Only one third of milk producers start breeding heifers at the optimum age for economic heifer rearing yet research and development by the partners in this programme show that a live weight at first calving of 540-580 kg at 23 to 25 months of age can provide savings on feed inputs and grazing, gives better reproductive performance and decreased incidence of lameness. In addition fewer herd replacements are required and there are savings on housing facilities, labour inputs and other overhead costs.

This article reviews the research and development which underpins this calving target and the feeding guidelines developed by John Thompson and Sons Ltd, in collaboration with AFBI and CAFRE, as part of the heifer rearing blue-print.

Effects of age at first calving on subsequent performance

Research over a number of years has indicated that, economically, the optimum age at first calving is between 23 and 25 months of age.  Whilst first lactation milk yields may be increased by delaying age at first calving beyond 25 months of age, this is outweighed by the additional rearing costs and poorer reproductive performance which impacts on overall lifetime yield.

For example a recent analysis by AFBI of data from herds of the Irish Holstein Friesian Association found that -  moving from 24 to 30 months of age, increased first lactation milk yield by 452 litres but increased calving index by 20 days. By the 3^rd calver stage differences in cumulative milk solids yield from first calving had disappeared.  Work in GB has also shown fewer problems around calving and increased herd life (+0.2 lactations) with heifers calving at 24 compared with 36 months of age.

Age at first calving has a considerable impact on the number of replacements carried on a dairy farm (Table 1).  Compared with calving at 24 months, 3-year old calving increases the number of replacement stock on a 100-cow dairy farm by 30 heifers.  This will increase farm stocking rate by 0.4 cow equivalents per hectare and by 25 kg of organic manure N per hectare.  To maintain the same organic manure loading as a farm calving heifers at 24 months, an additional 10 hectares of land would be required for a 100-cow dairy herd.  Calving at older ages also places greater demands on housing facilities, labour inputs and other overhead costs.

Table 1.  Number of heifer replacements required per 100 cows according to age at first calving

What is the optimum calving size for Holstein Friesian heifers?

Body size targets for Holstein-Friesian dairy herd replacements have been developed from a number of research studies carried out at AFBI Hillsborough over recent years on the feeding and management of dairy heifers. The main findings of this work are summarised as follows:

Live weight at calving on performance

• Milk production. Heifers reared to calve at heavier (620 kg) compared with moderate weights (540 kg) produced 800 litres more milk in their first lactation.  However the effects did not extend into subsequent lactations.
• The additional milk produced by rearing heavier heifers (620 compared with 540 kg) resulted from greater body condition score in early lactation, resulting in poorer reproductive performance. In addition, foot claw condition was poorer with heavier heifers (20% more heifers with heel erosion during rearing period) and this was associated with increased lameness (50% higher (poorer) locomotion scores).

Growth rates during the rearing period and subsequent performance

• Increasing growth during the first year of life (above 0.7-0.8 kg per day) increased skeletal size. However, very high planes of nutrition can have detrimental effects on udder development, particularly with strains/breeds of lower mature weight.
• Increasing growth during the second year of life (above 0.8 kg per day) has been found to increase body condition score at calving, reducing dry matter intake in early lactation.
• Feed inputs to achieve moderate (540-580 kg), but not heavy (600 kg+), live weights at first calving are significantly less. In studies at AFBI Hillsborough, this has amounted to 500-800 kg less concentrates over the 2-year period. Also, in-calf heifers reared to moderate weights required less grazing area (12% less)
• Forage type (silage versus straw) fed during the rearing period has been found to have only small effects on subsequent performance in the milking herd

Management At Mating

To achieve an average first calving age of 24 months, insemination needs to start from 13.5 months of age.  Once the decision is made to start mating, on average it will be 11 days before the average heifer is on heat.  Allowance must also be made for missed heats and returns to service.  Good fertility records and observation are essential and submission rates can be improved through using heat detection aids such as tail paint, kamars, teaser bulls, and pedometers.

Currently, 30% of producers surveyed across Northern Ireland estimate that they commence breeding at 13-14 months of age.  A further 30% of producers commence breeding heifers at 15-months of age with 40% of producers delaying the start of breeding to 16 months or above.  So overall, only one third of producers are aiming to start breeding at the optimum age for economic heifer rearing, with a significant number delaying their breeding programme and consequently incurring increased costs.

In terms of deciding when to commence breeding, body size and live weight as well as age are considered important by the majority of producers (90%+).  On dairy farms, visual assessments only are made of heifer growth with little weighing or measuring being undertaken (less than 5% of producers).  Whilst producers are confident of their ability to identify the optimum size for breeding, the AFBI/CAFRE survey shows (1) there can be a large difference between actual live weight and that predicted by producers using visual assessment, (2) body sizes targeted by producers to commence breeding tend to be larger than optimum.

Monitoring growth

Monitoring heifer growth by measuring heifers is key to making informed management decisions. Over the last 10 years, AFBI Hillsborough has collected a comprehensive dataset on body size measurements during each stage of the rearing period. This has enabled development of a simple and easy assessment of live weight, through the measurement of heifer girth. Measuring tapes scaled with the live weight of Holstein Friesian heifers are now available to enable producers to monitor progress of heifers against targets at key times, facilitating a cost effective rearing regime. The optimum live weights developed from the Hillsborough research programme are presented in Table 2.

Table 2.  Target live weights for Holstein Friesian heifers

More information on heat detection and dairy herd fertility management is available through the CAFRE Dairy Herd Fertility Challenge or at: http://www.ruralni.gov.uk/index/learning/challenges.htm