Benchmarking Data & Formulas

As outlined by Mills et al. (2018), there are several types of data that can be used, but different producers must collect the same data components so that the information can be compared between farms. There are also different ways of adjusting the data to account for the weight of animals being treated, the number of days in the production cycle, and the type of antimicrobial being used.

Here are some examples of different metrics that can be used to measure antimicrobial use⁸:

• 1. Total mg

  Total mg = (total mls, number or kg of product sold or used) x (mg/ml, mg/unit, or mg/kg of active ingredient)

  • Likely the simplest metric to use 
  • Ignores the size/weight of the animals as well as the number of animals that are on the farm. Therefore, you can’t compare farms with different numbers of animals or farms that raise different age classes of animals 
  • Doesn’t take into account different dose rates and lengths of treatment

  1. • As such, this metric may be lower for antimicrobials of the highest importance, which tend to be more potent and given in smaller volumes, but which may have a greater impact on the emergence of AMR. 

• 2. Total mg/kg

  Total mg/kg = total mg of antimicrobials / total weight of animals at risk of treatment (kg)

  • Another simple metric that takes into account the animal weight and number of animals 
  • It may be difficult to get individual weights on each animal, but using standardized weights may not be accurate. 
    • In addition, different antimicrobials are used in different age classes of animals
    • Using an inaccurate weight for the animals at risk of treatment would lead to an over or underestimation of actual antimicrobial use in mg/kg as shown in the table above
  • Does not take into account different dose rates and course lengths across different antimicrobials
    • As such, this metric may be lower for antimicrobials of the highest importance, which tend to be more potent and given in smaller volumes, but which may have a greater impact on the emergence of AMR
      • For example, Ceftiofur (Excenel, Zoetis) is dosed at 6.6 mg/kg in cattle, whereas long acting oxytetracycline (Oxyvet 200 LA) is dosed at  20 mg/kg in cattle

   

  Here is an example calculation below for 2 farms of similar numbers of animals with differing sizes:

  Farm	Total mg of antimicrobial used	Number of beef cattle	Mean cattle weight (kg)	Total mg/kg using meancattle weight for each farm	Total mg/kg using standardized weight of 300 kg10
  Farm 1	2,000,000	200	700	14.3	33.3
  Farm 2	2,000,000	200	400	25	33.3

• 3. Defined Daily Dose (DDD)

  DDD = total use (mg) / (daily dose (mg/kg) x total weight of cattle at risk of treatment (kg))

  • This calculation can be carried out for each antimicrobial category and added together to get the total amount
    • Can be calculated using mg/kg/d (ddd) where each kg of the animal is taken into consideration or using g/animal/d using standardized weights (DDD)
  • There are several advantages to using this metric including:
    • Accounting for animal weight and number of cattle
    • Accounting for dose rate differences across different antimicrobials
    • Also used by several countries and is a defined metric in Canada with specific calculations for broiler chickens and turkeys⁹, pigs⁹, and cattle¹⁰
      • This can allow comparison between different countries
  • This metric is more complex than the previous two, and has some additional limitations including:
    • If using standardized weights or defined dose rate, they may not be completely accurate
    • Some routes of administration for specific drugs do not have defined dose values
    • Doesn’t take into account the number of courses an animal receives 

• 4. Course dose metrics

  Course dose metric = total use (mg) / (course dose (mg/kg) x total weight of cattle at risk of treatment (kg))

  • This calculation is carried out for each antimicrobial category and added together to get the total amount
    • Can be calculated using mg/kg/course (dcd) where each kg of the animal is taken into consideration or using g/animal/course using standardized weights (DCD)
  • The advantage of using this metric over DDD is that it assigns the number of courses an animal receives and takes into account the daily dose and the course length 
  • It is also supported by several countries and is a defined metric in Canada with specific calculations for broiler chickens and turkeys⁹, pigs⁹, and cattle¹⁰
    • This can allow comparison between different countries
  • It has similar limitations to DDD, however, as mentioned above it takes into account the course of treatment

Applying Defined Daily Dose and Defined Course Dose 

To set up a benchmarking process, it is important to ensure that reliable antimicrobial use data is available. One potential source of data is veterinary clinic dispensing data¹¹, assuming that all antimicrobial products are purchased from participating clinics. 

Below is an example of how AMU could be calculated using a dairy farm with 200 milking and dry cows and a veterinary clinic using DDD and DCD:

• Step 1: Determine the quantity of each antimicrobial used

  The following formula is used to calculate the necessary amount (mg) of antimicrobial needed. This calculation needs to be completed for each antimicrobial dispensed to the farm:

  Quantity (mg) of antimicrobial = number of units of product (e.g. box, bottle, tube, etc.) x antimicrobial concentration (mg/unit)

  Convert to g of antimicrobial = mg of antimicrobial / 1000

  Using the table below, we evaluate the example dairy farm that has 200 milking and dry cows and the number of products that were dispensed to this farm from the veterinary clinic:

  Antimicrobial	Route	Number of units purchased	Volume of unit	Total volume	mg antimicrobial per unit	Total quantity of antimicrobial (mg)	Total quantity of antimicrobial (g)
  Ceftiofur	Injectable	15	100 mL	1,500 mL	50 mg/mL	75,000	75
  Oxytetracycline	Injectable	10	250 mL	2,500 mL	100 mg/mL	250,000	250
  Cephaspirin (lactating)	Intramammary	10	12 tubes	120 tubes	200 mg/tube	24,000	24
  Cephaspirin (dry cow)	Intramammary	4	144 tubes	576 tubes	300 mg/tube	172,800	172.8
  Florfenicol	Injectable	3	100 mL	300 ml	300 mg/mL	90,000	90

• Step 2: Convert to number of Canadian defined daily doses or defined course doses

  It is important to standardize both the number of cows and the number of days for which the antimicrobial dispensing data is available. For dairy farms in Canada, it is typically standardized for 100 cows and 365 days to report the antimicrobial usage in the number of DDD or DCD per 100 cow-years. 

  Below is a formula used to calculate the DDD or DCD per 100 cow-years:

  Antimicrobial	Route	Total quantity of antimicrobial (g)	Standardized number of cows	Standardized time under evaluation	DCDbovCA value (g per animal per course from Lardé et al.	Number of lactating and dry cows on farm	Number of days antimicrobial data collected for farm	DCD/100 cow-years
  Ceftiofur	INJ	75	100	365	3.92	200	365	9.57
  Oxytetracycline	INJ	250	100	365	7.29	200	365	17.15
  Cephaspirin (lactating)	IMM	24	100	365	0.40	200	365	30
  Cephaspirin (dry cow)	IMM	172.8	100	365	1.20	200	365	72
  Florfenicol	INJ	90	100	365	12.00	200	365	3.75
  Total DCD/100 cow-years								132.47