Evaluating Milk Peak and Persistency Using DHIA Data Part 3

The Herd Summary 202 is a good starting point for detecting potential problems in the herd persistency for a lactation group's milk peak. However, it may mask any management problems or efficiencies within different cow groups or Days in Milk (DIM).

In order to further investigate how cow groups peak and/or are persistent, producers can use PC DART management software to depict graphs that offer a quick visual check on lactation curves, milk peak, and persistence by DIM for lactation groups or individual cows.

The groups' lactation curves are compared to reference lactation curves, highlighting key stages like the peak milk production period (early lactation), the gradual decline in milk yield (mid-lactation), and the late lactation phase. 

Any deviation from peak milk or persistency is clearly visible. This is an excellent tool to identify potential problems like sudden drops in milk yield, or to assess the effectiveness of feeding and management strategies.

 In PC DART, navigate to 'Cow Graphs' and then 'Lactation' box with 'Average Curves'. Use filters to select Lactation. In the 'Average' section, choose either the 'Actual' or 'Projected' option. The 'Actual' option will produce a group lactation curve with variation/differences between tests/DIM included, while the ‘Projected’ option removes the group’s DIM variation/difference and calculates an average group curve. Check the 'Std' box to depict the reference lactation curve along with the group lactation curve. Choose the lactation and season of calving.

Graphs 1, 2, and 3 below depict 'Average Curves' examples for a dairy farm's Lactation 1, 2, and 3+ (in black) and lactation curves for cow groups (in red).  

The x-axis on the graph represents the DIM, showing how milk production changes over time, and the y-axis displays the average milk production level for the group at different points in their lactation. 

Here are some general points to observe to help evaluate lactation curves:

• The reference lactation curve/line (black line) should be used as a guideline to evaluate the adequacy of milk peak and persistency. Use this reference curve to compare the shape of the group curve.
• The lactation group's peak milk DIM can be estimated from the graphs and compared with standards/recommendations for different lactations. Peak milk should be between 50 to 90 DIM for 1^st lactation cows and 45-70 DIM for 2^nd and 3+ lactation cows.
• Observe whether the milk peak is above or below the reference lactation curve. The graph's background grid gives you an idea of how many lbs of milk below or above the reference line the group is.
• Similarly, how much below the reference curve is the group's milk yield descending line? The difference in pounds of milk can be estimated by the background grid. This difference corresponds to an opportunity for improvement.
• Observe the post-peak persistency. How quickly are the groups dropping milk yield? The slope of the group’s curve depicts this. If the persistency is weaker than the reference curve, the group curve (in red) slope would be steeper, resulting in a 'funnel' shape between the reference and group curves, with the group curve below the reference curve. This would represent a loss in milk production. If the persistency is strong, the group curve would be above the reference curve. This would be a benefit to the producer.

In the example graphs below, three lactation groups will be compared for a sample herd.

The average 1^st lactation curve has lower daily milk yields and a lower peak production than the 2^nd and 3^rd + lactation curves, but drops more gradually across the entire lactation compared to later lactations. This is typical for first-lactation cows, which usually have lower peak production and decrease more gradually to the end of lactation.

The potential milk production for a cow or a group of cows’ lactation is determined largely by peak milk production. If the peak is on the referenced lactation curve or above, the lactation is on or exceeds the cow's or group's potential. However, if the cows do not make a peak, they will hardly offset this loss, and the entire lactation will be below expected milk production.

In graphs 1, 2, and 3, we notice that all three groups have a common trend. They miss pre-peak persistency (lacking a rapid rise to peak). The Lactation 1 Cows (Graph 1) are on a slow start into their lactation as they peak a little bit lower than desired and at 90 DIM, which is at the end of the expected period. The milk peak for the first calf heifer should be between 50 -90 DIM.

Both 2^nd and 3^rd + lactation cows peak within the recommended DIM period, more on the lower DIM (at 50 DIM) (Graph 2). The milk peak is about 4 lbs. lower than the reference curve would suggest for the 2nd lactation cows and about 6 lbs. lower for the 3rd + lactation cows (Graph 3).

During the post-peak period, the 1^st lactation group exhibited a slightly stronger persistency and exceeded expectations in milk production for 100 days only, from 110 to 210 DIM. This group is underperforming throughout the rest of their lactation.

The 2^nd lactation cows are below the reference curve for the whole lactation. The milk yield is about 6 lbs below the reference curve until 150 DIM. After that, there is a slight improvement at the end of the lactation. Unfortunately, this group never produces enough milk to follow or exceed the expected performance.  Lactation 3 cows underperform for the first 150 DIM, half of the lactation. During their second half of the location, their performance is within the expected range.

The potential milk production for a cow or a group of cows' lactation is determined largely by peak milk production. If the peak is on the referenced lactation curve or above, the lactation is up to the cow's or group's potential. Second and third-lactation cows underperform during the first 150 DIM. This is the time when the cows should produce the most milk, and the producer should receive most of the milk income during the cow's lactation.

In this example, increasing milk peak by 4–6 lbs. for the 2^nd and 3^rd + lactation groups would increase the persistency throughout the whole lactations. If the groups' milk peaks meet the reference curve peak, their production would increase by about 4-6 lbs. of milk. This would be about 800 -1,500 lbs. of increased milk yield per lactation per cow.

Consistently low post-peak persistence is most common due to inadequate nutrition. During the onset of lactation, the demand for nutrients is high. The cows will draw energy from their body reserves to produce milk to their genetic potential. Post-peak rations should be formulated to provide nutrients for milk production and to replenish body reserves in later lactation.

However, if cows have inadequate body condition (below 3.0) and are fed poor rations that do not satisfy the energy and nutrient requirements, nutrients will be partitioned away from milk production. When this happens, milk production and persistency rapidly decline. 

Part 4. Recommendations for Improving Milk Peak and Persistency