DH-126
animal sciences dairy
COOPERATIVE EXTENSION SERVICE, PURDUE UNIVERSITY, WEST LAFAYETTE INDIANA 47907
Why Butterfat Tests Vary
W. M. Dillon and P. E. Pritchett, Department of Animal Sciences, Purdue University
Why do butterfat tests vary? The reasons are many—some traced back to the farm itself, others occurring at the bulk plant, and still others related to sampling and testing procedures. The purpose of this publication is to discuss the major causes of fat test variation and suggest ways of minimizing those that are controllable by the dairyman, the milk plant manager, the sampler and the tester.

HERD- AND FARM-RELATED CAUSES OF VARIATION
Breed and Cow Differences
The first reason for variation is simply due to breed differences. Average percent butterfat content in milk for the different breeds of cows are: Ayrshire, 4.0 percent; Brown Swiss, 4.2 percent; Guernsey, 4.9 percent; Holstein, 3.7 percent; and Jersey, 5.1 percent.
Within each breed, however, individual cows may vary considerably from these averages. For example, about one-sixth of the Holstein cows can be expected to test less than 3.3 percent, and one-sixth more than 4.1 percent.
Also, cows will usually test higher in the first month of lactation than in the second through fifth months. After that, they will increase gradually up to the end of lactation.
Ration Deficiencies
More and more, low tests are becoming associated with ration deficiencies, particularly insufficient crude fiber intake. Here are some reasons why:
•	Length and coarseness of crude fiber in the ration are as important as the amount of fiber fed; thus, chopping forage or silage too fine may be causing the low test.
•	Too much corn or other high-energy feed can reduce crude fiber levels below 17 percent of the total ration on a dry basis.
•	Lush pasture is low in fiber and can lead to a lower fat test, even though pounds of milk increases.
•	Pelleted grain mixtures, especially when fed in large amounts, probably do not contribute enough fiber to the total ration.
•	Improper ration formulation also results in a low fat test on a herd basis.
Calendar Variations
Variations may also be seasonal and daily. For example, tests usually run slightly lower in summer and higher in winter. Day-to-day fluctuations of individual cows can be from 1 to nearly 2 percent.
Milking Practices
Poor milking practices can cause fat tests to vary because of incomplete milking. The first milk drawn from the udder may test as low as 1 percent, while strippings may test from 7 to 10 percent. Also, unequal milking intervals will usually cause cows to test higher after the short interval and lower after a long one.
Health Problems
Disease and poor herd health can greatly affect fat content. Mastitis generally results in lowered tests. Cows in heat may have very erratic fat tests.