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• Accelerated growth programs aim to improve the nutritional status of the very young calf to restore biologically normal growth.
• Practical systems must be designed to allow effective weaning without a growth slump.
• Data on subsequent milk production and longevity are needed before a complete economic comparison of early weaning versus accelerated growth programs.

Implications and applications of accelerated growth schemes for young calves have drawn considerable interest recently, primarily as a result of provocative research from the laboratory of Dr. Mike Van Amburgh at Cornell University. Perhaps the most important commercial driver in the renewed interest in calf nutrition and management has been the high demand, short supply, and resultant sky-rocketing prices for pregnant heifers during the last 2 years. My objective in this paper is to review some key biological principles of growth and how nutritional management may impact growth of young calves during the first 2 months of life.

CURRENT MANAGEMENT PRACTICES VERSUS BIOLOGY OF THE CALF

During the first 2 to 3 weeks of life, the calf's digestive system is immature and is designed to digest milk-based nutrients efficiently. Conventional heifer calf rearing schemes rely on restricted feeding of milk or milk replacer (typically 8 to 10% of body weight (BW) to encourage early intake of starter. From an economic perspective, the incentive has been to wean calves as quickly as possible (without sacrificing health) from more expensive milk or milk replacer to less expensive concentrate-based feeds (starter) and forages. Health of calves has been perceived to improve once calves were weaned from milk, which likely is a factor of the extensive detoxifying ability of the rumen, the bulking effect of solid feeds in the intestine, and improvements in energy balance. Requirements for labor per calf also decrease considerably when calves no longer have to be fed liquid diets individually and can be housed in groups.

True early-weaning systems in the United States work well where implemented properly. These systems allow producers to wean healthy calves at less than 5 weeks of age and at minimal cost. However, excellent management is required and many producers have shied away from such early weaning programs. Consequently, recent national surveys show that producers on average do not wean heifer calves until 8 weeks of age, yet calf mortality still is over 10%. While excellent managers have much lower death losses, it is evident from this unacceptably high mortality rate that there is room to search for better systems to decrease mortality and morbidity.

Common guidelines for feeding milk or milk replacer to young calves typically specify feeding 8 to 10% of BW. These rates are much lower than ad libitum feeding rates. Calves allowed to suckle their mothers typically consume 6 to 10 meals per day, and may consume 16 to 24% of their BW as milk by the second week of life. Studies with hand feeding of milk also show that ad libitum intakes of milk are in excess of 18% of BW. For example, a New Zealand study fed calves whole milk at rates of 11.3%, 13.9%, 15.9%, or 19.4% (ad libitum) of BW. Average daily gains (ADG) during weeks 2 to 6 of life were 0.90, 1.10, 1.37, and 02.07 lb/day, respectively. Consequently, what is referred to currently as accelerated growth is, in fact, restoration of biologically normal growth in early life before starter consumption becomes the predominant source of nutrients. It is a management decision to feed smaller amounts of milk or milk replacer twice daily to encourage dry feed intake. Rather than accelerated growth, therefore, I use the term "enhanced early nutrition" to more appropriately refer to achievement of biologically appropriate growth for young calves. Another term used is "intensified" growth.

At higher rates of milk or milk replacer feeding, intake of starter clearly is decreased. Lower starter intake slows the rate of rumen development, which has been assumed to contribute to calves "stalling out" when weaned from milk. However, the greatest stimulus for increases of dry feed intake even in early weaning systems is removal of the liquid portion of the diet, which causes feed intake to nearly double in the first day. Establishment of a mature, functional population of microorganisms capable of fermenting both starchy and fibrous carbohydrates occurs gradually during the first 6 to 8 weeks of life, and microbial protein production in the rumen increases progressively with age in young calves. Because of the immaturity of rumen digestion at this point, the "stall-out" that may occur at weaning in calves fed more liquid diet results from the inability to eat and digest enough dry feed quickly enough to keep them growing at the higher rate after they are weaned. Consequently, practical systems employing enhanced early nutrition principles must strike a balance between greater liquid intake rates to increase early growth while still encouraging starter intake to avoid stall-out at weaning. Research still is ongoing in these areas but it is apparent that such a balance can be reached.

Enhancing early growth rates has the potential to capitalize on the efficiencies inherent in the rapid early growth capacity of young calves. Proportional rates of increase in wither height and BW are highest during the first 2 months of life, and the feed cost per unit increase in wither height is lowest during the first 2 months. Efficiency of dietary protein use for body protein gain is highest in young calves and decreases with body size. Feed efficiencies (lb milk DM per lb BW gain) in the New Zealand study referred to above in which calves were fed whole milk at rates of 11.3%, 13.9%, 15.9%, or 19.4% (ad libitum) of BW were 1.58, 1.48, 1.34, and 1.23, respectively. The latter values compare favorably with feed efficiencies for young pigs and lambs. Increasing efficiencies with increasing feeding rate are attributable to increased gain per unit of maintenance. This "dilution of maintenance" concept is similar to the efficiencies gained by genetic improvement of milk production or by use of bST for lactating cows; more milk is produced for a given "overhead charge" of maintaining the cow's body.

