Consequences of nutrition and growth retardation early in life for growth and composition of cattle and eating quality of beef

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Recent Advances in Animal Nutrition in Australia, Volume 15 (2005)

Summary

Severe growth retardation of cattle early in life is associated with reduced growth potential, resulting in smaller animals at any given age. Growth potential diminishes as age of onset of nutritional restriction declines, and severe, prolonged intra–uterine growth retardation may result in slower growth of cattle throughout life. Severe weight loss during the months immediately after weaning or slow growth after early–

weaning also limits compensatory growth. Carcass composition of small and large newborns is similar at heavier market weights. At equivalent weights, calves grown slowly to weaning subsequently have carcasses of similar or leaner composition than those grown rapidly, unless high energy concentrate feed is provided post–weaning, causing increased fatness.

Adverse effects of early–life growth on eating quality at market weights are not evident. When differences occur, they suggest that cattle restricted early in life may have slightly more tender meat. We propose that within pasture–based systems, plasticity of carcass tissues, particularly muscle which maintains a stem cell population, allows cattle growth–retarded early in life to attain normal composition at equivalent weights in the long–term, albeit at older ages. However, nutrition during recovery or following early–weaning is important in determining the subsequent composition of young, light–weight cattle relative to heavier counterparts.

Keywords: cattle, foetus, placenta, birth weight, growth, weaning, nutrition

Introduction

In Australia, growth of cattle to market weights is typically a prolonged process subject to short– and long–term environmental variation. Most notably, cattle experience variable nutrition due to climatic extremes. Growth of the bovine foetus has well–studied consequences for survival, and can be slowed during the latter half of gestation by restricted nutrition and/or inadequate placental development. Similarly,

influences of pre–weaning nutrition on growth to market weights of cattle are well–characterised. However, consequences of foetal calf growth for subsequent growth and of foetal and neonatal calf growth for carcass characteristics of cattle are less well understood.

This review focuses on research into consequences of cattle nutrition and growth early in life for subsequent growth and carcass composition of cattle and eating quality of beef. It includes initial findings from our recent studies on consequences of growth during pregnancy and to weaning of cattle sired by genotypes with extreme propensities for muscle and intramuscular fat development. Growth and nutrition of the bovine foetus and factors affecting it are also briefly discussed and, where instructive, results for sheep are also presented, as development has been extensively studied in this ruminant species.

Normal bovine conceptus growth and metabolism

It is important to recognise that, unlike postnatal growth in which energy and nutrient availability influence growth and body composition of cattle directly, environmental influences on foetal growth and development, and hence birth characteristics, are regulated via the dam and the placenta, the nutritional conduit between dam and foetus.

Most growth of the bovine foetus occurs during late gestation (Winters et al. 1942; Lyne 1960; Ferrell et al. 1976; Prior and Laster 1979). Foetal growth follows a flattened sigmoid pattern during the latter half of gestation as it proceeds from an early exponential phase through a rapid, linear phase, and then begins to diminish as term approaches (Greenwood and Bell 2003a, 2003b). Foetal nutrient uptake becomes a quantitatively important contributor to maternal nutrient requirements only after mid–gestation (Ferrell et al. 1983). Unlike the sheep, in which the placenta attains most of its mass of dry tissue, protein and DNA by mid–gestation (Ehrhardt

Consequences of nutrition and growth retardation early in life for growth and composition of cattle and eating quality of beef

P.L. Greenwood

¹

, L.M. Cafe

^1,2

, H. Hearnshaw

²

and D.W. Hennessy

²

Cooperative Research Centre for Cattle and Beef Quality, University of New England, Armidale NSW 2351

1NSW Department of Primary Industries, Beef Industry Centre of Excellence, University of New England, Armidale NSW 2351, [email protected]

2Formerly, NSW Department of Primary Industries, Agricultural Research and Advisory Station, Grafton, NSW 2460

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and Bell 1995), the bovine placenta normally continues to increase in weight until near term (Prior and Laster 1979; Ferrell 1989). As a result, it has been suggested that placental growth may be less sensitive to nutritional deficiencies in cattle than in sheep. Placental weight and birth weight are highly correlated in cattle (Anthony et al. 1986b; Echternkamp 1993; Zhang et al. 1999), and the functional capacity of the placenta is closely related to placental perfusion. Bovine uterine and umbilical blood flow increases exponentially during the second half of gestation, which equates to relatively constant rates of umbilical blood flow on a foetal weight–specific basis during this period (Reynolds et al. 1986). Detailed accounts of placental function and metabolism are provided by Ferrell (1989) and Bell et al. (2005).

