The study in this research paper was undertaken with a hypothesis that accelerometer data can be used to improve monitoring of energy balance in dairy cows. Animals of high (select, S) and average (control, C) genetic-merit lines were allocated to two feeding systems, by-product (BP) and homegrown (HG). This culminated in four production systems referred to as BPS, BPC, HGS and HGC. Cows between their first and fourth lactations were included and a total of 8602 records were used. The target crude protein (CP) and metabolisable energy (ME) content in the BP diet was 185 g/kg DM and 12.3 MJ/kg DM while it was 180 g/kg DM, and 11.5 MJ/kg DM for the HG diet, respectively. Milk yield, body energy content (BEC) and animal activity were monitored while the animals were all housed for winter. Results showed that cows on homegrown feeds were significantly (P < 0.05) more active than cows on by-product feeds as indicated by higher motion index and number of steps per day. Feeding duration was not significantly different (P > 0.05) between cows under by-product feeding system irrespective of the energy balance of the cows. However, there were significant differences for cows under homegrown feeding system. Cows in negative energy balance had a longer feeding duration per day than cows in positive energy balance. Milk yield was negatively correlated (P < 0.05) to motion index and number of steps per day but not to lying time and feeding duration. The results showed differences in cow activity were related to diet content and body energy status. This is useful in precision farming where feeds are provided according to specific animal behaviour and feed requirements.

Strong genetic selection on production traits is considered to be responsible for the declined ability of dairy cows to ensure reproduction. The present study aimed to quantify the effect of genetic characteristics (breeds and genetic merit for production traits) and feeding systems (FS) on the ability of dairy cows to be inseminated. An experiment was conducted during 9 years on Normande and Holstein cows assigned to contrasted pasture-based FS. Diets were based on maize silage in winter and grazing plus concentrate in spring in the High FS; and on grass silage in winter and grazing with no concentrate during spring in the low FS. Within breed, cows were classified into two genetic groups with similar estimated breeding values (EBV) for milk solids: cows with high EBV for milk yield were included in a Milk-Group and those with high EBV for fat and protein contents were included in a Content-Group. Holstein produced more milk throughout lactation than Normande cows (+2294 kg in the High FS and +1280 kg in the Low FS, P<0.001) and lost more body condition to nadir (−1.00 point in the High FS and −0.80 kg in the Low FS, P<0.001). They also showed a poorer ability to be inseminated because of both a delayed commencement of luteal activity (CLA) and delayed first service (more days from start of the breeding season to first service, DAI1). Cows in the Milk-Group produced more milk than cows in the Content-Group, but milk solids production was similar. Cows in the Content-Group had earlier CLA than cows in the Milk-Group (P<0.01). Genetic group neither affected ovulation detection rate nor DAI1. Within breed and FS, cows with high genetic merit for milk yield had later CLA and DAI1. Cows in the High FS produced more milk and lost less condition to nadir than cows in the Low FS. FS did not affect dairy cows’ ability to be inseminated. However, cows with higher milk protein content, and presumably better energy balance, had earlier CLA (P<0.01) and DAI1 (P<0.10). In addition, higher milk yield was associated with poorer ovulation detection rate and oestrus intensity (P<0.05). The study showed that at similar EBV level for milk solids, selection for increased milk fat and protein content resulted in improved cyclicity and similar oestrous expression and submission rates compared with selection for increased milk yield.

