat percentage.) 
As an interbreed proposition there 
may be a relation between size and fat 
percentage such as to result in a dif¬ 
ference in the value of the constant 
K t for the different breeds. An in¬ 
crease in maintenance requirements 
tends to increase the value of Km and 
an increase in production tends to 
decrease the value of Km * Since the 
maintenance requirements vary 
directly as the weight of the cow, it 
follows that the value of Km, and 
hence also the value of Kt, will vary 
unless production [milk yield X (2.66 
+ 0] varies also directly as the weight 
of the cow. The relation between 
size of cow and production, as between 
different breeds, has not been ade¬ 
quately determined. 5 As a practical 
way of considering the matter, how¬ 
ever, we may calculate the values of 
Kt for Holstein cows and Jersey cows, 
as representing the extremes in size 
and percentage fat content of milk 
among our common dairy breeds, and 
note how the values of K T for these 
two breeds compare. 
Table IV gives the values of cal¬ 
culated on the basis of Haecker’s quan¬ 
titative relations as, 
0. 049(2. 66 + Qmilk yield+(0. 00792X365X weight) 
milk yield (2. 66 + 1) 
5 Various studies of the relation between size of cow and yield indicate clearly that production varies 
with size. Brody, Ragsdale, and Turner (/), from the yearly official test records of Jersey cows found that 
the relation between size and fat yield is expressed by the general equation, F=a W+b, in which F is fat 
yield, in pounds, W is weight of cow, in pounds, and a and b are constants. Accordingly, the nutrients 
for maintenance per pound of fat yielded are and increase with increasing value of W; that is.. 
a W+b 
the smaller cow is more efficient than the larger. Where age is held constant a has a value near 0.2 and b 
near 250, and the variation in nutrients for maintenance per pound of fat, with variations in weight of the 
cow, is of considerable magnitude. When cows of ages 2 to 9 years are included the following equation, 
derived from their Table II and using their notation, describes the relation: F= 0.373 W+ 104. 
Pearson (unpublished data, Illinois Experiment Station) in a study of the yearly records of 642 grade 
Holstein cows, unselected as to age, found a positive correlation between weight and milk yield, r=0.304 
±0.024. The regression line is linear, and the mean milk yields of the several weight classes are closely 
expressed as 6.15 times the weight. The mean fat percentage was 3.42±0.0l and since fat percentage is 
independent of weight, Pearson's data may be translated in terms of fat, using the above notation as: 
F= 0.210 W. From the present standpoint this equation differs fundamentally from the one above for 
Jersey cows. Whether the difference is a matter of breed (heredity) or environment can not be said. 