Foetal mass increases many–fold from mid to late gestation and this increase is accompanied by increased rates of uterine and umbilical uptake of oxygen and nutrients, of urea export by conceptus tissues, and of foetal whole–body protein synthesis in cattle and sheep (Bell et al. 1986; Reynolds et al. 1986; Kennaugh et al.

1987; Bell et al. 1989; Ferrell 1991b). During late pregnancy in both of these species, 35–40% of foetal energy is taken up as glucose and its foetal–placental metabolite lactate, and a further 55% is taken up as free amino acids. In contrast to its importance as an energy source in the maternal ruminant, umbilical uptake of acetate is estimated to account for only 5–10% of foetal energy consumption. About 60% of amino acids are used for tissue protein synthesis, which accounts for approximately 18% of foetal energy expenditure (Kennaugh et al. 1987). The remaining 40% of amino acids are rapidly catabolised, accounting for at least 30% of the oxidative requirements in the well–nourished sheep foetus (Faichney and White 1987) or, in the case of glutamine and serine, are taken up and metabolized by the placenta (Battaglia and Regnault 2001).

Intrauterine growth retardation

Maternal nutrition

In cattle, severe nutritional restriction for at least the last half to one–third of pregnancy is usually required to reduce foetal growth. Significant reductions in birth weight were caused by prolonged underfeeding of heifers from weaning until parturition (Wiltbank et al.

1965), and underfeeding of heifers and cows during the second and third trimesters (Ryley and Gartner 1962;

Hodge and Rowan 1970; Freetly et al. 2000; Hennessy et al. 2002) or during late pregnancy only (Hight, 1966;

Tudor 1972; Bellows and Short 1978; Kroker and Cummins 1979). The effect of nutritional restriction on birth weight was more pronounced in heifers than cows when the period of restriction encompassed mid and late gestation (Hennessy et al. 2002) rather than late gestation only (Tudor 1972). However, birth weight was not significantly affected by nutritional restriction from mating to 140 days of gestation in heifers (Cooper et al.

1998), during the final 12 weeks of pregnancy (Hodge et al. 1976) in heifers or during the second trimester in mature cows (Freetly et al. 2000).

Foetal growth capacity can interact with available nutrition in determining the extent to which foetal growth is retarded. Birth weight of calves of Hereford dams sired by double–muscled Piedmontese bulls was more affected by restricted nutrition during mid and late pregnancy than those sired by Wagyu bulls, and birth weight of male calves was affected more than that of female calves (Hennessy et al. 2002). When assessed within parity and sire–breed, nutritional restriction resulted in reduced birth weights of Piedmontese–sired calves from heifers and cows, but only of Wagyu–sired calves from heifers. Effects of foetal growth potential, or foetal nutrient demand, on the nutritional reserves of pregnant cows were also evident (Greenwood et al.

2002b). Dams mobilised more muscle to support growth of male than female foetuses and tended to mobilise more muscle to support growth of Piedmontese– than Wagyu–sired foetuses; while heifers mobilised less fat and muscle to support foetal growth than cows.

During the final one–half to one–third of pregnancy, feed energy available to the dam appears to have more influence on birth weight than the availability of protein, although results are variable (Holland and Odde 1992). Variation in feed energy available to the dam during this period can result in differences in birth weight ranging from 0–8.2 kg (Dunn et al. 1969; Tudor 1972; Laster 1974; Corah et al. 1975; Bellows and Short 1978; Kroker and Cummins 1979; Bellows et al. 1982).

Variable dietary protein supply during the third trimester may (Bellows et al.1978) or may not (Anthony et al.

1986a; Holland and Odde 1987) alter birth weight of calves, and restricted or supplemental dietary protein during early or mid pregnancy had little effect on birth weights (Perry et al. 1999, 2002). However, chronic restriction of energy supply to heifers from weaning until parturition resulted in birth weight differences of up to 10 kg and chronic restriction of protein supply over this period resulted in birth weight differences of up to 7.3 kg (Wiltbank et al. 1965).

Placental weight and birth weight are highly correlated in cattle. However, because the bovine placenta may continue to increase in mass until near term, it is not clear whether the placenta regulates bovine foetal growth to the same extent as it does in sheep (Ferrell 1989). Placental characteristics may be altered by nutrition during early and mid pregnancy without significantly affecting foetal size (Rasby et al.

