Are Your Cows Meeting Their Contribution Potential for Your Bulk Tank?

When trying to diagnose a milk production problem in a herd, we are primarily interested in finding which cows or groups of cows are currently underperforming in relation to the entire herd. Two things come into play when looking at herd milk production: actual milk production and herd dynamics. Cows with normal lactations tend to peak between 41 and 100 days in milk, and usually have a slow decrease in production over time. Conventional dairies will have cows at different stages of milk production for any given time period, striving for a continuous supply of milk. One way to account for herd dynamics and milk production is through the concept of Contribution Potential.

Contribution Potential represents the relative contribution of a group of cows to the bulk tank milk weight. Simply stated, it shows how much milk one group of cows is putting in the bulk tank in relation to other groups of cows within the herd. Contribution Potential for a specific group of cows is determined by the following simple formula:

Contribution Potential % = % of total herd milk production – % of lactating herd size

For example, a herd may have 25% of all lactating cows within 41 to 100 DIM for a given test day, and this group of animals accounted for 30% of all the milk produced on that day. This would make the group’s Contribution Potential = 30% - 25% = +5%.

Using Contribution Potential allows us to identify a group of cows that may be underperforming for two distinct reasons, either low milk yield or low cow numbers. In the case of low milk yield, a herd may have a large number of cows in a specific DIM group, but those cows are not producing enough milk to meet their Contribution Potential. In the latter case, a group of cows may have above average milk production, but there not enough cows in the group to meet their Contribution Potential. To better understand these scenarios, examine the data from actual herds in the tables below.

In this example, we have a Holstein herd milking 3x with some bST usage. There are several groups of cows who are underperforming compared to herdmates. First, if we examine the herd population dynamics, there is an uneven distribution of cows with only 24% of the lactating herd < 100 DIM, where we would normally expect ˜ 30 – 35%. The lower number of cows in early lactation dramatically reduces the overall average for the lactating herd. Correcting the distribution of cows in the herd would reduce the DIM from the current 194 to ˜170 DIM, which would increase milk yield approximately 4 lbs per lactating cow per day. Second, when we examine milk production by DIM group, we can see that milk production is lower than expected for all groups beyond 41 DIM. More specifically, the 3rd lactation and greater cows are significantly lower than the rest of the herd. If the milk production of the 3rd + lactation cows followed a lactation curve similar to the 2nd lactation cows, then the milk yield average would increase ˜5 lbs per lactating cow per day. In combination, the uneven distribution of cows by DIM and the low milk yield for 3rd + lactation cows are potentially costing 9 lbs of milk per lactating cow per day.

In the second example, we have a high-producing Holstein herd milked 3x with full label bST usage and an average of 88 lbs of milk per lactating cow per day. However, even with this high level of production, there are underperforming groups of cows compared to herdmates. Examining the herd population dynamics, there is an uneven distribution of cows within the herd. There are disproportionately fewer 2nd lactation cows and fewer cows from 101 to 199 DIM. There are many potential reasons for this uneven distribution including issues with expansion and past reproductive problems in the herd. Regardless of the reasons, the uneven distribution of cows within the herd is impacting potential milk yield. If we were able to redistribute the cow numbers more evenly within the herd, we would gain approximately 2 to 3 more lbs of milk per lactating cow per day.

In both example herds, groups of cows were not meeting their Contribution Potential as a result of either low cow numbers or low milk yield or both. By identifying these groups of underperforming cows, we can better focus our investigation of the limitations to milk yield. The point to understand in diagnosing milk production problems in herds is that examination of milk yield alone and out of context with the distribution of cows by DIM and by lactation can be misleading.

By using Contribution Potential, we account for both milk yield and the distribution of cows within the herd in identifying underperforming cows. Our research on Contribution Potential examined the DHIA records of 269 herds representing over 24,000 cows in Southeastern Pennsylvania (Goodling et al., 2006). The average Contribution Potential by DIM group from that study is shown in Figure 1. The basic shape of the graph is similar to a lactation curve, but represents only current test day production and not lactation averages. Contribution Potentials are positive for DIM groups 0-40, 41-100, and 101-200, and negative for DIM groups 201-300, and 301+ (Figure 1). These findings are not surprising since milk production decreases as DIM increases, and most herds tend to have a larger percentage of animals with later days in milk.

When Goodling et al. (2006) examined the effects of herd size on Contribution Potential, there was a trend for Contribution Potential of late lactation cows (i.e. > 200 DIM) to increase with increasing herd size (Figure 2). These results suggested that larger herds are better able to sustain milk volume in later stages of lactation compared to smaller herds. Effects of herd size on Contribution Potential were also examined, and there was a trend for Contribution Potential to increase in later lactation cows as herd size increased, suggesting larger herds are better able to sustain milk volume in later stages of lactation compared to smaller herds.

In summary, trouble-shooting milk yield problems in herds must account for both milk production and herd population dynamics. Using Contribution Potential can easily identify groups of underperforming cows in the herd. For information on Contribution Potential and trouble-shooting milk yield problems, please contact Ken Griswold and Rob Goodling.

--------- Dr. Ken Griswold, Dairy Educator, Lancaster County