Appendix 10 - Calculations of Cases of Mastitis

Assumptions:

1. Incidence rate of clinical mastitis = 45 cases / 100 lactations

   Estimate based on the three major studies which provided data on clinical mastitis incidence rates (5407, 5422, 20). In the control cows in these studies, there were 441 cases in 975 lactations = 45 cases / 100 lactations.

2. Proportion of cases during treatment period = 78%

   Of the 441 cases reported above, 345 (78%) occurred during the treatment period.

3. rbST increases risk of clinical mastitis by 25%

   In Section 7.1.2 of this report the estimate of the increased risk of clinical mastitis associated with rbST use was 25% - 30%. 25% represents a slightly conservative estimate.

4. 77% of milk production occurs after day 60

   This estimate is based on a standard lactation curve with peak production of 39Kg/day and a persistency value of 0.133

5. rbST increases milk production by 10%

   In section 4.1.2 the increase in FCM was estimated to be 11.4% in primiparous cows and 15.6% in multiparous cows. However, it was noted that these results were obtained in institutional herds under very intensive nutritional management. Consequently, a more conservative estimate of 10% has been selected.

6. 100 cow dairy herd producing, on average, 8000 l per lactation

   This is just a hypothetical herd for the purpose of the following calculations.

Additional Cases of Mastitis if Producer Keeps Milking 100 cows:

45 cases distributed as:

10 cases before 60 days
35 x 1.25 = 43.75 cases after day 60
Total = 53.75 cases

If the producer keeps milking 100 cows, we can expect an extra 8.75 cases of mastitis per 100 lactations. This represents an overall increase of 19.4%

Additional Cases of Mastitis if Producer Keeps Constant Milk Production:

100 cows would produce:

100 x 8000 x 0.23 = 184,000 l before day 60
100 x 8000 x .77 x 1.1 = 677,600 l after day 60
Total production = 861,600 l (or an overall increase of 7.7%)

If the producer reduce the herd size by 7.7% to keep milk production constant, he would expect to have:

53.75 / 1.077 = 49.9 cases of clinical mastitis for a net increase of 4.9 cases of mastitis per 100 lactations. This represents an overall increase of 10.9%