1990), and protein supplementation of cows during early or mid pregnancy may also alter placental characteristics without necessarily affecting birth weight (Perry et al.1999, 2002).

Because development and growth of vital organs precedes development of bone, muscle and fat (Palsson 1955), respectively, the mass of the late maturing carcass tissues are generally considered more susceptible to the effects of nutrition during late pregnancy when it

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impacts most on foetal growth. However, more subtle effects on organ and tissue development due to nutrition during early pregnancy may occur, with potential for long term consequences for health, as shown in sheep (Greenwood and Bell 2003a, 2003b;

Bell et al. 2005).

Thermal environment

Foetal growth in cattle can be restricted (18% lower foetal weight) by chronic heat stress of pregnant cows, and provision of shade resulted in a 3.1 kg increase in birth weight (Collier et al. 1982). Severe cold stress of cattle may also reduce foetal growth if inadequate nutrition is provided to meet metabolic requirements additional to foetal requirements for growth and development (Andreoli et al. 1988), but more moderate cold stress of sheep ewes in late gestation increased birth weight by 15% (Thompson et al. 1982).

It is believed that temperature regulates blood flow to the periphery and lungs in order to preserve or dissipate body heat, resulting in increased or decreased blood flow and nutrient supply to the gravid uterus (Reynolds et al. 1985). In the sheep, chronic heat stress during early to mid gestation restricts placental development, thus imposing a limitation on subsequent foetal growth irrespective of nutrition later in pregnancy (Bell et al. 1987).

Parity

Heifers give birth to smaller calves than cows (Holland and Odde 1992) because the size and nutritional requirements for growth of heifers limit nutrient availability for placental and foetal growth. Severe maternal nutritional restriction impacts more on birth weight of calves of heifers than of cows, particularly among male calves and those of sires with inherently high birth–weight offspring (Hennessy et al. 2002). In adolescent sheep fed to attain excessive fatness prior to and during gestation, placental and foetal growth and birth weight are reduced (Wallace et al. 1996, 1999).

Litter size

Twin calves are rare in cattle unless exogenous regulation of ovarian function or embryo transfer is practiced. Individuals within litters have reduced foetal growth compared to singletons due to a reduced number of placentomes and mass of placenta per foetus (Hafez and Rajakoski 1964; Greenwood et al. 2000b) and because of greater total nutrient requirements of the litter. On average, twin calves are 7.4–9.8 kg lighter than singletons (Gregory et al. 1990, 1996; de Rose and Wilton 1991; Cummins 1994; Wilkins et al. 1994).

Restricted nutrition limits foetal growth earlier and more severely in twins or higher multiples than in singletons, although stocking rates of pregnant cows fed pasture did not significantly influence twin birth weights (Wilkins et al. 1994).

Foetal and maternal genotype

Foetal genotype is most important in determining foetal growth during early and mid pregnancy, whereas maternal genotype is more important in determining foetal growth during late pregnancy when most foetal growth normally occurs and foetal growth is increasingly subject to external influences mediated via the dam. The effect of foetal and maternal genotype on foetal growth was most convincingly demonstrated in cattle by Ferrell (1991a), who implanted Charolais (heavier birth weight) or Brahman (lighter birth weight) embryos into Charolais and Brahman cows. At 232 days of pregnancy, each foetal genotype was similar in size, irrespective of dam breed. However, by 274 days of gestation, Charolais foetuses in Brahman cows were 7 kg lighter than those in Charolais cows. In contrast, Brahman foetuses in Charolais cows were only 2 kg heavier than those in Brahman cows. Similar results were obtained by Joubert and Hammond (1958) for birth weights for South Devon and Dexter cattle and their reciprocal crosses.

Growth and development from birth to weaning

Calves undergo a transition at birth from a diet comprising primarily glucose and amino acids to one which is quantitatively greater and is proportionately higher in fat. This is associated with maturation of the digestive, metabolic and endocrine systems.

Evidence in sheep suggests severely growth–retarded newborns are immature with respect to energy metabolism and have more foetal–like metabolism than their well–grown counterparts (Rhoads et al. 2000a,b;

Greenwood et al. 2002a).

