|Dairy Herd Improvement Associations (DHIA) specialize in providing management
data for milk producers to use in managing their dairy cows. This work is epitomized
by the monthly "test" of cows in a herd, not only for the amount of production,
but also for a number of components in the milk such butterfat, protein, solids-not-fat,
and somatic cell count.
More recently technology has been developed to test for milk
urea nitrogen (MUN). Milk urea nitrogen is a strong indicator of the nutritional
status of the heard - especially the protein and energy portions of the ration. The
reason this is so important is because feed expenses account for over 50 percent of the
cost of milk production. Underfeeding or overfeeding these two major portions of the
ration mean lost income opportunities for the producer. In addition, excess nitrogen
in the feces and urine may contribute to environmental problems.
Most producers group their cows
into production strings. A production string is any grouping of cows that are
managed the same, especially in regards to feeding. Typical groupings will be based
on production, stage of lactation, reproductive status, and/or age. Other terms for
production strings are pens, corrals, or groups.
DHIA typically tests all of the cows in a herd on a once a month basis. There are
times, however, when a producer would like to know specific information more frequently.
The information may or may not be necessary on an individual cow basis. It
may have importance on a production string basis, however. The producer could use
the results of his sampling to measure the effects of changes in feed or milk component
analysis, discovery of pathogenic organisms, or simply changes in the cows comprising the
To address these needs, DHIA West embarked on a project to determine the best
method to sample groups of cows. Input was solicited from a number of industry
experts and three methods were identified.
The first method suggested was to merely
allow the milk to drip from the milk line before it reached the tank. This method,
known as the "drip" method, took advantage of the area where the milk line is
drained at the end of wash-up. Typically there is either a turn valve or a series
of connections at the low point in the milk line that are loosened to allow the line to
drip dry after the wash-up. By loosening the connections or turning the valve
slightly, milk drips out of the line. A new container is used for each production
string that is milked. A sample is then taken from the milk collected in the
A second method utilized some relatively inexpensive items readily available in the
industry. A 23-gauge hypodermic needle is pierced through an envelope Teflon gasket.
This specialized gasket replaces a normal gasket in the pipeline and is held in
place with the regular pipeline clamp. This method is referred to as the
Once the needle is inserted into the line, a steady stream of milk is produced every
time the milk pump kicks on. Tubing was added to the needle to allow the milk to
drip into a Whirl-pak bag, held in place by alligator clips. The bag is
replaced at the end of the milking of each group of cows.
It is very important that this sampling device is located after the milk pump and
filters. In addition, it needs to be positioned in a location where the only time
there is pressure on the line, is when the pump is on.
The final method used was a technique referred to as the "Southern Counties"
method, named after the DHIA in which the method was first devised. It takes
advantage of the amount of sample remaining after the lab vial is full on the normal DHIA
test day. The remaining milk from each cow is co-mingled in a container and a sample
is taken after each pen is milked. The remaining milk is discarded so that there is
no carryover between pens.
To analyze which one of these methods was most desirable, DHIA West in
conjunction with the University of California Cooperative Extension and California State
University, Fresno conducted a trial at the CSUF dairy. Over a period of ten
milkings, two milkings a day for five days, the herd was sampled using the techniques
outlined earlier. All individual cow samples as well as the production string
samples were analyzed for butterfat, protein, lactose, solids-not-fat, somatic cell count,
and milk urea nitrogen.
The weighted average of the results of the individual cows comprising each pen were
used as the control. The component results of each sampling method were compared to
the weighted average. The mean and the standard deviation of the differences were
determined for each method.
Discussion & Conclusion
Based on the results, the drip and Southern Counties method generally showed a lower
mean difference than the needle method. However, among the sampling methods, the
standard deviation of the needle method was less than the other two methods for most milk
components (Table 1).
The drip as well as the Southern Counties method tends to disproportionately weight the
sample, thereby causing greater variation in the results. With the drip method, the
milk that is held in the portion of the pipeline by the drain when the pump is not engaged
will have the greatest effect on the sample. in addition, the Southern Counties
method can only be performed on test day.
There would be limited applications for the Southern Counties method because some
components are already being determined as part of the DHIA monthly test. However,
if the producer did not want to pay the cost of the analysis for each cow, the Southern
Counties method would be an appropriate method for string averages on selected components.
Because results of the needle method have been shown to have less variation than the
other methods, does not require expensive equipment, and is relatively simple to perform,
the conclusion is that the needle method is the most desired method for measuring the
changes in production string management.
Table 1. Mean and Standard Deviation of Differences between Production String Sampling
Methods and the Weighted Average of Individual Cows Comprising a Production String