The concept of feeding greater amounts of milk to very young calves to capture this biological efficiency is, in itself, not a new idea. Numerous studies have examined increased rates of feeding milk or milk replacer. Other studies have examined ad libitum intakes of acidified milk replacer. Calves generally grew more rapidly during the liquid feeding period, but had lower starter intakes. Typically, the early growth advantage was not maintained as calves went into the heifer growing phases. Several differences between these early studies and current research must be noted, however. First, the milk replacers fed did not contain sufficient protein to support high rates of lean tissue growth, as discussed below. Second, in most cases calves were weaned to a starter feed or forages that did not contain sufficient protein and digestible energy to maintain the rapid rates of growth. Finally, the high fat content of whole milk or the milk replacers fed in these studies may have decreased starter intake, since it has been well documented that higher fat intakes decrease dry feed intake.

IMPACT OF EARLY NUTRITION ON GROWTH AND BODY COMPOSITION

The nature of the current conventional system for raising young calves in the US has obscured the distinction between what is normal biological growth for the animal and what is imposed from a management standpoint. Furthermore, differences in calf performance from alterations in nutrition most commonly have been measured as differences in ADG. The ADG may not reflect true changes in growth of the calf because of counteracting effects of changes in gut fill from differences in starter intake. For example, increased liquid feeding may increase growth of lean carcass, but the corresponding decrease in starter intake decreases rumen size and gut fill. As a result, differences in ADG of BW may be minimal despite substantial improvements in growth of the frame and muscle of the calf. It is important, therefore, to understand how nutritional alterations may change body composition of young calves. Results of three such studies we have conducted to understand impacts of nutrition on body composition are reported elsewhere in this edition of the Illinois Dairy Report.

The overall feeding rate determines energy intake and sets limits on the growth possible. This principle can be seen in data from Table 1, based on requirements for metabolizable energy (ME) and apparently digestible protein (ADP) calculated according to National Research Council (2001) guidelines. Note that as ADG increases, the required ME intake also increases and more milk replacer powder must be fed to provide that energy.

Table 1. Effect of rate of BW gain with constant initial BW (100 lb) on protein requirements of pre-weaned dairy calves.

The amount of protein required is largely driven by the rate of growth, because maintenance requirements are small and in theory could be met with as little as 8.3% CP in milk replacer (Table 1). The National Research Council (2001) states that 85 g of CP are required per pound of ADG. Consequently, the amount of CP required by the calf increases as it is fed more energy and rates of gain increase (Table 1). Note in Table 1 that the content of CP needed in milk replacer approaches a maximum in the range of 28% CP.

Also note in Table 1 that the calculated content of CP in milk replacer needed for calves gaining 0.5 lb/d is about 18%. On the basis of calculated protein required for maintenance and growth of calves fed typically recommended amounts of milk replacer (1 lb powder per day or 10% of BW), 18% CP likely is sufficient to meet absolute requirements for maintenance and growth. Body size of calves impacts this relationship only slightly because differences in maintenance requirements are small relative to requirements for tissue gain.

Data from our recent body composition research indicate that dietary CP (or protein to energy ratio), but not feeding rate, exerts a pronounced effect on composition of whole-body gain in young calves. Fat content of BW gain decreases as dietary CP intake increases. In contrast, increasing the feeding rate has pronounced effects on overall growth rate but has little impact on composition of gain assuming that dietary CP is adequate. A low to moderate fat content (12 to 15%) may be optimal for high rates of lean tissue growth.

The principles discussed to this point assume that a milk replacer containing only milk proteins is fed. As yet, no alternate protein sources have proven to be equal to milk proteins in promoting growth and health of calves. Use of alternate proteins that are cheaper but which may compromise growth and health of young calves seems a misguided approach for cost reduction, given that 55 to 65% of the costs of raising replacement heifers is attributable to feed costs and that 95% of the feed cost occurs after weaning.

Recent studies on body composition highlight an important distinction between the goals of increased liquid feeding for heifer calves and goals in the production of veal calves. Near ad libitum feeding of milk replacers containing moderate CP and high fat promote fattening at the expense of lean tissue deposition, which is desirable for veal calves but not for replacement heifers. Increased growth in heifers should emphasize growth of skeleton and muscle so that increases in stature are attained. Use of high-protein milk replacers with low to moderate fat contents should achieve these goals in heifer calves. Thus, these recent results have demonstrated that it is possible through dietary manipulation to produce high rates of lean growth without fattening.

It must be noted that to this point no comparative studies have been conducted between calves fed on enhanced early nutrition programs and calves on conventional early weaning programs. While differences in the nature of the absorbed endproducts from the diet (more volatile fatty acids and less glucose and fat in transitioning and weaned calves) might modify the quantitative results, there is no reason to expect that the general relationships would be different from calves fed only milk replacer. However, it is important that appropriate experiments be conducted to verify these relationships in calves fed both milk replacer and starter.

In true early weaning systems, starter intake begins to increase by the second week of life. Differences in nutritional status between accelerated schemes and early weaning schemes are most pronounced during the first 2 weeks of life. The key questions that remain to be answered is whether better nutrition during these critical 2 weeks of life translates into improved function and development of the immune system and whether there are long-term advantages in milk production ability or longevity in the milking herd. These questions need substantial additional research before a total economic analysis of profitability can be conducted.