Many changes we make to dairy cows through genetic selection are easy to see or quantify. Our cows are much taller, thinner, and have far better udders than cows from past eras. We’ve also doubled production over the last 50 years through genetic selection and improved management.
What can’t be as easily quantified is how we’ve changed a cow’s underlying biological functioning. Evaluating cow hormone and nutrient profiles can provide some insight into how cow physiology is changing because of genetic selection.
We, at Penn State, collaborated with scientists from the University of Bern in Switzerland to determine how genetic merit for various traits is associated with metabolic profiles from blood samples in early lactation cows. We are not the first research group to conduct such an analysis, though every study considers slightly different traits.
I’ve included a table to help provide a quick overview of how genetic merit for yield, feed efficiency, body condition and angularity, and fertility are associated with cow metabolic function. The primary parameters we considered were blood glucose levels (blood sugar), indicators of negative energy balance (nonesterified fatty acids and beta-hydroxybutyrate), growth hormone, and body heat production (indicated by thyroid hormone).
An up arrow indicates an increase in a given measure with the number of arrows representing the relative strength of the relationship; the opposite is true of down arrows. The results reported in our table are generally consistent with those of other research groups, particularly for yield. Feed efficiency is a more recent focus of genetic research.
Our results indicate that genetic selection shifts how a cow utilizes blood glucose. Milk yield is strongly influenced by how much lactose a cow produces, and lactose production requires glucose. Cows genetically inclined to produce high volumes of milk need a lot of glucose, which results in lower levels of glucose in blood. That leaves less energy for the cow to deposit body condition.
Those cows with high genetic merit for feed efficiency appear to be particularly efficient at diverting energy to the production of milk, fat, and protein. Our results also detected a stage of lactation effect.
The relationship of milk yield and blood glucose was significant during the first 30 days in milk, whereas the relationship of blood glucose with feed efficiency was significant through 60 days in milk. We only measured blood parameters through the first 60 days of lactation, but I expect more feed-efficient cows to have lower blood glucose throughout lactation based on research from other sources.
Results for growth hormone levels largely mirrored those of blood glucose levels. This is not surprising because growth hormone stimulates proliferation of mammary cells and is thought to partition nutrients toward milk production.
Somewhat surprising to us was the relationship of dry matter intake with glucose and growth hormone. I expected that cows who were genetically inclined to eat more would tend to have higher blood glucose; however, there was not a strong relationship between the two. We know that cows with high genetic merit for feed intake also milk more.
Perhaps the additional energy intake of cows with high genetic merit for feed intake simply supports milk production and that negates any potential influence of higher intake on blood glucose levels. Likewise, higher genetic merit for dry matter intake was relatively independent of early lactation growth hormone levels.
Negative energy balance
As we know, nearly all cows enter a period of negative energy balance in early lactation because their energy intake is insufficient relative to their energy demands for milk production and other body functions. In our table, an up arrow indicates more severe negative energy balance and is unfavorable.
Severe negative energy balance can lead to health disorders such as ketosis and reduced fertility. Thus, it is important to understand how selection alters energy balance. Alternatively, too much of a positive energy balance in later lactation may lead to excessive body condition and less efficient use of feed energy.
In our analysis, higher genetic merit for yield appeared to extend the period of negative energy balance. The relationship between yield and energy balance was stronger from 31 to 60 days than in the first 30 days of lactation. This may have indicated that nearly all cows have negative energy balance in early lactation, but the length and severity shifts for higher genetic merit.
The relationship of negative energy balance and feed efficiency depended to some degree on how we defined feed efficiency. Dry matter efficiency, or DME, is a common measure of gross feed efficiency, which is calculated as energy corrected milk yield divided by feed intake.
A higher DME is generally considered more efficient. The challenge with DME is that a cow with negative energy balance has an inflated DME because it is supporting milk production with the mobilization of body condition. A very sick cow can have a high DME because it is simply not eating, but, of course, such a cow is not truly efficient. High genetic merit for DME was associated with more severe negative energy balance.
Residual feed intake (RFI) is a measure of feed efficiency that indicates how much more (or less) feed a cow eats than expected based on factors such as milk yield, body weight, and body weight change. Because RFI factors body weight change, it was less related to negative energy balance in early lactation.
Heat production is considered by many to be a wasted use of body energy except when it facilitates proper body temperature during cold weather. As you can see, cows with higher genetic merit for yield and feed efficiency have lower heat production, indicating that more efficient cows loose less energy through heat production.
Fertility’s tug of war
I’ve ignored the last line of our table so far — fertility. The high growth hormone, low blood glucose, low heat production profile that we’ve associated with greater productive efficiency is, unfortunately, also associated with poorer reproductive performance. We have done a good job over the last five years maintaining — even improving — the genetic trend for cow fertility. Those results are particularly impressive in light of the apparent physiological challenges to fertility when we select for higher yields.
Understanding how selection alters a cow’s underlying physiological function could help us design more precise selection programs. For instance, selection that limits negative energy balance would not unfavorably impact feed efficiency and would benefit cow fertility. Nevertheless, the antagonism with blood glucose levels shows that there are always trade-offs to be made in our selection programs and highlights the importance of multiple trait selection and balanced breeding.