Target Driven Replacement Heifer Rearing

Key Points
• Replacement heifer rearing is often neglected on dairy farms even though it has long term effects on the performance of the milking herd.
• Knowing the target weights for replacement heifers on your farm, monitoring their performance against these targets and taking corrective action to get them back on target where necessary are critical to improving your financial bottom line.
• Good calf rearing practises are vital in the successful rearing of replacement heifers. 
• Knowing the quality of your forage during the winter periods is vital in determining the correct concentrate supplementation rates.
• Bodyweight and condition score is more important than age in determining the correct timing to begin mating your heifers.

Replacement heifers are the future of your dairy herd.  Often the rearing of heifers on farm is not given the attention it deserves despite the fact that how a heifer is raised can have long term effects on their performance in the milking herd.  The net cost of rearing a replacement heifer is now estimated to be €1,533 per head by Teagasc.  Achieving the target weights for the breed during the first two years of life will go a long way to ensuring lower costs and maximising future performance in the rearing of replacement heifers.  Table 1 shows the target weights for a number of breeds.

Table 1. Target weights (Kg) for different breeds of replacement heifers at different stages during the rearing period


% mature live weight

Holstein Friesen

New Zeland / British Friesen

Jersey X Holstein Fr.

Birth   38 36 34
6 weeks   63 60 56
3 months   90 85 80
6 months 30 155 148 138
12 months   280 267 250
15 months 60 330 315 295
21 months   490 470 437
24 months (pre-calving) 90 550 525 490

Birth until weaning
It has been proven that calves with higher growth rates early in life produce more milk and have improved fertility performance in their first lactation.  To achieve these higher growth rates there are a number of critical factors:
• All calves should receive adequate colostrums-the quantity, quality and timing of colostrum fed will impact greatly on the ability of the young calf to withstand disease in its first few weeks of life.
• Quality milk replacer is preferable to whole milk-proven in trials to result in heavier calves at weaning, reduces the risk of nutritional upsets, encourages intake of starter ration (earlier weaning) and less risk of disease.
• Clean fresh water and a long fibre source (good quality straw is preferable to hay) should be available at all times.
• Calves should have ad-lib access to good quality starter concentrate from day 10. They need to be consuming 1kg/head/day at weaning.

Three to ten months
The three to ten month period is termed the ‘critical period’ of the heifer’s development. During this period, which starts before puberty and continues for several oestrous cycles, the rate of growth of the mammary gland exceeds the rate of growth of the other body organs. High feeding levels during this period will increase fat deposition in the udder potentially reducing the development of the udder milk production cells. The recommendation is to keep growth rates close to but not above 0.8kg/day during the period.

Traditionally maiden heifers are targeted to be mated at 15 months and to calf at 24 months.  The latest research from Teagasc shows that bodyweight and body condition score (BCS) at mating are far more important than age with heifers mated at target bodyweight and BCS producing significantly more milk in their first lactation.  It is in fact possible to mate heifers at 13 months and calf them at 22 months provided them achieve the correct weight for their breed and a minimum BCS of 3.25 at 13 months.

Over wintering and other periods
This section focuses on spring born heifers but similar principals hold true for heifers born at other times of the year.  The growth rates and concentrates allowances recommended for spring born weanling heifers during their first wintering period are outlined in Table 2.  As can be seen from the table the weight at housing and the silage being fed will have a large influence on the quantity of supplementary concentrates required.  This emphasizes the importance of testing your forage and monitoring the weights of your animals.   The concentrate fed should have a high energy content and a crude protein content of 15-16%. 

Table 2.   Growth rates required (Kg/day) and recommended concentrate allowance (kg/head/day) for spring born replacement heifers over wintered on different quality silages assuming an early March turnout.

Light On Target
Live Weight    
At Housing 210 240
At Turnout 280 280
Indoor Period    
No. days 120 120
Target Growth Rate* 0.6 0.3
Concentrate with    
72% DMD silage 0.4 0.0
68% DMD silage 1.0 0.0
62% DMD silage 1.4 0.2

*Based on 330 Kg target weight at start of breeding season (start date mid April).

Heifers usually do not require supplementation during the grazing season provided they have sufficient access to good quality grass and they have achieved their target turnout weights.

Good quality silage (70+ DMD) is generally sufficient for the in-calf heifer provided they are up to target weight at housing. Heifers that are below target live weight may be fed up to 2kg concentrate per head per day until 6-8 weeks pre-calving. From then until calving, a quality pre-calving mineral is recommended as a routine supplement during the last 6-8 weeks of pregnancy. 

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