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J. Dairy Sci. 2007. 90:3909-3916. doi:10.3168/jds.2006-731
© 2007 American Dairy Science Association ®
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Metabolic Regulation in Danish Bull Calves and the Relationship to the Fertility of Their Female Offspring

C. Hayhurst*,1, M. K. Sørensen{dagger}, M. D. Royal* and P. Løvendahl{dagger}

* Department of Veterinary Clinical Sciences, University of Liverpool, Leahurst, Neston, South Wirral, CH64 7TE, United Kingdom
{dagger} Department of Genetics and Biotechnology, Danish Institute of Agricultural Sciences, Research Centre Foulum, PO Box 50, DK-8830 Tjele, Denmark

1 Corresponding author: Catherine.Hayhurst{at}liverpool.ac.uk


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES
 
The objective of this work was to estimate the genetic variation of free fatty acids (FFA), glucose, growth hormone (GH), and insulin in juvenile male dairy calves and to assess the relationships, if present, with the fertility of their female offspring. This study used data from 1,498 (269.5 d of age ± 11.1) male calves from a multiple ovulation and embryo transfer breeding scheme (data collected from 1997 to 2002). Calves were Danish Holstein (n = 1,047), Danish Jersey (n = 200), and Red Dane (n = 251), and were sampled following an overnight fast at approximately 9 mo of age. Plasma samples were assayed for basal FFA, glucose, GH, and insulin. Estimated breeding values of female fertility (high values indicating better fertility), based on progeny-test results for approximately 100 daughters per sire, were available for a subset (n = 810) of the male calves as adult sires. Data from Danish Holstein alone or Danish Holstein, Red Dane, and Danish Jersey combined (all breeds) were analyzed for each trait. In both data sets, the estimates of heritabilities of glucose (0.27 ± 0.06), FFA (0.11 ± 0.05), and insulin (0.21 ± 0.06) were moderate, and that of GH (0.09 ± 0.05) was low. Correlations of estimated breeding values for fertility traits with glucose and FFA breeding values were negative, indicating that male calves with high glucose or FFA had female offspring with reduced fertility. Selection for bull calves with lower concentrations of glucose and FFA following an overnight fast could result in female offspring with genetically better fertility. Glucose and FFA may therefore be of interest to enhance selection for improved female fertility, as a measurement in young bulls.

Key Words: fertility • metabolic hormones • heritability • correlation


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES
 
In the past, most dairy cattle breeding programs have focused on increasing production. In fact, milk yield has increased dramatically over the last 30 yr, particularly in the Holstein-Friesian population. Although, this remains a major economic factor today, there is growing evidence of unfavorable genetic correlations of production traits with others of economic importance such as fertility (e.g., Pryce et al., 1997; Veerkamp et al., 2000; Royal et al., 2002a). Therefore, broader selection goals have been introduced and are being continually developed to include traits, such as fertility, that are associated with longevity (Miglior et al., 2005). Although many countries now have the opportunity to select for fertility, the available selection tools are not perfect. Therefore, the challenge facing the dairy industry is to regain a balance between milk yield and fertility by placing more emphasis on selection for fertility while maintaining selection for milk yield.

The Danish dairy industry has published a fertility index since 1995 (Pedersen and Jensen, 1996). The current fertility index combines information on several fertility measures (days from first to last insemination in heifers and cows, days from calving to first insemination in cows, nonreturn rate in heifers and cows, estrus expression in heifers and cows, and fertility treatments in cows), which are combined and weighted according to their economic values. A greater fertility index indicates better fertility. One limitation of all current fertility indices worldwide is that the traits are measured in mature daughters of bulls and have low heritability, so genetic progress in fertility is slow. The addition to female fertility indices of an appropriate indicator trait that is measurable in the juvenile male could increase the rate of genetic improvement.

Subsequent to work by Land (1973), who first proposed that sex-linked characters in the female are expressed in the male, several studies in calves and lambs have looked at potential physiological juvenile indicator traits for female reproduction (e.g., Haley et al., 1989; Mackinnon et al., 1991; Royal et al., 2000). These studies have focused on reproductive hormones (testosterone, LH); however, no known studies have investigated the potential of metabolic hormones as genetic indicators of fertility.

