Why are 25% of dairy replacement heifers calving over 26 months?
Martina Gormley, Teagasc Dairy Specialist, tells us only 75% of heifers calve down at 22 to 26 months of age. 25% or around 80,000 heifers calve over 26 months in age.
The first question is what is your herd mature cow weight? Next what is the herd EBI Maint SI and does this match your herd average weight? Knowing these two pieces of data will help you determine what weight your heifers should be at different times throughout the rearing period.
The maintenance subindex within the EBI is calculated from the carcass weight of slaughtered cull cows multiplied by an economic value. Figure 1 belows shows a higher maintenance sub-index equates to a smaller cow and a lower or minus index equates to a bigger cow.
Figure 1 Mean cow-liveweight across different maintenance subindex values
As maintenance sub-index increases by €1 it is expected cow live weight will reduce by 4.98 kg for a 3rd lactation Holstein-Friesian cow in mid-lactation. These changes do not cause a change in beef sub index in EBI.
Table 1 below shows the target replacement weights for heifers at different ages. Novembers target weight for weanlings and incalf heifers are highlighted in yellow. Over the last number of years, cow size has reduced on many farms. Where this has happened herds typically range from a maintenance subindex of 10 to 20. If herd mature size is close to 600kg (Maintenance subindex 10) the target for weanlings now is around 235kg and 470kgs for incalf heifers.
Table 1 Maintenance subindex (SI) and heifer target weights per month
| Age | Month | % Mature Weight | Maint SI 5616 kg | Maint SI 10591 kg | Maint SI 20541 kg |
|---|---|---|---|---|---|
| Birth | February | - | 42 | 38 | 34 |
| 3 months | May | - | 123 | 118 | 108 |
| 6 months | August | 30% | 164 | 177 | 162 |
| 9 months | November | - | 246 | 236 | 216 |
| 12 months | February | - | 308 | 295 | 270 |
| 15 months | May | 60% | 370 | 355 | 325 |
| 19 months | September | - | 452 | 433 | 397 |
| 21 months | November | - | 493 | 473 | 433 |
| 24 months (pre-calving) | February | 90% | 554 | 532 | 487 |
Calving pattern, grass & silage quality are often highlighted as the reasons for later calving of heifers. These factors absolutely effect heifers reaching target weights. With many heifers now housed it is time to test your silage and feed heifers accordingly.
Another factor is the perception that your heifers are not on target and or that the mature herd weight is heavier than they actually are. This should be addressed through weighing the herd and heifers. Over feeding heifers is costly and you are not rewarded for this. Not submitting heifers for breeding that are on or near target is also costly. From weaning to calving down heifer concentrate feeding can range from 150kg to 500kg on farms. Depending on concentrate price this could easily cost €120 to €150 per heifer. For 20 replacements, that is €3,000 per year. This would cover the reseeding cost for the land that the heifers are grazing. Take the guesswork and extra cost out of heifer rearing by testing your silage now, weigh your heifers and have a look at your herd EBI Maintenance SI as an aid to determine target weights.
The Teagasc Dairy Specialists issue an article on a topic of interest to dairy farmers every Monday here on Teagasc Daily.