The increased demand for animal-derived protein and energy for human consumption will have to be achieved through a combination of improved animal genetic merit and better management strategies. The objective of the present study was to quantify whether differences in genetic merit among animals materialised into phenotypic differences in commercial herds. Carcass phenotypes on 156 864 animals from 7301 finishing herds were used, which included carcass weight (kg), carcass conformation score (scale 1 to 15), carcass fat score (scale 1 to 15) at slaughter as well as carcass price. The price per kilogram and the total carcass value that the producer received for the animal at slaughter was also used. A terminal index, calculated in the national genetic evaluations, was obtained for each animal. The index was based on pedigree index for calving performance, feed intake and carcass traits from the national genetic evaluations. Animals were categorised into four terminal index groups on the basis of genetic merit estimates that were derived before the expression of the phenotypic information by the validation animals. The association between terminal index and phenotypic performance at slaughter was undertaken using mixed models; whether the association differed by gender (i.e. young bulls, steers and heifers) or by early life experiences (animals born in a dairy herd or beef herd) was also investigated. The regression coefficient of phenotypic carcass weight, carcass conformation and carcass fat on their respective estimated breeding values (EBVs) was 0.92 kg, 1.08 units and 0.79 units, respectively, which is close to the expectation of one. Relative to animals in the lowest genetic merit group, animals in the highest genetic merit group had, on average, a 38.7 kg heavier carcass, with 2.21 units greater carcass conformation, and 0.82 units less fat. The superior genetic merit animals were, on average, slaughtered 6 days younger than their inferior genetic merit contemporaries. The superior carcass characteristics of the genetically elite animals materialised in carcasses worth €187 more than those of the lowest genetic merit animals. Although the phenotypic difference in carcass traits of animals divergent in terminal index differed statistically by animal gender and early life experience, the detected interactions were generally biologically small. This study clearly indicates that selection on an appropriate terminal index will produce higher performing animals and this was consistent across all production systems investigated.

The present study was undertaken to examine the effect of cow genetic merit on enteric methane (CH4) emission rate. The study used a data set from 32 respiration calorimeter studies undertaken at this Institute between 1992 and 2010, with all studies involving lactating Holstein-Friesian dairy cows. Cow genetic merit was defined as either profit index (PIN) or profitable lifetime index (PLI), with these two United Kingdom genetic indexes expressing the expected improvement in profit associated with an individual cow, compared with the population average. While PIN is based solely on milk production, PLI includes milk production and a number of other functional traits including health, fertility and longevity. The data set had a large range in PIN (n=736 records, −£30 to +£63) and PLI (n=548 records, −£131 to +£184), days in milk (18 to 354), energy corrected milk yield (16.0 to 45.6 kg/day) and CH4 emission (138 to 598 g/day). The effect of cow genetic merit (PIN or PLI) was evaluated using ANOVA and linear mixed modelling techniques after removing the effects of a number of animal and diet factors. The ANOVA was undertaken by dividing each data set of PIN and PLI into three sub-groups (PIN:<£3, £3 to £15 and >£15, PLI:<£23, £23 to £67 and >£67) with these being categorised as low, medium and high genetic merit. Within the PIN and PLI data sets there was no significant differences among the three sub-groups in terms of CH4 emission per kg feed intake or per kg energy corrected milk yield, or CH4 energy (CH4-E) output as a proportion of energy intake. Linear regression using the whole PIN and PLI data sets also demonstrated that there was no significant relationship between either PIN or PLI, and CH4 emission per kg of feed intake or CH4-E output as a proportion of energy intake. These results indicate that cow genetic merit (PIN or PLI) has little effect on enteric CH4 emissions as a proportion of feed intake. Instead enteric CH4 production may mainly relate to total feed intake and dietary nutrient composition.