Following parturition, many cows enter a period of negative energy balance (NEB) that can last for several weeks (Butler, 2000). Furthermore, the duration and severity of NEB postpartum, often determined by changes in BCS, is unfavorably correlated (phenotypically and genetically) with the interval to first ovulation (Butler, 2000; de Vries and Veerkamp, 2000; Dechow et al., 2002; Royal et al., 2002b). During this period of NEB, changes are seen in the concentrations of FFA, glucose, growth hormone (GH), insulin, IGF-I, and other regulatory hormones (Hart, 1983; Butler, 2000; Roche et al., 2000). Synthesis of GH by the anterior pituitary gland increases (Diskin et al., 2003) causing an increase in lipolysis, which results in elevated levels of circulating FFA (Hart, 1983). Some FFA in turn is transported to the liver, where it can accumulate and lead to liver ketosis (Bobe et al., 2004). Furthermore, concentrations of circulating insulin and glucose decrease, and liver GH receptors decrease causing IGF-I production by the liver to decrease (Butler et al., 2003).

It has been suggested that FFA, glucose, GH, and insulin could be used as indicators of energy balance (Reist et al., 2002), and thus for NEB. However, in addition to their involvement in metabolic regulation, these metabolites and hormones per se have links with many aspects of reproduction including follicle growth and steroidogenesis (reviewed by Webb et al., 2004) such that altered concentrations during NEB could impair follicle growth and steroidogenesis (Roche et al., 2000). This therefore highlights the possible route for a genetic link between energy balance and fertility.

To be an efficient juvenile indicator trait for female fertility, it is important that the parameter in question (e.g., a metabolite or hormone) has moderate heritability and sufficient genetic correlation with female fertility. To date, there have been no studies investigating the genetic relationship of metabolic regulation in calves with female fertility and few estimating the heritability of these possible indicators. Previous studies have found estimates of the heritability of GH ranging from 0.04 to 0.60 in 9-mo-old dairy calves (Løvendahl et al., 1994; Sørensen et al., 2002) and of glucose at 0.41 in 3- to 15-mo-old dairy calves (Rowlands et al., 1983).

The aim of this study was to estimate the genetic variation in FFA, glucose, GH, and insulin plasma concentrations in 9-mo-old male dairy calves and to assess the strength of any genetic link with the fertility of their female offspring.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES
 
For this study, data collected during a Danish project investigating physiological indicator traits in Danish dairy cattle were used (1997–2002; Danish Institute of Agricultural Sciences, Foulum, Denmark; Løvendahl and Sørensen, 2001). All procedures involving animals were approved by the Danish Animal Experiments Inspectorate and complied with the Danish Ministry of Justice Law no. 382 (June 10, 1987) and Acts 739 (December 6, 1988) and 333 (May 19, 1990) concerning animal experimentation and care of experimental animals.

Animals and Experimental Design
A cohort quantitative genetic study was designed. All animals were male calves from the Danish progeny-testing scheme (269.5 d of age ± 11.1). Breeds represented included Danish Holstein (n = 1047), Danish Jersey (n = 200), and Red Dane (n = 251), as shown in Table 1Go. Physiological data including plasma samples were collected, and progeny-test results for these animals were collated at a later stage (Danish Cattle Federation, Aarhus, Denmark). Ancestry was traced back at least 3 generations to construct a pedigree file with 59,243 animals.


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  		Table 1. Number and age of male calves of each breed sampled at each of the 4 stations (A to D)
 
Housing and Management
Calves were housed at 4 experimental stations (A, B, C, and D) where they arrived ideally before 3 mo of age; calves arriving up to 5 mo of age were still accepted for further testing. Calves were born in several private herds (468 herds submitting from 1 to 37 calves each) and transferred to the experimental stations following the necessary health checks. A diet based on dried grass pellets supplemented with barley straw was fed ad libitum. Pellets were offered from feed troughs, and water was freely available throughout.

Experimental Procedure
Two sampling protocols were used during this study. Initially, blood samples were taken by jugular venipuncture (protocol 1; 1997–1999) and later by jugular cannulation (protocol 2; 1999–2002).

