:feed cost of milk production as affected by 
THE PERCENTAGE FAT CONTENT OF THE MILK 1 
By W. L. Gaines 
Professor and Chief, Milk Production, University of Illinois 
This paper is an elaboration of a sub¬ 
ject which was touched incidentally 
by Gaines and Davidson (8, p. 596) 2 
in a study of the relation between per¬ 
centage fat eonteht and yield of milk. 
In the last analysis feed cost in this 
paper refers to financial cost. Ap¬ 
proach is made, however, through cer¬ 
tain physiological principles and rela¬ 
tionships which permit deductions as to 
feed cost in terms of nutrients. 3 From 
feed cost in terms of nutrients to rela¬ 
tive feed cost in terms of dollars is a 
comparatively simple and direct step 
in the present case. 
From a nutritional standpoint the 
feed cost of producing milk falls in two 
categories: (1) Maintenance require¬ 
ments, that is, the nutrients required 
by the cow as not lactating; (2) lacta¬ 
tion requirements, that is, the addi¬ 
tional nutrients required as lactating. 
The term “maintenance” as here used 
includes the requirements for all meta¬ 
bolic activities except those associated 
with lactation. Such usage is common 
in our present-day feeding standards 
for cows in milk. 
According to our feeding standards 
the nutrients required for maintenance 
and for lactation are approximately 
equal in the case of an 1,100-pound cow 
producing 9,277 pounds of 4 per cent 
milk per year. For a lower yield the 
maintenance requirement is the larger 
and for a higher yield the lactation 
requirement is the larger. Roughly 
speaking, in very good dairy practice 
the two categories of feed cost—main¬ 
tenance and lactation—are of equal 
weight. Together they constitute the 
feed cost of milk production, and the 
problem is to express them as a func¬ 
tion of the percentage fat content of the 
milk. We may consider, first, the lac¬ 
tation requirements; second, the main¬ 
tenance requirements; third, the milk- 
production requirements; that is, the 
sum of the lactation and maintenance 
requirements; and fourth, the transla¬ 
tion of this sum into terms of dollars. 
THE LACTATION REQUIREMENTS 
In the development of feeding stand¬ 
ards the lactation and maintenance 
requirements were early distinguished. 
Haecker carried the development fur¬ 
ther by showing experimentally that 
the lactation requirements are de¬ 
pendent on the composition of the milk as 
well as on the amount of milk. Haecker 
{If., p. 50) made the fundamental ob¬ 
servation “ * * * that there was a 
remarkable uniformity in the net nutri¬ 
ment required for the production of 
a unit of milk solids when nutrients 
consumed and milk solids produced 
were reduced to carbohydrate equiva¬ 
lent.” He accordingly formulated his 
feeding standard on the basis of the 
composition of milks of varying fat 
percentage as indicated by analyses 
made in his own laboratory. His 
formulation on this basis has stood the 
test of several years of practical use 
and the scrutiny of considerable ex¬ 
perimental investigation. 
Haecker’s observation quoted above 
is a key for attack of the lactation re¬ 
quirements. His “ carbohydrate equiv¬ 
alent” of the milk solids is equivalent 
to energy valu£ of the milk solids. And 
the energy value of milk solids in a 
given amount of milk may be simply 
expressed as a function of the fat per¬ 
centage of the milk. If we take one 
pound as our unit amount of milk, and 
measure energy value in large calories, 
letting E represent energy value, and, 
t, percentage fat content of the milk 
we have, 4 I£=49.64 (2.66 + £). If 
Haecker’s generalization is correct the 
nutrients required for lactation are 
directly proportional to 49.64 (2.66+0, 
and may be expressed as Kl (2.66+0 
where K L is a constant. His pub¬ 
lished data permit of detailed exam¬ 
ination as to how well this generaliza¬ 
tion is supported by his experimental 
observations. 
Haecker’s published data pertain 
to 142 individual feeding trials with 
1 Received for publication April 1, 1924—issued March, 1925. 
2 Reference is made by number (italic) to “Literature cited,” p. 601. 
2 The term “nutrients” as used throughout this paper refers to total digestible nutrients of the feed, diges¬ 
tible fat being multiplied by the factor 2.25. 
* For the derivation of this equation see Gaines and Davidson (S, p. 562). 
Journal of Agricultural Research, 
Washington, D. C. 
( 593 ) 
Vol. XXIX, No. 12 
Dec. 15, 1924 
Key No. 111—14 