Crossbreeding of Holstein–Friesian dairy cows with both early maturing (e.g. Aberdeen Angus (AA)) and late maturing (e.g. Belgian Blue (BB)) beef breeds is commonly practised. In Ireland, genetic merit for growth rate of beef sires is expressed as expected progeny difference for carcass weight (EPDCWT). The objective of this study was to compare the progeny of Holstein–Friesian cows, sired by AA and BB bulls of low (L) and high (H) EPDCWT for performance and carcass traits. A total of 118 spring-born male progeny from 20 (9 AA and 11 BB) sires (8 L and 12 H) were managed together from shortly after birth to about 19 months of age. They were then assigned to one of two mean slaughter weights (560 kg (light) or 620 kg (heavy)). Following slaughter, carcasses were graded for conformation class and fat class, the 6th to 10th ribs joint was dissected as an indicator of carcass composition, and samples of subcutaneous fat and musculus longissimus were subjected to Hunterlab colour measurements. A sample of m. longissimus was also chemically analysed. Slaughter and carcass weights per day of age for AAL, AAH, BBL and BBH were 747, 789, 790 and 805 (s.e. 10.5) g, and 385, 411, 427 and 443 (s.e. 4.4) g, respectively. Corresponding carcass weight, kill-out proportion, carcass conformation class (scale 1 to 5) and carcass fat class (scale 1 to 5) values were 289, 312, 320 and 333 (s.e. 4.0) kg, 516, 522, 542 and 553 (s.e. 3.5) g/kg, 2.5, 2.4, 3.0 and 3.1 (s.e. 0.10), and 3.4, 3.5, 2.9 and 2.8 (s.e. 0.11). There were few breed type × genetic merit interactions. Delaying slaughter date increased slaughter weight, carcass weight and all measures of fatness. It also reduced the proportion of carcass weight in the hind quarter and the proportions of bone and muscle in the ribs joint. None of these effects accompanied the increase in carcass weight due to higher EPDCWT. It is concluded that BB have superior production traits to AA. Selection of sires for higher EPDCWT increases growth rate, kill-out proportion and carcass weight of progeny with little effect on carcass or muscle traits. The extra carcass weight due to higher EPDCWT is more valuable commercially than a comparable carcass weight increment from a delay in slaughter date because it comprises a higher proportion of muscle.

In genetic improvement programmes for beef cattle, the effect of selecting for a given trait or index on other economically important traits, or their predictors, must be quantified to ensure no deleterious consequential effects go unnoticed. The objective was to compare live animal measurements, carcass composition and plasma hormone and metabolite concentrations of male progeny of sires selected on an economic index in Ireland. This beef carcass index (BCI) is expressed in euros and based on weaning weight, feed intake, carcass weight and carcass conformation and fat scores. The index is used to aid in the genetic comparison of animals for the expected profitability of their progeny at slaughter. A total of 107 progeny from beef sires of high (n = 11) or low (n = 11) genetic merit for the BCI were compared in either a bull (slaughtered at 16 months of age) or steer (slaughtered at 24 months of age) production system, following purchase after weaning (8 months of age) from commercial beef herds. Data were analysed as a 2 × 2 factorial design (two levels of genetic merit by two production systems). Progeny of high BCI sires had heavier carcasses, greater (P < 0.01) muscularity scores after weaning, greater (P < 0.05) skeletal scores and scanned muscle depth pre-slaughter, higher (P < 0.05) plasma insulin concentrations and greater (P < 0.01) animal value (obtained by multiplying carcass weight by carcass value, which was based on the weight of meat in each cut by its commercial value) than progeny of low BCI sires. Regression of progeny performance on sire genetic merit was also undertaken across the entire data set. In steers, the effect of BCI on carcass meat proportion, calculated carcass value (c/kg) and animal value was positive (P < 0.01), while a negative association was observed for scanned fat depth pre-slaughter and carcass fat proportion (P < 0.01), but there was no effect in bulls. The effect of sire expected progeny difference (EPD) for carcass weight followed the same trends as BCI. Muscularity scores, carcass meat proportion and calculated carcass value increased, whereas scanned fat depth, carcass fat and bone proportions decreased with increasing sire EPD for conformation score. The opposite association was observed for sire EPD for fat score. Results from this study show that selection using the BCI had positive effects on live animal muscularity, carcass meat proportion, proportions of high-value cuts and carcass value in steer progeny, which are desirable traits in beef production.

