We have previously summarised a study that showed heifers that were heavier before first mating produced more milk in first lactation and accumulated milk over 3 lactations compared with heifers that were lighter.
This week's post is taking it one step further, did heavier heifers survive longer in the herd than lighter heifers?
Key Points
In general, the crossbred cows (FX, FJ and JX) had better stayability than the full breeds (F and J).
Heifers that were heavier were more likely to remain in the herd for successive calvings compared with heifers that were lighter.
For the majority of heifers in NZ, being 'too heavy' is unlikely to be an issue, whereas, being 'too light' is of much greater concern.
A measure of cow survival that does not require super accurate recording of cull data is called stayability. Stayability is defined as the probability (or chance) of a cow surviving to each calving year given that she calved the year before.
Holstein-Friesian (F)
Holstein-Friesian crossbred (FX)
Holstein-Friesian-Jersey crossbred (FJ)
Jersey crossbred (JX)
Jersey (J)
The descriptions of the 5 breed groups as well as their average weights and milk production is given in more detail in Part 1 of this post.
Below is a graph showing the average stayability to each year for each breed.
Let's take Holstein-Friesian-Jersey (FJ) crossbreds as our example (grey bar); 93% of the FJ that were reared ended up calving as 2-year-olds and 77% ended up calving as 3-year-olds. By third calving as 4-year-olds only 64% of the FJ remained in the herd.
In general, the crossbreds (FX, FJ and JX) had better stayability than the full breds (F and J), although the differences were rather small.
Live weights of the heifers were known and used to study the relationship between weight and stayability to each of the first three calving years for each of the 5 above breed groups.
The question being asked here is "Does an increase in weight at 15-months of age, relate to stayability as heifers get older?"
Overall, heifers that were heavier were more likely to remain in the herd for first calving as a 2-year-old, second as a 3-year-old, and third calving as a 4-year-old compared with heifers that were lighter.
This graph here shows that relationship for FJ heifers, if we follow the blue line, we can see that as heifers got heavier at 15-months of age, they were more likely to calve as 2-year-olds. From around 255 kg up to 415 kg the predicted stayability was more than 90%. Similar relationships were found for stayability to 3 and 4 years of age.
You can see at the end of the graph (from around 400 kg onwards) that there was a decline in stayability. What this means was that for heifers that were at the heaviest end of the weight range in the study, there was a slight decline in stayability. However, it is not worse than that of the very light heifers.
Are you enjoying this summary? Have a read of one of these while you're here!
It is likely that the very heavy heifers at 15m were also very heavy around their first calving. In Part 2 of this series, we reported that the heaviest heifers in this study had the highest milk production, perhaps these heifers were producing milk at the expense of reproduction?
The way this study was conducted (taking pre-recorded data and analysing it) means that the reasons behind the results found can only be hypothesised. An intensive study where blood samples and body condition scores could be taken would be needed to untangle why the very heavy heifers did not have as a good a stayability as the more average sized heifers.
The other breed groups had similar relationships as the FJ which has been displayed in the graphs here. Please let us know if you would like to see the specific graphs for any of the other breed groups.
Let's take a closer look at some examples...
The average weight of FJ heifers was approximately 300kg at 15 months of age (See Part 1), at this LWT we would estimate that 94% would be present at the herd at first calving.
For heifers that were 20kg lighter than average (280kg) at 15m, we would expect that approximately 93% would be present in the herd at first calving (as a 2 year old; Figure 3).
Heifers that were 20kg heavier than average (320kg) at 15m of age, we would expect that approximately 94% would be present in the herd at first calving (Figure 3).
These differences for a 20kg jump in LWT at 15 months of age are very small, especially compared with the milk production response. (found in Part 2 Figure 2).
There appears to be more of an increase in stayability for a jump in LWT as we go from 2yo to 3yo to 4yo, but it is still rather small (especially when we think of it on a per herd basis!).
For example, if we take a herd of 500 cows that has an annual replacement rate of 20%, each year we are keeping 100 heifer calves (give or take to make the maths more simple!).
Based on the numbers in Figure 3 above (if they were all 300kg at 15m), out of these 100 heifer calves, we would expect 94 of them to calve as 2-year-olds. We would then expect when we get to 2nd calving as 3-year-olds that we would only have 78 out of the original 100 left and by the time we get to third calving as 4-year-olds we would be left with 65 of the original 100 that we reared.
If we were then to have reared these 100 heifers to be 20kg heavier at 15m (320kg) we would expect to have 94 calve as 2-year-olds, 79 as 3-year-olds and 66 as 4-year-olds. A difference of one whole cow by third lactation!
If anyone wants to do these calculations in their own herd, we would love to hear from you! Flick us a tweet or facebook message @agrisciencer
What does this all mean?
Importantly, the results show that an increase in weight for a light heifer had more of an impact on stayability to future lactations than an increase in weight of a heavy heifer.
For NZ dairy heifers that were below average and average weight (for their breed group), we can expect some benefits over the first 3 lactations by improving rearing practices to result in heavier heifers before first calving.
This study did not look into management options to achieve this but some options could include preferentially feeding the lightest heifers or by increasing the feed to all heifers.
Additionally, in Part 2 we mentioned that the relationship between weight and accumulated milk yields was less of a straight line than that between weight and first lactation milk production. The reason this occurred may in fact be due to the slight decline in stayability of the very heavy heifers, i.e. they were producing lots in each lactation but their chance of surviving to 2nd or 3rd lactation was slightly lower than an average or slightly above average heifer.
The conclusions from Part 2 still stand now that we have looked at stayability:
For the majority of heifers in NZ, being 'too heavy' is unlikely to be an issue, whereas, being 'too light' is of much greater concern.
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