Low Nitrogen Sires - can we select for reduced nitrogen excretion?
Updated: Jan 20, 2020
Last week we summarised a study on using duration controlled grazing as a potential option to reduce nitrogen (N) losses to the environment.
This week we are exploring a different tool in the toolbox; breeding cows to have lower milk urea nitrogen (MUN) in the hope it reduces urinary nitrogen (UN).
Breeding cattle for lower milk urea nitrogen (MUN) is one tool in the toolbox with the potential to reduce nitrogen losses.
The heritability of MUN was moderate (0.19 - 0.28) which indicates that we can select cows for high or low MUN.
The relationships with the traits included in Breeding Worth (BW) were mostly favourable which suggests that selecting for low MUN will not negatively impact genetic gain.
What is urinary nitrogen and milk urea nitrogen?
Dairy cattle use N from the diet for body maintenance (muscle maintenance), milk production and tissue growth. Nitrogen that is surplus to these requirements is excreted in the urine (urinary nitrogen or UN) and the faeces (or poop!).
Ammonia that is produced mainly in the rumen and is converted from ammonia to urea in the liver. From there it diffuses into the blood stream (known as blood plasma urea) and is further diffused into milk (milk urea; MU) in the udder and urinary urea in the kidneys.
Several studies based on nutritional differences have reported that there is a positive relationship between UN and milk urea nitrogen (MUN). As UN is difficult and expensive to collect and analyse, MUN (collected from milk samples) can be used to predict the amount of UN that a cow is excreting.
Below in Figure 1 are the results from 4 studies illustrating the relationship between MUN and UN when there were dietary differences causing MUN differences. As can be seen in the graph, the slopes of the lines are very similar for all the studies ranging from 13 to 16 g of UN per cow per day with an increase in MUN of 1 mg/dl.
As NZ has a predominantly pasture-based dairy system, our cows consume a diet that is fairly high in protein. Generally speaking, this results in a higher MUN for pasture-fed cows compared with those fed a total mixed rations diet. Based on the above relationships, a higher MUN is related to a higher UN load on pasture, and methods to reduce UN loading are being sought after.
What is the average MUN for NZ dairy cattle?
This study looked at the herd test data from 133,624 cows over 4 calving seasons (2013/14 to 2016/17).
The mean MUN by breed is illustrated in Figure 2 below. For all 133,624 cows, the mean MUN was 14 mg/dl. Holstein-Friesian cows had a slightly higher MUN than Jersey cows did.
The authors made a point of emphasising that changing breeds from Holstein-Friesian to Jersey would not change the modelled UN excretion. This is because more Jersey cows would be needed to consume the same pasture that the Holstein-Friesians would have consumed (smaller cow=less demand=more cows required to balance supply and demand).
Can we select cows for low MUN?
The results from this study indicate that the inheritance of MUN in NZ dairy cows is similar to that in overseas cows.
The heritability estimates ranged from 0.19 to 0.28.
As we discussed in our calving difficulty post, heritability is a measure of how much a trait is influenced by genetics or the environment. The range of possible values for heritability is 0 to 1 (or 0% to 100%).
A heritability of 0 suggests that all the variation (or differences) in a trait are caused by environmental factors (e.g. nutrition, the weather etc.) and a heritability of 1 suggests that all of the variation in a trait is caused by genetics.
The range of 0.19 to 0.28 for the heritability of MUN puts it in the "moderately heritable" band. This means that some of the differences between cows in their MUN is due to their genetics, meaning that if we can identify which cows/bulls have the "good" genetics for low MUN then we can breed our cows for low MUN.
To put these values into perspective, the heritabilities used for the estimation of breeding values (BV) used in Breeding Worth (BW) are 0.36 for milk volume, 0.33 for fat yield and 0.31 for protein yield (source).
Sire BVs for MUN were estimated and ranged from -2.8 to +3.2 with the average BV of 0.15.
A negative MUN BV would be favourable (i.e. lower MUN) and a positive MUN BV would be undesirable (i.e. higher MUN).
By breed, the Jersey sires had lower mean MUN BV than Holstein-Friesian sires did, but the range in BVs were similar. For Holstein-Friesian sires the range was -2.4 to +3.2 and for Jersey sires was -2.8 to +2.3. This illustrates that selecting bulls to sire low MUN cows can be done in both breeds, as both had bulls with negative MUN BVs.
What about other traits? Can I select cows for low MUN and high BW?
Now here is where it gets a bit confusing. A bull/cow that has favourable genetics for MUN would have a lower/negative BV and a bull/cow with undesirable genetics for MUN would have a higher/positive BV.
It then becomes a bit counter-intuitive when we start talking about relationships with BW and other traits as a "positive relationship" does not necessarily mean a good thing and a "negative relationship" does not necessarily mean a bad thing. I will try my best to explain it clearly!
The authors used the published BVs from 2017 for the sires used in their study and investigated the relationships between their MUN BVs and their other BVs that are important to BW.
What they found was mostly favourable relationships, in that selection for low MUN BVs would be associated with favourable outcomes for the other BVs and BW.
For example, there was a small negative relationship between MUN BVs and BW. This means that selecting sires based on them having a negative (favourable) BV for MUN would be associated with selecting sires with a higher (favourable) BW.
This is shown in Figure 3 below for BW and fertility BV with the purple shading representing the desirable animals.
The relationships between MUN BV and liveweight, volume and somatic cell score BVs were all positive. This meant that selecting animals for low or negative MUN BV was associated with animals that also had lower BVs for liveweight, volume and somatic cell score. This is shown in Figure 4 below with the purple shading representing the desirable animals.
Lastly, there was a negative (favourable) relationship between MUN and the percentage of protein in the milk. In other words, cattle with lower MUN have a higher percentage of protein in the milk. This indicates that cows that are selected for lower MUN are partitioning the N from the diet differently to those with higher MUN genetics.
What does this all mean?
If the relationships in Figure 1 (which show that MUN and UN are related when dietary differences cause differences in MUN) are the same when MUN is changed by genetic selection, then there is significant potential to reduce UN excretion by breeding cows for lower MUN BVs.
This would mean that for the same N intake, these cows would be excreting less N in urine and would be partitioning it elsewhere.
Next steps for this work would be to validate that cows genetically selected for low MUN will have a corresponding reduction in UN in our pasture-based system. A massive task!
Here is an article by Rural Delivery from 2017 that goes into a bit more detail on this work by CRV Ambreed (https://www.ruraldelivery.net.nz/stories/CRV-Ambreed-Low-N-Dairy-Sires). We can't wait to see the results!
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