January is a busy month for pregnancy testing spring-calving herds. The pregnancy results (6-week in-calf rate and not in-calf rate) have a large impact on farm milk production, and profitability for the subsequent season.
In most cases, non-pregnant (empty) cows are culled when pasture cover or feed reserves become low, or at the end of the milking season. She is then replaced by a two-year-old heifer in the new season.
In some cases, farmers choose to dry off their “best” empty cows at the end of the milking season, and carry them into the following season, with the assumption that she will have a successful mating and calve again.
For these reasons, we have asked Rachel Gardner to summarise her recent publication on milk production and survival of carryover dairy cows. Rachel completed her Masters in Animal Science at Massey University in 2017. She is now in her third season contract milking 600 cows, with her partner, in greytown.
So, is it worth carrying over an empty cow? Will she have a production advantage when compared to herd-mates? And will she survive for successive seasons, or will she be empty after the first mating, once she’s returned to a milking herd?
Key Points
Carryover cows produced 13% more milksolids than non-carryover cows in the same lactation, when they returned to a milking herd.
Cows that were carried over produced 4% more milksolids than cows of the same age, when they returned to a milking herd.
Survival of carryover cows were significantly lower than non-carryover cows of the same age and lactation.
Carryover cows that calved in October and carryover cows that were Holstein Friesian breed, had the lowest survival.
Farmers invest considerable money and time into rearing or purchasing each heifer that enters their herd. Therefore, it is disappointing when a cow is identified as empty, especially if the animal is early in her productive life.
Both options (empty cows culled and replaced with a heifer OR utilising carryover cows) incur feed costs. However, carrying over cows reduces animal “wastage”, which can be seen by consumers as a environmental benefit.
What was their milk production when they returned to the herd?
Lactation records for carryover cows were analysed across three seasons, after they had returned to a milking herd. These were compared to cows of the same age, or cows in the same lactation, that were in the same herd (i.e. subject to the same management).
Across all comparisons, carryover cows produced significantly more milk (yield, fat, protein) in the first year when they returned to a milking herd, and for two subsequent seasons.
Same lactation number, but different ages
What this comparison is measuring is...
a cow that had her first lactation followed by a carryover year and then she came back into the herd for her second lactation
COMPARED WITH
a cow that did not have a carry over year but was in her second lactation i.e. she was a year younger than the carryover cow.
Carryover cows produced an extra 13% milksolids, compared to non-carryover cows in the same lactation, in their first lactation, once they’d returned to a milking herd.
This production advantage continued into their second and third lactation (7%, 3%, respectively).
Different lactation number, but the same age
What this comparison is measuring is...
a cow that had her first lactation followed by a carryover year and then she came back into the herd for her second lactation
COMPARED WITH
a cow that did not have a carryover year so was the same age as the carryover cow but had one more lactation
Milk solid production was 4% greater, in the year when the carryover cow returned to a milking herd. In the second and third season, carryover cows produced 3% and 2% more than cows of the same age.
Although the data did not include body condition score (BCS) information, it would be expected that a carryover cow would have greater BCS at calving, when compared to a cow that had been lactating in the previous season. These differences would contribute to the milk solid production differences between the two groups.
How about their survival?
Yes, the carryover cow can produce more milk, but do they survive, or do they become empty cows again?
On average, carryover cows survived 1,131 days (approx. 3yrs) after their second calving date, whereas, non-carryover cows survived on average 1,460 days (4yrs) after their second calving date.
This means that a carryover cow was expected to survive 329 days (0.90 years/lactations) less than a non-carryover cow after their second calving date.
Survival analysis showed that carryover cow survival was consistently lower than non-carryover herd mates. Figure 3 shows the probability of survival for carryover cows is significantly lower than non-carryover cows after their second lactation.
Did survival differ based on breed?
Additional survival curves showed that carryover cows that were Holstein Friesian (F) had the lowest survival, followed by Holstein Friesian x Jersey (FxJ), then Jersey (J).
However, the survival of non-carryover cows was still better than that of the carryover cows.
As can be seen in Figure 4 below, F carryover cows survived on average 1088 days after calving which was 327 days (0.9 years/lactations) less than non-carryover cows.
For Holstein Friesian x Jersey and Jersey this was a difference of 324 days (0.89 years/lactations) and 314 days (0.86 years/lactations).
Was survival different for early vs late calvers?
Unsurprisingly, carryover cows that calved in October (late calvers), when they returned to a milking herd had the lowest survival of 879 days (Figure 5; purple grid), when compared to carryover cows that calved in July (early calvers; purple solid).
Although, these “early-calvers” (July; purple bar) still had lower survival than “late-calvers” (October; blue grid) that were not carryover cows.
So, should we do it?
The answer depends on the farm system, and how much information you have on the empty cow. You may want to ask yourself: Was the empty cow sick during the mating period? Did she have an assisted calving? Was she cycling? Did you miss her heat during the AB period? Were your bulls in “working” condition, and checked for fertility? Was she in optimal BCS for mating and conception?
Simply, does she have a “reason” for being empty? The risk with carrying over empty cows is that you may breed for infertility. This is not what any seasonal dairy farmer wants to do. However, if you know why she may be empty and she is a high value, young cow, then carrying her over could be a great tool to reduce your herd wastage.
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