Medicating Groups of Pigs: What Weight of Pigs Do We Treat?

How do you get a proper dose of antimicrobials into a group of pigs with respiratory diseases? That was the question asked, and answered, by Dr Tom Kniffen, Technical Services Manager for Merck Animal Health at the World Pork Expo in Des Moines, Iowa, US earlier this month. Senior editor, Chris Wright, reports for ThePigSite.
calendar icon 29 June 2012
clock icon 6 minute read

Dr Kniffen used elements of statistics, pharmacokinetics, pharmacodynamics and pig weights to evaluate the dose of antimicrobials pigs in a group receive when they are all medicated with the same dose of an antimicrobial. This evaluation, which used a data set from a 500–pig farm, then led to recommendations on how to best to proceed.

The rationale for doing this evaluation was three–fold: firstly, antimicrobial use in animal agriculture is being scrutinized and challenged; second, in order achieve a better biologic result from more accurate dosing and third, to develop a more cost–effective strategy for using antimicrobials.

All antimicrobials work in one of two ways: concentration–dependent or time–dependent, according to Dr Knippen.

A few of the antibiotics used in pigs today depend completely on getting the right volume of the medication into the animal. You must give the correct full dose to get the proper response. If you want more therapy, you simply increase the concentration. While an increased concentration does raise costs and presents possible toxicity issues, it is also an easy way to get a better response.

The time–dependent antimicrobials, which represent a majority of the antibiotics used in pigs, work on the basis of how long they remain in the pig’s system. You still have to give the animal a full dose, but it is not the amount of antibiotic the pig receives but rather how long it stays in the pig’s system that makes it effective. To maximise a time–dependent antibiotic, you have to dose more frequently. In other words, you need more injections.

If you double the dose, you just increase duration in the pig by a half–life. In other words, if you double dose an antibiotic that normally lasts seven days, it will last nine days, not 14.

Dr Kniffen warned that before using a medication, one has to watch for resistant bacteria to be sure the vaccinations will even work.

Dosage Regimen

There are number of options that can be used, when injecting a group of animals:

  • Treat the average weight of the group. All the pigs get the same dose.
  • Treat the weight that the pigs should be, according to the standards.
  • Use the same weight for all the pigs.
  • Treat for the lightest pigs in the herd.
  • Treat for the heaviest pigs in the herd.

Dr Kniffen then showed different ways to analyse statistically the data on the 493 pigs in the herd being studied. These pigs were weighed at seven days and then again at 21 days, which is weaning age in the US.

The mean or average weight of the pigs at seven days was 5.9lbs. The weights actually ranged from a high of 8.3lbs to a low of 3.7lbs. At 21 days, this herd weighed an average of 13.9lbs, ranging from 8.3 to 19.3lbs. These were normal weight variations for a group of pigs.

But the question still remains: How do treat a herd with such a wide variation in weights?

The coefficient of variation, which is a measure of the amount of variation in the data, did not change from seven and 21 days of age. In other words, the big pigs at seven days were also the big pigs at 21 days and the lightest pigs at seven days were also the lightest pigs at 21 days.

Quartiles and Percentiles

These statistical terms, quartiles and percentiles, become very important concepts in how to treat groups of pigs. The quartile concept is simply breaking up the numbers into four groups, where each grouping represents 25 per cent.

If you treat the pigs using the 50th percentile weight as your guide, 5.9lbs at seven days, then the light pigs get too much medicine, while the heavy pigs do not get enough. This is due to the wide variations in the weights of the herd, which is relatively common. No matter what percentile level you choose as your guide for medicating, you will have the same problem every time.

However, if you use the quartile concept, and you take the 3rd quartile weight for this herd, which was 7.2lbs at seven days, you will be able to get a full dose into 80 per cent of the pigs. However, the lightest pigs will get 195 per cent of the dose. This should not be a problem with most antibiotics, except for the cost.

Using the 3rd quartile as the guide, you are using a higher weight than going with the 50th percentile, and therefore there will be under–dosing of the heavier pigs.

Hybrid Approach

You can also use a hybrid approach, Dr Kniffen said, which consists of using one syringe with a dose that uses the average weight of group, but which will be used to treat only the bottom 50 per cent of the pigs, the lightest ones.

Then you use a second syringe, using the 3rd quartile weight as the basis for the dose, to treat only the heaviest pigs.

During the question and answer period at the end of the session, it was pointed out that the hybrid concept is the one many pig producers in the US currently use.


Every pig needs to be weighed and the dosage calculated, Dr Kniffen said but in reality, that never happens.

If you treat using the mean weight, then 30 per cent of the pigs will be over–dosed and 30 per cent will be under–dosed.

To get a full dose of antibiotics into 75 per cent of the pigs, you have to treat for a pig weight 20 per cent above the group’s mean weight.

Finally, Dr Kniffen emphasised that you cannot treat pigs based on mean weight unless you want to get a full dose into only 50 per cent of the group.

June 2012

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