The major nutritional factors affecting pre–

weaning calf growth and composition at weaning are lactational performance of the dam and quality and availability of nutrients from pasture and/or supplementation prior to and following parturition. Most notably, maternal genotype, age, parity, body condition and liveweight interact with calf growth capacity and milk consumption capacity to influence lactational output. Calves become increasingly dependent on forage–based diets, which result in the production of volatile fatty acids that stimulate development and maturation of the rumen (Warner and Flatt 1965).

Early weaning is practiced primarily to allow the dam to recover body condition to maximise reproductive rates, particularly in harsher nutritional environments, or in high output or accelerated finishing systems.

Successful weaning at a very young age requires adequate growth and rumen development by weaning, and usually involves access by young cattle of low liveweight to a high protein, high energy concentrate supplement prior to, during and after weaning because

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they have a higher requirement for protein relative to energy than their heavier counterparts (Leibholz 1971a,b).

Long–term consequences of altered growth during early life of cattle

Consequences of foetal growth and nutrition

Our recent studies have demonstrated that foetal growth restriction resulting in 10.2 kg or 26% lower birth weight (Table 1) may limit the capacity of cattle to exhibit compensatory growth. Cattle significantly growth–retarded during foetal life due to severely restricted maternal nutrition from day 80–90 of pregnancy until parturition remained smaller at any given postnatal age compared to their well–grown or better nourished counterparts (Table 1). Whether this represents a permanent stunting or simply a delay of attainment of mature size of cattle is open to conjecture.

Growth of low birth weight cattle was significantly slower than that of high birth weight cattle at all stages of postnatal growth, although pre–weaning growth was likely to have been influenced by maternal nutritional status during pregnancy.

Low birth–weight male calves reared rapidly to weaning grew faster than their high birth–weight counterparts during artificial rearing, whereas the opposite occurred for female calves (Tudor and O’Rourke 1980). Calves that had birth weights 5.4 kg and 5.9 kg lower than those from cows well–nourished during late pregnancy were 16.5 kg and 17.2 kg lighter at weaning (Hight 1966, 1968a). However, it should be noted that, in assessing influences of foetal development on postnatal performance, it is not possible to fully separate out consequences of nutrition during pregnancy on the foetus when offspring remain with their dams to weaning because of carry–over effects on

maternal performance (see Greenwood et al. 1998 for an example of a rearing system designed to uncouple prenatal and postnatal influences).Despite this, the net effects of maternal nutrition during pregnancy on the calf remain of practical significance to livestock producers. In this regard, differences in weight of calves at birth following three levels of maternal nutrition during late–pregnancy disappeared by weaning when postnatal nutrition was of high quality and availability, although residual effects of the previous year’s nutrition did influence calf growth (Hight 1968b). Similarly, effects of variable nutrition during mid and/or late pregnancy on weight at birth were overcome by adequate nutrition postpartum, resulting in no differences in body weight at 58 days of age (Freetly et al. 2000). Twin cattle are lighter at birth and grow more slowly until weaning when they remain with their dams (Hennessy and Wilkins 1997). Compared to singletons post–weaning, they may grow more slowly (Gregory et al. 1996), at a similar rate (de Rose and Wilton 1991), or more rapidly (Wilkins et al. 1994; Clarke et al.

1994; Hennessy and Wilkins 1997), depending upon the rearing system and subsequent nutritional regimen.

However, twin cattle tended to consume less feed in the feedlot than singletons, mainly because of their lower liveweights (de Rose and Wilton 1991).

Few studies examined the long–term consequences of foetal nutrition and growth for body and carcass characteristics in cattle (Tudor et al. 1980) prior our recent studies (Cafe et al. 2004a,b,c;

Greenwood et al. 2004; Hearnshaw et al. 2004), but there have been an increasing number of studies with sheep (Villette and Theriez 1981; Nordby et al. 1987;

Greenwood et al. 1998; Krausgrill et al. 1999; Oliver et al. 2001; Gopalakrishnan et al. 2004; Paganoni et al.

2004a,b), and there is increasing interest in the influence of maternal nutritional restriction or stress during foetal development on health during adult life (Greenwood and Bell 2003a,b; Bell et al. 2005).

Table 1 Consequences of growth in utero for growth and liveweight characteristics of beef cattle to 30 months of age.