Eight days before the day of sampling (d 0) calves were grouped into batches of 5 to 24 and weighed (Figure 1Go). Normal feeding occurred on d –8, –7, –6, and –5. On d –4, –3, –2, and –1 calves were fed to cover maintenance only in a single meal given at 1530 h to avoid interference between expected feeding time and circulating hormone concentration and so that the time elapsed since the last meal (16.5 h) was the same in all animals on the day of sampling. On d –1 calves were cannulated; on d 0, sampling was carried out and normal feeding resumed at 1530 h.


Figure 1
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  		Figure 1. Feeding protocol before blood sampling on d 0.

 
Various blood samples were taken during the day, and Table 2Go indicates which samples were used for the current analyses. Cannulas were filled with isotonic saline supplemented with heparin between samplings. Blood was chilled on ice and plasma separated by centrifugation (2,000 x g, 4°C, 20 min) and stored frozen (–20°C) until assayed.


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  		Table 2. Sampling schedule on d 0 highlighting samples used for growth hormone (GH), glucose, insulin, and FFA measurement in protocols 1 and 2
 
Hormone and Metabolite Assays
Growth hormone and insulin concentrations were determined by validated, noncompetitive, time-resolved immunofluorometric assays as previously described by Løvendahl et al. (2003) and Løvendahl and Purup (2002), respectively. Free fatty acid concentrations were determined using a commercial assay kit (NEFA C; Wako Chemicals GmbH, Neuss, Germany) adapted to automation on a Advia 1650 Bayer Opera system (Bayer, Raleigh, NC). Similarly, a commercial assay kit (Glucose-hexokinase II; Bayer) adapted to the same automated system was used to determine glucose concentrations.

Fertility Breeding Values
Estimated breeding values for female fertility (FertEBV; estimated from approximately 100 daughters each, and calculated by the Danish Cattle Federation) were available for 810 male calves as sires (calves not available as sires had been culled). At the time of these analyses, the fertility index linked information on only 3 fertility measures routinely recorded in Denmark: days from first to last insemination in heifers, days from first to last insemination in cows, and days from calving to first insemination in cows. The 3 breeding values were weighted according to economic value and combined to give FertEBV. The FertEBV is standardized to give a mean of 100 and a standard deviation of 10; higher values of FertEBV are indicative of better fertility. Unfortunately, the individual components of the FertEBV were not available for analysis.

Statistical Analysis
Concentrations of GH, insulin, and FFA were log-e transformed to give approximately normally distributed residuals. The geometric mean was obtained by back transformation (ex) to give values in measured units.

A univariate mixed model was fitted to the data using the average information-REML method in the DMU software (Madsen and Jensen, 2002). Data for Danish Holstein, Red Dane, and Danish Jersey were analyzed jointly (all breeds), in addition to a subset containing Danish Holsteins only. The inclusion of various effects in the model were explored and the final model fitted to the hormone and metabolite data was:


Formula

where Yijklm = the hormone or metabolite concentration; fixed effects are {alpha} = intercept, Fi = station (i = 1 to 4), Bj = breed (j = 1 to 3), Pk = protocol (protocols 1 and 2), D = age of animal in days; b = regression coefficient, and where random effects are Al = breeding value [N (0, {sigma}2AA], where A is the numerator relationship matrix of animals available in the data, Bm = batch effect of sample [N (0, {sigma}2B)], and {varepsilon}ijklm = error term [N(0, {sigma}2E)].

Heritability (h2) was calculated as the proportion of phenotypic variance ({sigma}2p) attributable to additive genetic variance ({sigma}2a). The variance due to batch effect ({sigma}2b) in each case was small in relation to the total variance and was not included in the phenotypic variance:


Formula

The standard error of the heritability estimates were calculated using the Taylor series expansion and the following formulas:


Formula

The significance of the fixed and random effects used in the model was assessed using SAS software (version 8, SAS Institute Inc., Cary, NC) for each physiological trait analyzed, but using a common final model as above. Approximate genetic correlations (rEBV) were estimated by correlating the hormone or metabolite breeding values with the FertEBV. However, EBV are regressed toward zero and consequently, the correlation estimates will be biased toward zero as well. To compensate for this bias, a correction was needed taking into account the accuracies of the 2 sets of breeding values (r2IA1 = squared accuracy of trait-1 breeding values and r2IA2 = squared accuracy of trait-2 breeding values) as suggested by Blanchard et al. (1983) and by Calo et al. (1973):


Formula

Squared accuracies for indicator traits (glucose, GH, insulin, and FFA) were calculated as:


Formula

where {sigma}2a2 = the additive genetic variance for an animal for a particular indicator trait and pev = the accompanying predicted error variance of the additive genetic variance.

