Rearing heifers to join the herd at an age and body weight that will enable them to achieve their full lifetime potential, in terms of both yield and longevity, is key. Heifers that calve for the first time at 23-25 months of age have been shown to outperform their later calving compatriots in terms of fertility, milk production and survival over their first five years of life.¹ Age at first calving is a function of the age at the commencement of first breeding, combined with the reproductive efficiency of the animal. The decision on when to start breeding is primarily a management one, based on the age and size of the heifer. Heifers must be 55-60% of mature body weight at first breeding. This will typically require a growth rate up to first breeding (at 13-14 months of age) of around 0.7-0.8 kg/day (Table 1). Growth rates vary considerably according to plane of nutrition – enough protein and energy must be supplied to maximise the early growth potential of the young calf.

Nutrition and early-life programming
Early nutrition is also important for developmental programming. Feeding more milk and higher growth rates over the first eight weeks has been shown to increase organ weights (including the liver, spleen, kidneys and mammary gland), demonstrating these organs are responsive to nutrient intake during the preweaning stage.^2,3 The mammary glands of calves fed enhanced levels of milk (energy supplied increased from 4.2-8.4 Mcal of metabolisable energy [ME] per day) were heavier than the mammary glands of control calves (supplied with 2.8 Mcal of ME/day).² Furthermore, the parenchymal mass of the mammary glands of enhanced calves weighed 5.9 times more than the mammary parenchymal mass of control calves.² Clearly, the mammary gland is responsive to nutrient intake preweaning, and this supports data showing that increasing growth rate before weaning has a positive effect on lactation milk yield.^4,5
The calf’s primary source of nutrition until three to four weeks of age is milk, since starter intake is minimal.²⁴ By two to three weeks of age, starter intake will begin to increase, and by five weeks should increase to 0.5 kg/day. Therefore, during the first month of life, feeding more milk will help to maximise feed efficiency, growth and improve health.

Feed conversion efficiency
Growth rate is maximal and most efficient in pre-pubertal animals, during the first two months of life. The feed conversion efficiency (FCE) (body weight gain/amount of feed consumed) in the milk fed calf is at its highest, around 50%, dropping to 30% post-weaning, and less than 10% in older heifers around the time of first breeding.6 Based on a FCE of 50% during the milk feeding period, every 1kg of feed (milk and starter) could be expected to give approximately 500 g of growth (assuming adequate colostrum intake, an environmental temperature of 10 to 25°C, and good calf health).

Early Growth Potential
Young calves have an enormous potential for growth. Holstein calves fed milk replacer ad libitum have been shown to achieve pre-weaning growth rates of 0.89 kg/day, over 12 weeks - more than 1.5 times greater than restricted fed calves (5L daily from day 4 until day 21, then 6L daily until day 56; mean growth rate of 0.57 kg/day, over 9 weeks).⁷ The largest difference in growth was during the first 3 weeks of life when daily live weight gain was 0.72 kg/day in the ad libitum fed calves compared to only 0.17 kg/day in the restricted fed calves - a four-fold difference during this period.7 The minimal growth in the restricted fed calves was accompanied by a considerable loss in BCS of nearly 0.5 BCS points over the first 4 weeks.⁷ In contrast, a consistent increase in BCS from birth was recorded in the calves fed ad libitum.⁷ The young calf experiences significant health and environmental stresses during the first two weeks of life – calves must be supplied with enough milk from birth to minimise the impact of these stresses.

Nutrient Intake and Calf Health
Nutritional insufficiency has a suppressive effect on immune function. Dairy calves fed a high plane of nutrition (HPN) had a faster resolution of diarrhoea, maintained hydration, grew better and converted feed with greater efficiency following a Cryptosporidium parvum challenge at 3 days of age, compared with calves fed a low plane of nutrition (LPN).⁸ (HPN: 28% crude protein, 20% fat milk replacer at up to 0.30 Mcal/kg of MBW [MBW = body weight in kg0.75] as a function of birth weight, LPN: 20% crude protein, 20% fat milk replacer at 0.13 Mcal intake energy/kg of MBW as a function of birth weight).8 Similarly, Jersey calves fed a HPN (735 g dry matter/day) showed a better immune response and were more resistant
to a Salmonella challenge given 1 month after weaning compared to calves on a LPN (409 g DM/day).9 Feeding a higher plane of nutrition during the milk feeding period may improve post weaned resistance to disease.⁹

Dairy ingredients used in milk replacers
Dairy ingredients are the main source of protein in milk replacers and these include both skim and whey. Skim milk powder is whole milk with the fat removed, whilst liquid whey is the liquid fraction remaining after the precipitation of caseins (Figure 1). There are different methods to process liquid whey, resulting in various types of whey (whey powder, delactosed whey, concentrated when protein) that will range in protein, lactose and ash content. For example, whey powder contains 12.5 to 13% protein compared to concentrated whey protein that typically contains 35% protein. Thus, whey powder in a finished milk formula will not contribute the same level of dairy protein as a milk formula containing concentrated whey protein.
The quality and processing of the dairy protein (both whey and skim) will influence its digestibility. If good quality ingredients are used (whey or skim) that have been processed correctly – there will be little difference in calf performance. Research has shown that the protein digestibility, and growth and feed efficiency of Holstein calves fed milk replacer containing either whey protein concentrate, or dried skim milk does not differ (Table 2).^10,11