Validation of economic indexes under a controlled experimental environment, can aid in their acceptance and use as breeding tools to increase herd profitability. The objective of this study was to compare intake, growth and carcass traits in bull and steer progeny of high and low ranking sires, for genetic merit in an economic index. The Beef Carcass Index (BCI; expressed in euro (€) and based on weaning weight, feed intake, carcass weight, carcass conformation and fat scores) was generated by the Irish Cattle Breeding Federation as a tool to compare animals on genetic merit for the expected profitability of their progeny at slaughter. A total of 107 male suckler herd progeny, from 22 late-maturing ‘continental’ beef sires of high (n = 11) or low (n = 11) BCI were compared under either a bull or steer production system, and slaughtered at approximately 16 and 24 months of age, respectively. All progeny were purchased after weaning at approximately 6 to 8 months of age. Dry matter (DM) intake and live-weight gain in steer progeny offered grazed grass or grass silage alone, did not differ between the two genetic groups. Similarly, DM intake and feed efficiency did not differ between genetic groups during an ad libitum concentrate-finishing period on either production system. Carcasses of progeny of high BCI sires were 14 kg heavier (P < 0.05) than those of low BCI sires. In a series of regression analyses, increasing sire BCI resulted in increases in carcass weight (P < 0.01) and carcass conformation (P = 0.051) scores, and decreases in carcass fat (P < 0.001) scores, but had no effect on weaning weight or DM intake of the progeny. Each unit increase in sire expected progeny difference led to an increase in progeny weaning weight, DM intake, carcass weight, carcass conformation score and carcass fat score of 1.0 (s.e. = 0.53) kg, 1.1 (s.e. = 0.32) kg, 1.3 (s.e. = 0.31) kg, 0.9 (s.e. = 0.32; scale 1 to 15) and 1.0 (s.e. = 0.25; scale 1 to 15), respectively, none of which differed from the theoretical expectation of unity. The expected difference in profitability at slaughter between progeny of the high and low BCI sires was €42, whereas the observed phenotypic profit differential of the progeny was €53 in favour of the high BCI sires. Results from this study indicate that the BCI is a useful tool in the selection of genetically superior sires, and that actual progeny performance under the conditions of this study is within expectations for both bull and steer beef production systems.

An evolutionary algorithm was applied to a mechanistic model of the mammary gland to find the parameter values that minimised the difference between predicted and actual lactation curves of milk yields in New Zealand Jersey cattle managed at different feeding levels. The effect of feeding level, genetic merit, body condition score at parturition and age on total lactation yields of milk, fat and protein, days in milk, live weight and evolutionary algorithm derived mammary gland parameters was then determined using a multiple regression model. The mechanistic model of the mammary gland was able to fit lactation curves that corresponded to actual lactation curves with a high degree of accuracy. The senescence rate of quiescent (inactive) alveoli was highest at the very low feeding level. The active alveoli population at peak lactation was highest at very low feeding levels, but lower nutritional status at this feeding level prevented high milk yields from being achieved. Genetic merit had a significant linear effect on the active alveoli population at peak and mid to late lactation, with higher values in animals, which had higher breeding values for milk yields. A type of genetic merit × feeding level scaling effect was observed for total yields of milk and fat, and total number of alveoli produced from conception until the end of lactation with the benefits of increases in genetic merit being greater at high feeding levels. A genetic merit × age scaling effect was observed for total lactation protein yields. Initial rates of differentiation of progenitor cells declined with age. Production levels of alveoli from conception to the end of lactation were lowest in 5- to 8-year-old animals; however, in these older animals, quiescent alveoli were reactivated more frequently. The active alveoli population at peak lactation and rates of active alveoli proceeding to quiescence were highest in animals of intermediate body condition scores of 4.0 to 5.0. The results illustrate the potential uses of a mechanistic model of the mammary gland to fit a lactation curve and to quantify the effects of feeding level, genetic merit, body condition score, and age on mammary gland dynamics throughout lactation.