Prenatal growth/birth weight

Variable L o w High Significance of difference

(n = 120) (n = 120) (P)

Birth weight (kg) 28.6 38.8 <0.001

Pre–weaning ADG (g) 670 759 <0.001

Weaning (7 mo) weight (kg) 174 198 <0.001

Backgrounding ADG (g) 571 603 <0.001

Feedlot entry (26 mo) weight (kg) 481 520 <0.001

Feedlot ADG (g) 1480 1617 <0.001

Weaning to feedlot exit (30 mo) ADG (g) 696 743 <0.001

Weaning to feedlot exit (30 mo) gain (kg) 473 505 <0.001

Feedlot exit (30 mo) weight (kg) 647 703 <0.001

Values are predicted means from REML analyses including effects of birth weight, pre–weaning nutrition, sex/year cohort, sire–genotype and their interactions.

Also refer to Hennessy at al. (2002), Cafe et al. (2004a,b,c), Greenwood et al. (2004) and Hearnshaw et al. (2004).

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Our research has shown that a significant reduction in birth weight following severe maternal nutritional restriction did not influence indices of fatness, apart from P8 (rump) fat, in carcasses of Wagyu–

or Piedmontese–sired steers and heifers at 30 months of age beyond that normally attributable to differences in live or carcass weight (Table 2). Low birth weight cattle had a similar retail yield, fat trim and bone content at equivalent carcass weight, suggesting that there was little difference from their high birth weight counterparts in carcass composition. However, ossification score was higher in low compared to high birth weight calves, suggesting an impact of prenatal growth on maturity. Similarly, gross compositional differences were not evident in the whole body or the carcass of Hereford steers or heifers grown to 370–400 kg liveweight following restricted or adequate nutrition of their dams from 180 days of pregnancy to parturition that resulted in a 22% or 6.8 kg difference in calf birth weight (Tudor et al. 1980).

Research on twin cattle has also demonstrated that, despite significantly lower birth weights and reduced pre–weaning growth, compositional differences at equivalent slaughter weights or ages are small and not significant, twins generally having similar or leaner carcasses than singletons (de Rose and Wilton 1991;

Wilkins et al. 1994; Clarke et al. 1994; Gregory et al.

1996). In contrast, low birth weight lambs are more likely to be fatter than their normal birth weight counterparts at slaughter at weights up to 34 kg (Villette and Theriez 1981; Greenwood et al. 1998) due to higher post–partum weight–specific nutrient intake, lower maintenance energy requirements, limited lean tissue growth capacity and, probably, greater requirements for protein in the

diet during the early postnatal period (Greenwood et al.

1998, 2000a, 2002a). However, evidence is limited on whether increased fatness among low birth weight lambs persists through to heavier weights (Oliver et al.

2001) or to mature size, as may occur in low birth weight pigs (Bell 1992).

We also detected no adverse effects on objective measurements of beef quality due to restricted growth in utero that resulted in low birth weights (Table 3).

Peak force was lower, indicating slightly more tender meat in low compared to high birth weight calves, but compression, cooking loss and colour were unaffected.

Consequences of pre–weaning growth and nutrition

Consequences of nutritional restriction from birth to weaning for subsequent growth were reviewed by Allden (1970), Berge (1991) and Hearnshaw (1997). It is generally recognised that severe pre–weaning nutritional restriction limits the capacity of cattle to exhibit compensatory growth and achieve equivalent weight–for–age in later life. In reviewing a series of Australian studies on consequences of pre–weaning nutritional systems, Hearnshaw (1997) concluded that compensatory gain occurred most frequently when post–weaning growth rates were less than 0.6 kg/d, that compensation did not occur or was negligible at higher post–weaning growth rates and in feedlots. However, Hennessy and Morris (2003) found that calves reared slowly (464 g/d) from birth to weaning were 37 kg lighter at weaning compared to those reared rapidly (872 g/d), but 48 kg lighter after backgrounding and 46 kg lighter at slaughter at 17 months of age. Low pre–weaning

Table 2 Consequences of growth in utero for carcass characteristics of beef cattle at 30 months of age.

(n = 120) (n = 120) (P)

At equivalent age (30 months)

Carcass weight (kg) 364 396 <0.001

Retail yield (kg) 239 257 <0.001

At equivalent carcass weight (380 kg)

Eye muscle area (mm²) 90.4 88.9 0.25

P8 fat depth (mm) 21.3 19.6 0.048

Rib fat depth (mm) 10.9 10.5 0.35

Aus–Meat marble score 1.83 1.86 0.56

USDA marble score 447 444 0.98

Ossification score 206 195 0.009

Retail yield (kg) 249 247 0.20

Bone (kg) 66.9 67.6 0.10

Fat trim (kg) 54.6 56.0 0.58

Values are predicted means from REML analyses including effects of birth weight, pre–weaning nutrition, sex/year cohort, sire–genotype and their interactions, with carcass weight as a covariate (linear and, where significant, quadratic) to predict means at equivalent carcass weight.

