The cost of a day open varies from region to region due to changes in breeding cost, risk of culling and replacement costs, and reduced production. However, if replacements are not available, the cost of a day open decreases drastically, because there is no alternative. With that said, there is one more factor to be considered when calculating day open value – the value of marginal milk.
Marginal milk is a result of having a more productive, nearer-peak cow averageing on the farm. This is reflected in lower herd average DIM, and over time, increased milk per day per lactating cow in the herd, even though the housing, nutrition, and genetics have not changed. For some reason, marginal milk is not part of the day open calculation formula. The sample below demonstrates why it should be:
In a herd with average production of 11,800 kg, the lactation curve shows that at 200 DIM, average production is 35 kg/d. If preg. rate is improved such that days open can be reduced by 20 days, the lactation curve at 180 DIM will show that the average production is 36.5 kg/d. Therefore the value of Marginal Milk is 1.5 kg/d. And the suggested formula is:
((([Marginal milk amount* kg/d] x [$milk price per 1 kg**]) – [difference in feeding costs***]) x [days open****]) x [number of cows]
* In the above example it is 1.5 kg/d
** The price the farmer get paid for their shipped milk
*** Cows at 180 DIM with higher production eat more
**** The target of saved days open
So now lets put some numbers in the formula (In Euro):
(((1.5 x 0.33) – 0.1) x 20) x 360 = 2844
Marginal milk is yet another factor that hit the farmer’s pocket, and yet another reason to focus on a proper calving interval.
For further reading: “Economics of Improving Reproductive Performance in Dairy Herds”, by Stephen LeBlanc