Relationships between genetic merit for milk production and animal parameters and various parameters of reproductive performance were examined using multilevel binary response analysis in a study of 19 dairy herds for three successive years, representing approximately 2500 cows per year. The proportion of cows intended for rebreeding that were back in-calf again within 100 days of calving (ICR-100) and the proportion of cows that reappeared again with 365 (RR-365) and 400 days (RR-400) of a previous calving were considered in addition to the traditional measures of reproductive performance. Each 100-kg increase in genetic merit for milk yield was associated with an increased interval to first service (IFS) and calving index (CI) of 1.4 ( P < 0.001) and 1.8 days ( P < 0.001), respectively, a 0.5% increase ( P < 0.05) in calving rate to first insemination (CR-1) and 0.8% increase in RR-400. Each £10 increase in £PIN (the economically weighted yield selection index used in the UK that takes account of butterfat and protein yields) was associated with an increased IFS and CI of 1.5 ( P < 0.001) and 3.0 days ( P < 0.001), respectively. Cows with increased genetic merit for milk yield and £PIN were more likely to re-calve (RR-overall; P < 0.001). Each 1000-kg increase in 305-day milk yield was associated with an increased IFS and CI of 3.2 ( P < 0.001) and 7.8 days ( P < 0.001), respectively, and a 13.6 ( P < 0.001), 22.4 ( P < 0.001), 19.9 ( P < 0.001) and 19.0% ( P < 0.001) decrease in CR-1, ICR-100, RR-365 and RR-400, respectively. A 10-kg increase in maximum yield was associated with a 6.6-day increase in CI ( P < 0.001) and a 14.9 ( P < 0.001), 18.3 ( P < 0.001), 9.6 ( P < 0.05) and 14.2% ( P < 0.001) decrease in CR-1, ICR-100, RR-365 and RR-400, respectively. Fertility performance was also associated with season of calving, lactation number and dystocia score. Level of production had a larger effect on fertility performance than genetic merit for milk production suggesting that infertility at an individual cow level is more likely to be associated with increased production and an inability to meet the nutritional requirements of the cow.

One hundred and eight Holstein-Friesian cows in six herds were run on six separate farmlets over a 5-year period from 1995 to 1999 at Wollongbar Agricultural Institute, on the subtropical north coast of New South Wales, Australia. Three of the herds comprised high genetic merit (HGM) cows — Australian breeding value (ABV) of +49·1 kg for milk fat (F) plus protein (Pr) and three herds comprised low genetic merit (LGM) cows-ABV of 2·3 kg. Within genetic merit groupings, one herd was given 0·34 t (l), one herd was given 0·84 t (m) and one herd 1·71 t (h), of concentrate per cow per lactation. Within each genetic merit group, cows were matched for milk yield and live weight, and over all groups for time of calving and age at the commencement of the study. The 30 paddocks within each farmlet were matched between farmlets for pasture type and pasture growth rate and soil fertility. Half the cows within each herd calved over a 3-month period in spring and the other half in autumn. Strict management criteria ensured that there was no bias towards particular treatment groups.

HGM cows were ‘open’ (days from calving to conception) for 8 days longer than the LGM cows (99 v. 91 days). The lHGM cows took 11 days longer to commence luteal phase activity and 21 days longer to first observed oestrus post calving than hLGM cows (P < 0·001), with the other groups being intermediate.

After 24 days of mating, 22% of lHGM cows were pregnant, and this was less than half of the rate of the best herd-mLGM. After 9 weeks of mating, the chances of an LGM cow being pregnant was 87% greater than an HGM cow. After 12 weeks of mating, 70% of lHGM cows were pregnant compared with a mean pregnancy rate of 87% for the LGM cows.

The number of cows treated for abnormal ovarian activity (anoestrus, cystic) was highest (P < 0·001) in the HGM herds given ‘l’ and ‘m’ levels of concentrate compared with the remaining herds (0·24 v. 0·12 treatments per cow mated, respectively).

There was a significant positive relationship between live-weight change from 4 weeks before, to the start of, the mating period and the chances of a cow being pregnant at 24 days (P < 0·05) and at 6 and 9 weeks after the commencement of mating.

There was a significant negative relationship (P < 0·001) between the change in daily F plus Pr yield, from the start to 4 weeks after mating began, and pregnancy rate at 9 weeks. The change in F plus Pr yield was +63 g/day for cows pregnant at nine weeks as opposed to +154 g/day for cows not pregnant.