Refer to Table 1 for growth characteristics of the cattle.

Also refer to Hennessy et al. (2002), Cafe et al. (2004a,b,c), Greenwood et al. (2004) and Hearnshaw et al. (2004).

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growth rates did not influence feed efficiency in the feedlot compared with cattle grown more rapidly prior to weaning after variation in liveweight, which contributes to differences in energy requirements for maintenance and growth, were accounted for (Hennessy and Arthur 2004).

In our more recent studies on the consequences of growth of cattle during early–life, a difference in weaning weight of 73 kg resulted in a difference of 40 kg in liveweight and 24 kg in carcass weight at 30 months of age (Tables 4 and 5). The low weaning weight cattle grew more rapidly during backgrounding and at a similar rate in the feedlot, resulting in more rapid growth from weaning to 30 months of age. However, compensation in liveweight was not complete when the study ended.

This research confirmed earlier findings that severe, chronic nutritional restriction to weaning limits compensatory growth, which only occurred prior to feedlot entry and not in the feedlot, resulting in smaller cattle and carcasses and less retail yield of beef at an equivalent age.

Our research has also shown that, at equivalent carcass weight, there was more fat trim, less retail yield and there tended to be less bone in the carcasses of cattle grown rapidly to weaning compared to those grown slowly. This suggests that the greater fatness at weaning of the rapidly reared cattle persisted to 30 months of age (Table 5). However, because of failure to fully compensate in weight, carcasses from light weaners remained smaller and weight of retail beef was lower compared to the heavy weaners at the same age. We are presently analysing results from CT–scan and DEXA studies to ascertain whether results of carcass and bone assessments presented in Tables 2 and 5 are consistent with whole carcass composition and bone density measurements.

Earlier studies within pasture–based nutritional systems failed to demonstrate substantial differences in body– or carcass composition due to nutrition and growth from birth to weaning (Berge 1991; Hearnshaw 1997). These authors concluded that cattle from low pre–weaning nutrition groups generally have less fat

(n = 80) (n = 80) (P)

Peak force (kg) 4.05 4.38 0.038

Compression (kg) 1.49 1.53 0.50

Cooking loss (%) 22.9 23.0 0.59

Colour L (lightness) 39.1 39.4 0.58

Colour a (red/green) 26.1 26.2 0.82

Colour b (yellow/blue) 13.5 13.5 0.74

Values are predicted means from REML analyses including effects of birth weight, pre–weaning nutrition, sex, sire–genotype and their interactions, with carcass weight as a covariate (linear and, where significant, quadratic).

Refer to Tables 1 and 2 for growth and carcass characteristics of the cattle.

Table 3 Consequences of growth in utero for objective measurements of longissimus (striploin) quality of beef cattle at 30 months of age.

Pre–weaning growth

(n = 119) (n = 121) (P)

Pre–weaning ADG (g) 554 875 <0.001

Weaning (7 mo) weight (kg) 151 221 <0.001

Backgrounding ADG (g) 615 558 <0.001

Feedlot entry (26 mo) weight (kg) 483 517 <0.001

Feedlot ADG (g) 1527 1570 0.15

Weaning to feedlot exit (30 mo) ADG (g) 742 697 <0.001

Weaning to feedlot exit (30 mo) gain (kg) 505 474 <0.001

Feedlot exit (30 mo) weight (kg) 655 695 <0.001

Table 4 Consequences of growth from birth to weaning for growth and liveweight characteristics of beef cattle to 30 months of age.

Values are predicted means from REML analyses including effects of birth weight, pre–weaning nutrition, sex/year cohort, sire–genotype and their interactions.

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than those from high pre–weaning nutrition groups, but if compared at a constant carcass weight, differences in fatness usually disappear. As a result, calves with lower weaning weights take longer to reach carcass specifications than heavier calves.

Differences in objective measurements of meat quality between cattle grown slowly or rapidly to weaning were not evident (Table 6). Similarly, in earlier studies, objective measures of eating quality were not adversely affected by restricted pre–weaning nutritional treatments (Hearnshaw 1997; Hennessy et al. 2001; Hennessy and Morris 2003). When they were affected, meat of cattle from low nutrition groups was usually more tender than that of high nutrition groups (Hearnshaw et al. 1997; Hennessy et al. 2001).