In this study, the accuracies on the indicator traits were especially low (range: 0.04 to 0.20) because information came solely from a single test on the individual and the traits had low heritability. For the fertility traits, accuracies were from a progeny-testing scheme with around 100 offspring per sire giving accuracies of approximately r2IA = 0.80. Therefore, the estimates of genetic correlations were larger than the correlations between breeding values (rEBV).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES
 
The plasma concentrations of FFA, glucose, GH and insulin are given in Table 3Go. There was little difference between the plasma concentrations in the 3 breeds or between the 2 data sets investigated (all breeds and Danish Holsteins). Furthermore, the average FertEBV was similar for each breed and close to the standardized average for the FertEBV of 100. Plasma concentrations of FFA, glucose, GH, and insulin found in this study were comparable to those found in animals of similar age during mild feed deprivation (e.g., Løvendahl et al., 1994; Govoni et al., 2003; Taylor et al., 2004). The plasma concentration of FFA increases during a fast with a significant increase seen after a single night of fasting (Sinnett-Smith et al., 1987). Although in the present study prefast levels of FFA were not available, the FFA concentrations were significantly elevated after one night of fasting when compared with prefast levels in previous studies (Sinnett-Smith et al., 1987; Woolliams et al., 1992; Jorritsma et al., 2003).


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  		Table 3. Number of calves sampled, mean (SD) and geometric mean of fertility EBV (FertEBV), FFA, glucose, growth hormone (GH), and insulin in Danish Holsteins, Danish Jersey, Red Dane, and all breeds combined
 
Plasma glucose concentrations in ruminants are more stable than in monogastric species, with significant reductions seen after 24 to 48 h of fasting (Diskin et al., 2003; Chelikani et al., 2004). In the present study, the concentrations of glucose were similar to those seen in fed animals of other studies. Insulin falls in response to reduced glucose; therefore, because glucose concentrations appear not to have dropped, we propose that insulin concentrations in these animals would also not have dropped significantly. Growth hormone concentration shows greater variation and release tends to increase at night and during periods of energy restriction (Tannenbaum, 1988). The plasma concentrations of GH in this study (2.69 ng/mL in Danish Holstein; 2.56 ng/mL in all breeds) were similar to those reported by Chelikani et al. (2004) after a 12- to 24-h fast (1.50 to 1.80 ng/mL).

The fixed effects of station, breed, and protocol were significant (P < 0.05) in each analysis, but age was not significant (P > 0.05). The greatest differences in the hormones and metabolites measured were apparent among the 4 stations used rather than among the different protocols and breeds. Station C had the lowest average concentrations of insulin, glucose, and FFA, but the highest concentration of GH. This is perhaps due to management differences. In addition, this station contained only Danish Holsteins and, as such, the effects were inseparable. Farm environment can affect the extent to which an animal’s genotype is expressed (i.e., genotype by environment interaction; reviewed by Bryant et al., 2005). In the present study, this could have affected the hormone and metabolite concentrations in the males and, to a lesser extent, the FertEBV, because data collected on at least 100 females was used to calculate the FertEBV (Danish Cattle Federation). Although the regression of age was not significantly different from zero, it was decided that it should remain in the model due to the monthly sampling leading to a spread in the ages of the animals (220 to 313 d).