The results of the present study indicate that the reproductive performance of HGM cows, with a mean of 61% North American (NA) genes, is lower than LGM cows (22% NA genes) under a predominantly pasture-based system of farming. The influence on reproduction was possibly due to genes favouring partitioning of energy to milk yield rather than body-condition maintenance in the HGM cows and when food intake was inadequate, then being more willing to use body reserves.

These reproductive problems may be reduced by more intensive reproductive management. However, such practices are costly and time consuming. Another approach may be to ensure that live-weight loss over the mating period is minimized by strategic supplementary feeding.

This paper describes a two-part study evaluating the use of growth hormone (GH) response to infusions of somatostatin (SRIF) and GH-releasing factor (GRF) as an indicator of genetic merit in dairy cattle. The first part of the study involved the development of a standard test procedure. This was achieved in two sub-trials, the first testing three SRIF doses, 0·6, 3·0 and 15 µg/kg per h, given over an infusion length of 3 h. The second involved infusing 3·0 μg/kg per h SRIF for either 20 min, 1 h or 3 h. The GH response was measured in serum during the infusion and for 2 h following cessation, during which time a rebound response in GH occurred. As a consequence of these trials, a total of 40 British Friesian calves from two lines genetically divergent for yield traits were infused with 3·0 μg/kg per h for a period of 1 h and at the end of the infusion given 0·1 μg/kg GRF. Five hours later, GRF was administered without SRIF. The protocol was repeated on 2 days consecutively. Results suggested that calves of high merit had lower SRIF-induced baselines than their low-line contemporaries. There were no differences in the GH concentrations immediately following GRF administration, either with or without SRIF. The results from this trial, together with previously published work, indicated that calves selected for high milk fat plus milk protein production may be more sensitive to SRIF such that GH is regulated by lower concentrations of SRIF. However, the practical use of GH response to SRIF as an indicator of genetic merit for dairy cattle may be limited because of logistical difficulties in implementation.

To determine a best measure for predicting genetic merit for milking and nursing ability in beef cattle, restricted maximum likelihood (REML) estimates of variances and covariances for direct and maternal effect were obtained by using pre-weaning growth records of calves at various stages after parturition until weaning in Japanese Black cattle. Data used in this study were pre-weaning growth records of 1892 Japanese Black calves obtained from the Tottori National Livestock Breeding Station covering the period from 1960 to 1985. Traits analysed were adjusted monthly weights, adjusted monthly heights and cumulative daily gains (DGs) from birth to 1, 2, 3, 4, 5 and 6 months of age. Estimates of variances and covariances were obtained by REML using the DFREML program of Meyer. Results indicate that: (1) the direct effects on pre-weaning growth of calves are the smallest at birth to 1 month of age and then increase; (2) the maternal effects of the dams are the largest for the first 2 months, and then decrease; (3) the contribution of these two effects on calf growth is reversed at 3 or 4 months of age; (4) the non-additive maternal effects exist but are small; and (5) the genetic correlations between direct and maternal effects are low for weight and cumulative DG, whereas those for height are relatively high. These results suggest that DGfrom birth to 2 months of age (DGO-2) is most influenced by the additive maternal effect. Thus, DGO-2 is concluded to be the best measure for predicting genetic merit for milking and nursing ability in beef cattle.

The aim of this study was to examine the relationship between pedigree index for kg fat plus protein and the financial margins between milk returns and food, health and reproduction costs in individual lactations. Records of milk and milk component production, food intake, health and reproduction were obtained for individual Holstein-Friesian cows over 38 weeks of lactation over a 7-year period, at the University of Edinburgh/Scottish Agricultural College Langhill dairy cattle research centre. Records for a total of 302 lactations were collected. Milk prices, input costs and margins were calculated for individual animals, using 1989/90 values. Regressions of margin over food costs (MF) and margin over food, health and reproduction costs (MFHR) on pedigree index (PI) for kg fat plus protein were all positive and ranged from £2·50± 3·00 per kg PI for MFHR for heifers, to £6·50 ± 2·10 per kg PI for MFHR for all animals. The lower regression for heifers was attributed to higher reproduction costs in animals of higher genetic merit, though the standard error for the regression estimated in this smaller data set was high. Sensitivity analyses showed that the regression coefficients remained very similar, even with ±50% changes in food, health or reproduction costs. It was concluded that selection on PI for kg fat plus protein is likely to lead to increased margin over food, health and reproduction costs.