When compared at a constant carcass weight, meat quality differences became non–significant in about half of the studies (Hearnshaw 1997). However, meat quality may be compromised if slow growth of older cattle results in them being at least 8–9 months older at slaughter weight (Loxton 1997; Purchas et al. 2002).

This is likely to be a result of developmental delay and effects of animal age on connective tissue toughness, although it is unclear if similar age differences resulting from growth restriction earlier in life have the same effect.

In contrast to the above findings relating to body and carcass composition, severe nutritional restriction to weaning followed by concentrate (high energy) feeding from weaning to slaughter results in increased fatness at the same live and carcass weights compared to cattle that are well–nourished prior to weaning (Tudor et al. 1980). In this study, cattle restricted until weaning and subsequently grown on pasture to the same

slaughter weight as those that were well–nourished throughout did not differ in composition from those that were well–nourished. Factors likely to have contributed to increased fatness among the small weaners relative to that of large weaners after they were subsequently fed concentrates include: greater length of time on concentrate feed to reach the slaughter weight, greater weight–specific intake of nutrients following nutritional restriction, greater requirement for protein relative to energy at weaning and consequent imbalance in the concentrate diet during the early post–weaning phase, and limited capacity for lean tissue accretion.

Early–weaning and nutrition during the immediate post–weaning period

In Northern Australia, early–weaning at two months of age followed by slow growth (0.3 kg/d) during the first and/or second dry season after weaning restricted compensatory growth and slowed feedlot growth.

This resulted in cattle taking four months longer to attain market weight, with increased variability in live and carcass weights compared to their counterparts grown rapidly post–weaning (0.8 and 0.6 kg/d) (Anon. 1996). Rump fat thickness did not differ between the groups, although marbling score tended to be higher at carcass weights averaging about 290 kg among the animals grown more slowly immediately post–

weaning. This research also included animals grown at a moderate rate during the post–weaning dry season (0.5–0.6 kg/d), which was more cost–effective.

In a system aimed at production of twin cattle in a more

(n = 119) (n = 121) (P)

At equivalent age (30 months)

Carcass weight (kg) 368 393 <0.001

At equivalent carcass weight (380 kg)

Eye muscle area (mm²) 90.1 89.2 0.55

P8 fat depth (mm) 20.1 20.8 0.41

Rib fat depth (mm) 10.4 11.0 0.33

Aus–Meat marble score 1.92 1.77 0.15

USDA marble score 450 441 0.49

Ossification score 202 199 0.53

Bone (kg) 67.8 66.7 0.053

Fat trim (kg) 52.8 57.8 <0.001

Table 5 Consequences of growth from birth to weaning for carcass characteristics of beef cattle at 30 months of age.

Values are predicted means from REML analyses including effects of birth weight, pre–weaning nutrition, sex/year cohort, sire–genotype and their interactions, with carcass weight as a covariate (linear and, where significant, quadratic) to predict means at equivalent carcass weight.

Refer to Table 4 for growth characteristics of the cattle.

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temperate part of Australia, neither productivity nor economy were enhanced by early weaning one calf of twins born to cows grazing high quality pastures (Wilkins et al. 1994).

Recent research in the USA has attempted to elucidate the effects of weaning regimens and post–

weaning nutrition on growth, carcass and eating quality characteristics of cattle. In particular, systems to maximise feed efficiency and marbling without causing excessive carcass fatness and physiological maturity have been studied. Early weaning of steers onto concentrates resulted in improvements in feed efficiency in the feedlot, carcass quality grades and carcass marbling scores compared to weaning at heavier weights with or without provision of a concentrate supplement prior to weaning (Myers et al. 1999). In contrast, carcass characteristics of beef steers weaned at 108 d of age did not differ at equivalent slaughter weights compared to those weaned at 202 d, although they had better panel scores for tenderness and juiciness, following concentrate feeding from weaning to slaughter (Schoonmaker et al. 2001). Similarly, weaning at 119 d of age onto a forage–based diet or onto various allowances of concentrate diets until 218 d, after which all cattle were fed a concentrate diet ad libitum, did not influence marbling scores at 218 d or at slaughter at carcass weights of approximately 300 kg (Schoonmaker et al.

