I frequently receive questions regarding differences in milk butterfat levels between the dairy producer's milk processor and DHIA. Tracking down the reasons can be very educational. I would like to cover some of the areas that everyone should consider when trying to evaluate these differences.
Some find it hard to believe, but daily fat tests can vary by as much as plus or minus 0.5%. The magnitude of the swing can be influenced by such things as herd size and milking schedules. For example, large herds (larger sample size) and regular milking times will decrease the variability.
In general, DHIA and bulk tank samples are usually not measuring exactly the same milk. DHIA samples are taken at one or two milkings (out of a possible 2 or 3) and are averages of individual cow values (not a weighted average). Bulk tank samples may be taken from milk picked up every-other-day, every day, or partial pick-ups and are weighted averages of all cows milked during that time period. Cows may freshen or dry up and not be consistent with the animals tested by DHIA. In general, I don't expect these tests to be identical.
I have had dairy farmers tell me that the instrumentation is faulty for the entity they feel is incorrect. I did a little experiment where I had the truck driver for a major processor in Utah collect one sample for himself and one for me. I took my sample to the DHIA laboratory and matched my results with the processor's results for the same sample. In 85 samples, the average for the processor was 3.53% and the DHIA lab was 3.56% These averages are essentially identical considering that they were analyzed on two different machines. However, on a day-to-day level, samples varied by a maximum difference of plus or minus 0.15%. It doesn't mean one lab is better than another, just that they are different.
Sources of Variation
1. Sample collection and handling - The sample used by either the processor or DHIA has to be collected by either a truck driver or DHIA technician. If the starting sample is not properly collected or mixed in order to get a representative, composite sample, then a source of variation has been added. Bulk tank milk should be agitated for 5 minutes before a sample is collected.
Other sources of variability can be the milking practices of the dairy farmer on the day of a DHIA test. An example might be if the vacuum is not adequately adjusted to compensate for having meters on the systems. Butterfat may be lowered under this situation. Milking intervals, frequency of sample collection, such as if the sample was taken from the morning or evening milking, or one milk sample out of three for DHIA tests such as AM-PM sampling, and measuring devices can all contribute to the differences.
In general, improper sample collection and handling is a major source of differences.
2. Bulk tank problems (cooling) - If the bulk tank is improperly cooling or churning the milk, it can cause huge discrepancies between the processor and DHIA. I know from experience that this happens. If insufficient milk is in the tank when the compressor comes on, then freezing may occur. This tends to lower the fat test from the processor. Also, excessive agitation of the milk at warmer temperatures can make butter. You should never see clumps of butterfat floating in the milk or on the paddles. This will also decrease the fat test from the processor.
3. Day-to-day variation - Weather, feeding practices, ration composition, season of year, stage of lactation, health of cows, and age of cows are all factors which may cause differences. Keeping a daily farm diary can be very useful in troubleshooting problems, if they arise.
While normal differences between DHIA and the processor will occur, major discrepancies should be investigated immediately. Focusing on sample collection and handling, and bulk tank problems can usually solve most problems. If you can't remedy the problem, develop a team approach for attacking your problem. Be hesitant about finger pointing or placing blame because both entities may be correct.