Two studies were conducted to examine the effects of selection for milk-fat yield on pancreatic responses to exogenous glucose, as measured by circulating insulin concentrations following an intravenous glucose load, in heifer calves. The first study involved 6-month-old Friesian heifers, eight each from the Massey University high breeding index (HBI) and low breeding index (LBI) lines. Average breeding indices (BI) for milk-fat yield of the calves based on ancestry BI were 136 (s.d. 2) and 112 (s.d. 2), respectively. Oestrous cycling of the heifers was controlled by intravaginal insertion of progesterone-impregnated controlled internal drug-releasing devices (CIDRs). Challenges of glucose (0, 75, 150 and 300 mg/kg live weight) and insulin (0, 0·1, 1, and 10 fig/kg live weight) were administered via indwelling jugular cannulas and were followed by serial blood sampling to measure responses. Plasma insulin concentrations immediately following the glucose challenge were significantly (P < 0·05) greater in HBI than in LBI heifers, peak insulin concentrations (above baseline) in the HBI group being approximately twice those in the LBI group. Plasma glucose concentrations were marginally lower in HBI heifers, probably due to the secondary effect of the relatively greater hyperinsulinaemia in this group. Although the exogenous insulin challenge significantly reduced circulating glucose concentrations in all heifers, there was no difference between the lines in the magnitude of insulin-induced hypoglycaemia.

The second study was designed to determine whether insulin responses to an exogenous glucose load were influenced by stage of the oestrous cycle in heifers of the two lines. In this study, eight yearling HBI heifers and eight LBI heifers (with BI of 135 (s.d. 4) and 111 (s.d. 1) respectively) were administered an intravenous glucose load (150 mg/kg live weight) on tivo occasions, the first being after 17 days of CIDR treatment (simulating the luteal phase of the oestrous cycle) and the second 46 h after withdrawal of the CIDRs (when the heifers were expected to be in oestrus). HBI heifers again showed significantly greater circulating insulin concentrations than LBI heifers following the glucose challenge but the magnitude of this effect was not influenced by time relative to CIDR withdrawal. It is concluded that heifers selected for high milk-fat yield have an increased pancreatic response to exogenous glucose which is apparently independent of the stage of the oestrous cycle. This difference may provide a useful means of identifying heifers of superior genetic merit for milk-fat production prior to their first lactation.

Fifty-five calves aged 105 days of age, of both sexes, belonging to two genetic groups (H, high or L, low) differing in their predicted breeding value (PBV) by 78 kg for fat plus protein yield, were individually penned for 3 weeks with the final 2 weeks on a diet designed to provide energy close to their maintenance requirements. The calves were then cannulated in the jugular vein and blood was sampled every 15 min for 25 h. Following this a growth hormone releasing factor (GRF) preparation was administered on up to four occasions, being one of (per kg live weight) either 0·2 or 0·4 μg GRF (treatments SGRF or DGRF respectively) or 0·2 μg thyrotropin releasing hormone (TRH). On each occasion blood samples were taken at −15, 4, 8,12,16, 20 and 45 min relative to the time of administration and up to four further occasions, one prior to and three within 32 min of administration. Samples were assayed for growth hormone (GH).

GH concentration of troughs prior to an episode of GH release was 1·19-fold greater in H compared with L calves with concentrations decreasing to 0·9 of their previous values each h. Trough and peak concentrations had repeatabilities of 0·21 and 0·26 respectively (both P < 0·05). There was no association between PBV and either peak concentrations, number of pulses (4·84 per 25 h) or mean GH concentration (13·3 μg/l).