2003). However, more rapid fattening of the carcass was evident among young cattle weaned onto a high–

concentrate diet provided ad libitum compared to those normally–weaned or fed forage ad libitum following early weaning. This regimen resulted in the early–

weaned, ad libitum fed cattle having smaller carcasses and smaller longissimus muscles with less intramuscular fat when the groups were slaughtered at a constant subcutaneous fat depth (Schoonmaker et al. 2004). A restricted allowance of a concentrate diet after early weaning also increased intramuscular fat content compared to early weaned cattle fed ad libitum post–

weaning, although other carcass characteristics did not differ between these groups (Schoonmaker et al. 2004).

A recent study by Tomkins et al. (2005) was undertaken to assess the impact of weaning in seasonal dry periods of varying severity in Northern Australia.

During growth to about 500 kg liveweight, growth rate of cattle that lost weight during the first four months post–weaning was not significantly different from that of cattle grown slowly or rapidly during the same period, suggesting that compensatory growth did not occur or was short–lived at pasture. At the end of the study (600 days post–weaning) animals from the rapid post–

weaning growth rate group were significantly heavier than those from the weight loss group, but did not differ from the slow growth group. Carcass weights, dressing percentage, P8 fat depth, marbling, eye muscle area and objective measures of meat quality for the longissimus muscle did not differ significantly between groups.

Conclusions

Severe growth retardation of cattle early in life is associated with reduced growth potential, resulting in smaller animals at any given age. The occurrence of long–term compensatory growth diminishes as the age of onset of a nutritional restriction resulting in severe growth retardation declines, such that intra–

uterine growth retardation can result in slower growth throughout postnatal life. However, severe weight loss during the months immediately following weaning of cattle well–grown to normal weaning weights, or slow growth of early–weaned calves, can also limit compensatory growth.

Carcass composition of low and high birth weight calves is similar at the same carcass weight. At equivalent carcass weights, calves grown slowly from birth to weaning have carcasses of similar or leaner composition than those grown rapidly unless high energy concentrate feed is provided post–weaning, in which case the slowly grown, small weaners can become fatter at equivalent weights post–weaning than the heavier weaners.

(n = 80) (n = 80) (P)

Peak force (kg) 4.23 4.20 0.73

Compression (kg) 1.51 1.52 0.98

Cooking loss (%) 23.1 22.8 0.43

Colour L (lightness) 39.3 39.1 0.67

Colour a (red/green) 26.2 26.1 0.77

Colour b (yellow/blue) 13.6 13.4 0.53

Values are predicted means from REML analyses including effects of birth weight, pre–weaning nutrition, sex, sire–genotype and their interactions, with carcass weight as a covariate (linear and, where significant, quadratic).

Refer to Tables 4 and 5 for growth and carcass characteristics of the cattle.

Table 6 Consequences of growth from birth to weaning for objective measurements of longissimus (striploin) quality of beef cattle at 30 months of age.

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Adverse effects of prenatal, pre–weaning and early post–weaning growth on eating quality at market weights are not evident and when differences occur, they suggest that cattle restricted early in life may have slightly more tender meat than cattle well–grown during the same period.

We propose that within pasture–based production systems for beef cattle, the plasticity of the carcass tissues, particularly of muscle which maintains a population of stem cells throughout postnatal life, allows animals that are growth–retarded early in life to attain normal composition at equivalent weights in the long–

term, albeit at older ages. However, nutrition during recovery from growth retardation or following early–

weaning is important in determining the subsequent composition of young, light weight cattle relative to their heavier counterparts.

Acknowledgements

The financial and in–kind support of the Cooperative Research Centre for Cattle and Beef Quality, NSW Department of Primary Industries, CSIRO Livestock Industries and the University of New England is gratefully acknowledged. We also wish to acknowledge the considerable efforts of research, technical and/or farm staff of the NSW Department of Primary Industries Agricultural Research and Advisory Stations at Grafton and Glen Innes and its Beef Industry Centre of Excellence in Armidale, at the Beef Quality CRC

‘Tullimba’ Feedlot, at CSIRO Livestock Industries, Queensland Bioscience Precinct, St Lucia and Tropical Beef Centre, Rockhampton, and at the University of New England Meat Science Complex, in the conduct of the Beef CRC research described in this review. The contributions of Dr Greg Harper and Mr Peter Allingham in providing critical comment on this review are also gratefully acknowledged.

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