Most of us will have heard that the weight and condition score of heifers at their first calving is important for their future milk production and reproductive performance.
In general, the heavier they are at or near calving, the better they will perform - have a read of this summary to see more about this --> "Do heavier heifers produce more milk?"
This begs the question, what should we do with them when they return from grazing?
Key Points
Grazing heifers separate from mature cows had a greater effect on their weight gain than for mature cows grazing separate from heifers.
The benefit of greater weight gain up to calving for heifers grazed separately did not translate into better milk production.
As always, it is important to consider experimental design and the number of animals used in order to draw conclusions.
We recently ran a poll over on Twitter asking how dairy farmers manage their heifers over winter before calving. Over half answered that they graze them completely separate from the main herd and bring them together either once calved or when they are in the springer mob before calving.
Reasons why they were grazed separately included; it was easier, to give them the best grass, or for numbers (mob sizes).
A small number (11%) grazed their heifers and cows together. Not many answers were provided as to why all heifers and cows were together over winter.
We wanted to investigate and see what studies have been done in NZ that compared wintering strategies for heifers before they enter the dairy herd.
We only found one study which was completed in Taranaki in 1990. This study was run from the 28th June to December.
There were 90 cows and 20 heifers that were allocated into 3 treatments as outlined in the figure below.
Half of the heifers (10) were kept in their own mob, the other half of the heifers were added to a mob of half the cows (55 total; 10 heifers & 45 cows) and grazed together until calving. The remaining 45 cows made up the 3rd mob of cows grazed separately.
To ensure that all three mobs were given the same access to feed, they were all grazed in the same paddock that was split into three sections; each a different size so that each cow was offered 9 kgDM per day of similar pre-grazing herbage mass. This means that each of the three mobs were grazed at the same stocking rate even though the mob sizes were all quite different.
Pre- and post-grazing herbage masses were similar for all 3 treatments.
What was their weight gain over winter?
The weight gained from June 20th until calving date was greater for those that grazed separately than for those that grazed together for both heifers and mature cows.
The effect of grazing separately or together was greater for heifers than for mature cows, or in other words; grazing heifers separately had a greater effect on their weight gain than grazing mature cows separately.
The graph below shows the weight gained from June to calving for the 10 heifers grazed separately (blue) or the 10 heifers that were grazed with the older cows (black). Likewise, the weight gained for the 45 cows that grazed separately (grey) and the 45 cows that were grazed with the 10 heifers (purple).
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Did the weight advantage convert into better milk production?
Considering that the heifers that were grazed separately gained more weight up to calving, it would be easy to assume that they would be able to use this weight advantage to produce more milk than the heifers that were together with the older cows.
However, the benefit of greater weight gain of heifers up to calving did not translate into better milk production.
The milk production from calving to December was similar for heifers that were grazed separately or with the older cows (blue and black bars below). Similarly, there were no differences in milk production for cows grazed separately or with heifers (grey and purple bars).
As this study was done at the beginning of the '90's, milk fat was more important than milk protein, and there were no statistics presented on milksolids production. Below is a graph I have attempted to replicate here from a rather small and blurry version in the original paper...
It clearly shows that the milk fat production was initially greater for the heifers that were grazed together with the cows (black) than for the heifers grazed separately (blue). By November, both mobs of heifers were producing similar quantities of milk fat to each other.
You might be thinking that this graph looks rather unusual, (and you'd be right!) the milk production dropped after calving before increasing again in October. The authors pointed out that there was a severe feed shortage after calving and this resulted in the drop in milk fat production.
The study contained information on the liveweight in December and the weight gain from June to calving; but it did not state the weight in June (before the experiment started), but it was stated that each group of heifers started the experiment with the same weight. It was also not reported what the weight was at calving; except for a rather blurry graph I have attempted to replicate below...
We can see that the heifers that were kept separate over winter (blue) were initially heavier than the heifers that were kept with the older cows (black). Within the first month or two of calving, the heifers kept separate dropped in weight dramatically (approx 65 kg), whereas, those that were grazing with the cows before calving lost much less weight. By mid-September both groups were pretty similar in weight. Again this dramatic weight change would have been due to the severe feed shortage.
What appears to have happened is that the heifers that were grazed separately in their mob of 10 (blue) did not adjust as well to life in the main herd as those that spent their winter with older cows (black).
When new animals are introduced to a mob, there is time taken to re-establish a herd hierarchy. For the heifers that were grazed with the mature cows, this hierarchy change would have occurred early on in winter, whereas, for the heifers that grazed separately this hierarchy change would have occurred immediately after calving.
The feed shortage would have also increased competition between animals further exacerbating the hierarchal changes for the heifers that were newly introduced to the main herd.
The question then becomes, did these heifers not cope well because of having to join the older cows and form a new hierarchy? or was it just a numbers game, where they went from a mob of 10 to a herd of 110?
It is quite likely to be a combination of both and is an important thing to note when reading scientific publications. The number of animals in each treatment is important when interpreting research findings.
For the heifers we had a mob of 10 compared with a mob of 55... it makes it difficult to determine if the differences that were observed were because of the effect of grazing heifers separately or together with mature cows (we call this a true treatment effect)
OR
if the differences were observed purely because the mob size was different.
To answer this and to find out the true treatment effects this study would need to be repeated but maybe considering having even treatment sizes, or comparable mature cow numbers. For example, one option could be to have a first mob of 30 heifers separate, a second mob of 15 heifers + 15 cows (30 together), and a third mob of 30 cows separate.
So what does this mean?
When rising 2-year-old heifers are returning to the main farm after grazing, at some point they need to be introduced into the rest of the herd. Based on this small Taranaki study we have summarised here, it appears that there were some advantages to doing this well before heifers calved.
However, if your heifers have come home lighter than you would have liked (especially in areas affected by drought) it may be beneficial to graze the heifers separately on the best feed to get them that bit heavier to set them up for calving and their first lactation.
A fair number of comments we got back on twitter was that heifers were grazed with some mature cows over winter. These were usually 40-odd early calvers or lighter cows. I would say that this would be a potentially great idea to help heifers establish themselves in their new herd, whilst still giving them access to better feed.
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