GH released was only 1·11-fold greater after DGRF than SGRF. The difference in the regression coefficients for PBV between SGRF and DGRF was small, as it was for the difference between coefficients for the two sexes. The pooled coefficient was 0·00342 loge units per kg (s.e. 0·00157; P< 0·05) indicating a 1·31-fold greater response in H than in L calves. The correlation between responses to SGRF and DGRF was 0·482 and the repeatability of SGRF was 0·338; a pooled repeatability was estimated as 0·362(P < 0·01). The regression of GH release on PBV for TRH was 0·00345 (s.e. 0·00330). For all secretagogues, response depended on prior concentrations.

It was concluded that GH release following GRF administration was positively related to PBV in dairy calves and response was moderately repeatable; furthermore, although aspects of endogenous secretion may be related to PBV they suffer from measurement difficulties.

This study investigated the potential use of blood metabolite and hormone concentrations as genetic markers for milk fat production and their possible interactions with sex and age. Two groups of calves, one from the Massey University high breeding index (HBI) line of dairy cattle (seven males, eight females) and the other from the low breeding index (LBI) line (four males, 11 females), were studied at 3·5 months and 7 months of age. The average breeding indices (BI) of the calves based on ancestry BI were 138 (s.d. 4·4) and 111 (s.d. 2·3) respectively. Serial blood sampling regimens were conducted in relation to feeding (chaffed lucerne hay at 1·3 times maintenance energy requirement), during an intravenous urea load (120 mg/kg live weight) and during fasting (63 h) and refeeding. Urea spaces and fractional decay constants at each age were estimated by a single compartment distribution model based on plasma urea concentrations following the intravenous urea load.

Plasma concentrations of urea, creatinine and non-esterified fatty acids (NEFA) were not significantly different between the lines in any of the periods examined and at either age. In contrast, plasma concentrations of glucose and insulin were greater in the HBI calves than in the LBI calves although these differences were restricted mainly to the period immediately after feeding. Urea space at 7 months of age was also greater in the HBI animals than in the LBI animals. Plasma concentrations of all hormones and metabolites except insulin were significantly influenced by sex and/or age.

The study does not confirm previous findings that genetic merit for dairying is expressed in terms of plasma levels of urea and NEFA, particularly those during a fast. However, the results of the present study are consistent with previous observations of differences in glucose and insulin metabolism between the tivo Massey University genetic merit lines. These traits may therefore have potential as genetic markers for milk fat production.

Eighty calves of both sexes, 4 months of age, of two lines, differing in genetic merit for fat plus protein production by 70 kg per 305-day lactation, were given one of four diets differing in protein content to give an estimated range of protein intakes from 299 to 530 g/day. After 21 days the calves were fasted for 72 h. Samples of blood were taken before and after feeding in the morning and afternoon prior to fasting, seven further samples were taken throughout the fast and five samples were taken ranging from 0·5 to 7·5 h after food was re-introduced. The samples were analysed for concentrations of urea, free fatty acids (FFA), glucose, triglyceride and f5-hydroxybutyrate.

No association between metabolite concentrations and predicted breeding value (PBV) were found at any stage of the trial except for the effect of feeding on FFA concentration, the rate of increase of FFA during the fast and its ultimate concentration. These associations with FFA depended on the sex of the calf and were either absent or less marked in males. The results strengthen the possibility that FFA concentrations during fasting are related to PBV in females, and that sexual dimorphism may explain differing conclusions amongst previous trials.

The magnitude of the estimate of coheritability of urea concentration during fasting with PBV, pooled across experiments, is reduced from -0·193 to -0·066 (s.e. 0·037) by inclusion of these results. However, heterogeneity between experiments is evident and remains to be explained. This suggests that urea concentrations are not consistent predictors of genetic merit and cannot be considered to be reliable for use in practice.