Summary of Dr. Bill Weiss’s webinar “Estimating energy supply and requirements of dairy cows”

November 2, 2021

Kai Yuan PhD, Senior Technical Advisor

Energy supply and utilization is key to the lactation performance of dairy cows. In a recent Hoard’s Dairyman webinar, Dr. Bill Weiss from the Ohio State University discussed a few areas in the estimation of energy supply in dairy cows. Below is the summary of a few key points he presented:

1. What is Net Energy of Lactation (NEL)? 

One common misconception is that NEL represents energy in a diet that is used to produce milk. In fact, NEL is not specific to lactation energy; it simply represents the energy in a diet that is useful to the dairy cow. NEL is calculated as gross energy minus fecal energy, urine energy, methane energy, and heat increment. NEL is usually an estimated value, as we cannot easily measure it.

2. Factors affecting NEL

There are many factors that can affect NEL. For example, increasing diet DCAD level in lactating cow may increase NEL by improving rumen efficiency in feed digestibility. Management factors such as reducing crowding in pens and separating lactating heifers from mature cows may increase NEL by creating a more stable rumen fermentation environment that should help increase digestibility.

3. Variation in starch digestibility

Based on data collected by Ohio State Dairy Nutrition Lab from 1990 to present, the total tract starch digestibility ranged from less than 83% to over 95%. Just using the “average” number of digestibly is not good enough given the large variations. In 50% of diets, variation of digestibility is at least equivalent to 3.3 lbs of milk. Knowing the actual starch digestibility of a given herd is important.

4. Diminishing returns of pushing more starch

Higher starch decreases NDF and total diet digestibility. After reaching certain point, additional dietary starch may not provide as much energy to milk as you think because of the negative effects of high starch on feed digestibility and energy utilization.

5. Increasing diet CP often increases digestibility

Feeding crude protein is much more than just meeting the amino acid requirements. Level of dietary protein affects NEL by affecting dry matter digestibility. Because CP, especially RDP, is important in feeding rumen microbes that help digesting forages, increasing diet CP often increases feed digestibility and total digestible energy.

Some of the points presented by Dr. Weiss are consistent with our QLF recommendation on feeding adequate diet CP together with at least 7% fermentable sugar to drive rumen feed digestibility. This improved digestibility should help improve dietary net energy of lactation as a whole.

Note: The link to the recording:


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