Supplying Energy
Good calf performance depends on more than just the type of dairy protein – calves must be supplied with enough energy. The total amount of ME required is equal to the ME requirement for maintenance plus the ME requirement for growth. The energy requirement of calves at different live weights with different daily live weight gains¹² can be used to calculate the amount of energy needed to be supplied via milk and starter (Table 3). Since starter intake is minimal over the first two to three weeks of life²⁴ the best way to supply more energy to the young calf is to feed more milk solids (either by increasing volume offered or the concentration).

Meal Size
When supplying more milk, consideration must be given to meal size. It is often believed that the capacity of the abomasum is about 2L, and meal sizes above this volume will cause milk to enter the rumen.13 The abomasum has however, been shown to have a large ability for distension - three-week-old calves can consume up to 6.8 L of milk in one meal without milk entering the rumen or showing any indications of abdominal pain or discomfort.13 But these results do not support the feeding of milk meals as high as 5 to 6 L on a daily basis because the test meals were offered as single meals on three separate occasions, and it was not tested how much calves would have drunk if offered this amount two or three times a day.¹³
Another important consideration in relation to meal size is the rate of abomasal emptying. Calves can slow down the rate of abomasal emptying to control blood glucose, i.e. to control how quickly nutrients are presented to the small intestine. Feeding calves 4L per feed (8 L/day) compared to 2 L (4 L/day) slows the rate of abomasal emptying by 40%.14 A slower rate of emptying will give the bacteria present in the stomach more time to ferment the milk, and therefore may increase the risk of bloat.
A further consideration of meal size is insulin responsiveness – it has been shown that milk volume has no effect on insulin sensitivity¹⁵ yet in contrast feeding 4L per feed has also been found to decrease insulin sensitivity.16,17 A decrease in insulin sensitivity has been linked with an increase in fat deposition¹⁶ – early changes in fat deposition and insulin sensitivity could have important consequences on subsequent fertility. Taken together, the recommended maximum meal size is 3L per feed. Feeding calves larger volumes of milk for enhanced growth rates will therefore require an increase in milk feeding frequency throughout the day.

Weaning
Providing more milk will maximise feed efficiency, pre-weaning growth, health and lifetime milk production. But feeding more milk may delay solid feed intake, which can compromise rumen development leading to poor growth and feed efficiency in the weeks immediately after weaning. It is essential to balance the intake of nutrients from both milk and solid feed. For the high milk fed calf, starter intake can be encouraged by using social housing (pair or group), offering chopped forage and using a three-week weaning period.
High milk fed calves will not be driven by hunger to eat starter since the high milk volume will fulfil their nutrient requirement. Housing calves in pairs or groups from an early age (1 week) encourages starter intake during the milk feeding period due to social facilitation – a feeding response elicited by seeing another animal feeding. One study compared feeding calves 5L of milk/day with 9 L/day, and housing calves either individually or as a pair.¹⁸ For calves fed only 5 L/day – the type of housing made no difference to starter intake, since the calves were driven by hunger to eat starter feed.18 When calves consumed 9L/day, they were not driven by hunger to eat starter – but pair housed calves ate more than those housed individually due to peer stimulation.18 This resulted in a higher body weight gain (0.99 kg/day, day one to 44) for high-milk-fed calves housed in pairs.¹⁸
Offering forage alongside starter feed plays a role in rumen development – it helps stabilise the rumen pH, stimulate the muscular layer of the rumen, and maintain the integrity and healthiness of the rumen wall.¹⁹ Providing chopped forage (e.g. barley straw, 3-4cm chop length) separately from starter, during the milk feeding period stimulates starter intake, improves weight gain and improves forage intake after weaning due to the calves increased ability to ingest and digest forage.¹⁹
A rapid increase in starter intake at weaning, when milk is removed, does not allow enough time for the rumen to develop resulting in reduced digestibility of nutrients around weaning.^20,21 Reducing the amount of milk offered gradually over a three-week period encourages starter intake, helps rumen development and improves the digestibility of nutrients after weaning.²² To maintain a pre-weaning average daily gain of 1 kg/day, it has been shown that calves must be eating at least 1377 g/day of starter before weaning to maintain that rate of growth after weaning.²³
Calving heifers for the first time at 23 to 25 months of age, with adequate body size, increases longevity and maximises economic returns. It is essential to balance the intake of nutrients from both milk and solid feed, to ensure calves are supplied with the energy and protein required to hit the target growth rate.