The heritability estimates with standard errors are given in Table 4Go. In general, the heritability estimates for glucose, GH, and insulin were higher in all breeds combined than in Danish Holsteins, albeit not significantly. The standard errors were of similar magnitude in both cohorts. Free fatty acids, glucose, and insulin concentrations had moderate heritability in both Danish Holsteins and all breeds combined (heritability ± standard error range: 0.11 ± 0.05 to 0.30 ± 0.07). However, the heritability estimate for GH was low in Danish Holsteins and in all breeds combined (0.06 ± 0.06 and 0.11 ± 0.06, respectively), which is in agreement with others (Løvendahl et al., 1994; Grochowska et al., 2001). This may have been due to pulsatile GH secretion. Large secretory pulses of GH add noise to the data and thereby to the total variance, which reduces the heritability estimate (Theilgaard et al., 2007). This could partly be overcome by serial blood sampling over an extended period. Alternatively, the stimulated release of GH could be studied. This approach is more time effective because fewer blood samples over a shorter period are needed. Grochowska et al. (2001) estimated the genetic variation in peak GH release following thyrotropin-releasing hormone challenge and found a higher heritability than for baseline GH (Polish Friesian, male and female, n = 214, age 335 ± 8 d; h2 = 0.14 ± 0.11 vs. 0.02 ± 0.11). Similarly, Løvendahl et al. (1994) found a higher heritability in peak GH following growth hormone releasing factor stimulation (Danish Jersey, Red Dane, Danish Friesian, and Danish Red and White, male n = 284 and female n = 272, age 242 to 311 d; h2 = 0.42 ± 0.16 for males, h2 = 0.60 ± 0.16 for females) rather than for baseline GH (h2 = 0.04 ± 0.12 for males, h2 = 0.60 ± 0.16 for females).


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  		Table 4. Estimated heritability (h2 ± SE) for plasma FFA, glucose, growth hormone (GH) and insulin in Danish Holsteins and all breeds combined (Danish Holsteins + Danish Jersey + Red Dane)
 
This study has shown that the concentrations of FFA, glucose, and insulin in male calves were moderately heritable. This is, to some extent, due to them being easier to study, because levels are more stable leading to less "noise" in the data. The heritability estimate for glucose is lower than in a previous study by Rowlands et al. (1983), which looked at the genetic variation in glucose and other circulating metabolites in fed young British Friesian bulls (n = 428; 3 to 15 mo of age). The heritability estimate for plasma glucose in the Rowlands et al. (1983) study was high (h2 = 0.41 ± 0.17) although the standard error was large. Heritability estimates for glucose reported in a Danish study of Red Dane, Danish Friesian, Danish Jersey, and Danish Red and White 9-mo-old calves (h2 = 0.22 ± 0.08 for males, n = 451; h2= 0.28 ± 0.09 for females, n = 371; Løvendahl and Jensen, 1997) were similar to results in the current study. The heritability estimates they reported for insulin and FFA were greater than in the present study (h2 = 0.04 ± 0.08 for males, n = 334; h2 = 0.43 ± 0.11 for females, n = 300; h2 = 0.52 ± 0.16 for males, n = 198; h2= 0.32 ± 0.14 for females, n = 190, respectively; Løvendahl and Jensen, 1997) although the number of calves was low.

To be used as indirect selection criterion, in addition to having a genetic correlation to the trait of interest (in this case, fertility), the criterion must have a moderate heritability (Falconer and Mackay, 1996). Thus, the heritability estimates found here are encouraging. Although GH had low genetic variation, FFA, glucose, and insulin showed sufficient genetic variation to be suitable to be indirect selection criteria.

Correlations of FertEBV with the hormone and metabolite breeding values and the approximate genetic correlations are shown in Table 5Go. Growth hormone and insulin showed no significant correlation with FertEBV. The correlations of FertEBV with FFA were negative and significant (P < 0.001) in all breeds combined and in the Danish Holsteins subset. Furthermore, the correlation of FertEBV with glucose was negative in both the Danish Holstein subset and all breeds, although this estimate was not significantly different from zero in the Danish Holsteins. Therefore, on average, there was a small tendency for male calves with high glucose and FFA following overnight fast at 9 mo of age to produce female offspring with reduced fertility. The concentration of FFA following a fast indicates the extent to which an animal breaks down stored fat (lipolysis) during energy shortage. This correlation potentially indicates that calves that mobilize a large amount of FFA in response to a fast tend to have female offspring with reduced fertility. The plasma concentration of FFA, in addition to BCS, is a good indicator of the metabolic status of an animal, and increased concentrations of FFA are seen during NEB (Reist et al., 2002). In the past, cows have been selected for "dairy type," which is primarily lower BCS and higher milk yield. The length and severity of NEB is unfavorably correlated (phenotypically and genetically) to the interval to first ovulation postpartum (Butler, 2000; de Vries and Veerkamp, 2000; Dechow et al., 2002), so selection in the past for yield and dairyness may have inadvertently been for animals that have low BCS and experience severe or prolonged NEB.


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  		Table 5. Estimated correlations between fertility EBV (FertEBV) with plasma FFA, glucose, growth hormone (GH), and insulin breeding values (rEBV), correction factors and genetic correlations (rA) in Danish Holstein and all breeds combined (Danish Holsteins + Danish Jersey + Red Dane)
 
In this study, a short period of fasting may have resulted in lowered glucose levels. It is unlikely that the drop would be significant; however, there is clearly variation in glucose concentrations possibly due to differing responses to short-term feed deprivation. The negative genetic correlation between FertEBV and glucose would indicate that calves whose plasma glucose drops by a greater extent or is at a lower concentration tend to have female offspring with improved fertility, whereas calves whose plasma glucose drops less or is at a higher concentration tend to have female offspring with lower fertility. Blood glucose concentration is maintained within a narrow range by the alternating release of glucagon (when glucose drops below a certain concentration) and insulin (when glucose rises above a certain concentration; Jiang and Zhang, 2003). It is possible that calves whose glucose concentrations dropped the most following the overnight fast had a lower critical concentration below which the release of glucagon was stimulated. Consequently, in these animals, the plasma concentration of glucose was able to drop further before glucagon was released and both gluconeogenesis and glycogenolysis were initiated. The relationship of this in the male calf with better fertility in the female is unclear.

Concentrations of FFA and glucose have been found to be genetically correlated between male and female calves (9 mo of age) previously (Løvendahl and Jensen, 1997). It is likely that concentrations of FFA and glucose at 9 mo of age are genetically the same trait in males and females (Løvendahl and Jensen, 1997). Therefore, it is expected that female offspring from the males with high FFA and glucose after an overnight fast may also have a tendency to mobilize body reserves quickly during energy shortage, which might lead to a cow that will use most of her body reserves for milk production, go into extreme NEB, and subsequently have fertility problems such as a prolonged interval to first ovulation postpartum. Calves that are better at withholding their body reserves may go on to be cows that experience less severe NEB and fewer subsequent fertility problems.

The present study was designed to estimate correlations between metabolic traits recorded in a single blood sample or a few blood samples from young bulls and breeding values obtained from their female offspring during lactation. However, the correlations were from chains with several intermediate links, some of which are not necessarily strong. Separate investigations of each of the links, such as the genetic correlation between metabolic hormones in male and female calves, have been investigated at a genetic level in a previous study (Løvendahl and Jensen, 1997), and so have not been included in the current study. The larger question is how close metabolic measurements during lactation correlate with measurements during juvenile physiological states in fed or in feed-restricted animals. The literature is scarce on this and it is the subject of our current work. Furthermore, to assess the genetic correlations between concentrations of metabolic hormones in female calves with their fertility and concentrations of the same metabolic hormones during first lactation would require a very extensive experiment because fertility has low heritability and repeatability, leading to low reliability when seen as a performance test trait. Highly reliable breeding values for fertility are only available for males after progeny test and therefore these were used in the current study. This relationship between metabolic regulation in bull calves with their offspring’s fertility needs further investigation before use by the dairy breeding industry. Correlated responses to selection in other traits would need to be determined to prevent undesirable side effects to selection.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
ACKNOWLEDGEMENTS
 REFERENCES
 
This analysis has provided evidence to show that additive genetic variance is responsible for a substantial proportion of the phenotypic variation in a selection of metabolites and regulatory hormones in male calves. Furthermore, the results indicate that glucose and FFA in bull calves are negatively correlated to their EBV for fertility of their female offspring. Glucose and FFA, as a measurement in young bulls, may be of interest to dairy cattle selection programs to improve female fertility.


   ACKNOWLEDGEMENTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 ACKNOWLEDGEMENTS
REFERENCES
 
The authors are grateful to personnel at the 4 testing stations for care of animals and help during blood sampling, and to technical staff for testing animals (Jacob Jacobsen and Peter Johansen) and assaying blood samples (Janne Adamsen). This study was funded by Danish Cattle Federation (Aarhus N, Denmark), Dansire (Randers, Denmark), and the Directorate for Food, Fisheries and Agro Business, Denmark.

Received for publication November 6, 2006. Accepted for publication April 19, 2007.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES
 


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