Materials and methods breeding values estimated for them. While economic

224 P .R. Amer et al. Livestock Production Science 67 2001 223 –239 detail Cunningham, 1974; Barlow, 1982; Barlow breed parent in these herd types are shown in Table and Cunningham, 1984. Thus, it is timely for a 1. These breeding objective traits are divided into reassessment of the breeding objective for Irish beef five groups, with some repetition for traits included cattle. in the reproduction group. This repetition facilitates a The primary objective of this paper was to outline more simple combination of a reproduction sub- the approaches taken in the development of a index with sub-indexes for the other groups. Where breeding objective for Irish beef cattle. The sec- economic values vary across mating terminal sire ondary objective was to explore the potential mag- versus sire to breed female replacements and pro- nitude of variation in economic values when calcu- duction beef breed matings to dairy versus beef lated for some alternative breeds and production cows systems, sub-indexes can be presented to systems. This largely involved the use of published index users to allow flexibility in selection emphasis methodology and models, adapted to be relevant to across relevant groups of traits. Sub-indexes are Irish beef cattle production, although in some in- computed by setting economic values for all breed- stances, new models and approaches were used, and ing objective traits not in the sub-index of interest to these are also described here. zero Amer et al., 1998a. Traits expressed by calves prior to or at weaning are assumed to have both direct and maternal

2. Materials and methods breeding values estimated for them. While economic

values are typically the same for direct and maternal 2.1. Breeding perspective effects on the same trait, the number and timing of their expression is different across different classes In this study we take farm profit as the perspective of selection candidates and so both effects are from which the benefits of selective breeding are included in Table 1. defined. The number of animals per farm is assumed Cow mature weight takes three forms in the to be the long run constraint on farm size, as reproduction index. Cow mature weight influences opposed to the feed resources available, or the feed requirements for replacement heifers, mainte- absolute level of beef output. There are currently no nance requirements for mature cows neither in- limits to beef output per farm, rather, there is an cluded in the breeding objective as feed intake traits increased tendency for support systems to be linked in their own right, and also cull cow slaughter to extensive management practices such that stocking values. It was not appropriate to combine these into a rates are constrained below the economic optima single economic value because they have differential which would apply in the absence of such support rates and timing of expression which are accounted systems. Allowances for projected changes in beef for by using discounted gene flow coefficients. prices and the carcase payment system were also made to increase the relevance of the breeding 2.3. Economic values of feed intake and weaning objective to the time when genetic improvements are weight expressed in commercial animals. Calving is typical- ly in spring time in Ireland, with steers finished at 24 indoor finishing over winter or 30 finished off Feed intake for growing animals is included pasture months of age. directly in the breeding objective for both winter and summer. An alternative approach e.g., Amer and 2.2. Breeding objective structure Emmans, 1998 which involves adjustment to other breeding objective traits for expected correlated Genetic characteristics of beef cattle are expressed responses in feed intake is not appropriate because in herds of dairy cows progeny of beef matings to actual feed intake records are expected to be avail- dairy cows, beef cows, as well as growing and able on some selection candidates or their relatives finishing cattle. Traits used to describe the commer- via performance testing. Feed costs were calculated cial performance of animals with at least one beef on a per unit of effective energy EE basis Em- P .R. Amer et al. Livestock Production Science 67 2001 223 –239 225 Table 1 Outline of breeding objective structure including traits, abbreviations and expressions coefficients by breeding objective trait group Breeding objective Breeding objective trait Abbreviation Context Expressions coefficient trait group a Terminal sire Sire breeding b,c replacement females Growth Weaning weight direct WWd Beef X K ?X TB TB d Feed intake summer FIS Beef, dairy X K ?X TS TS d Feed intake winter FIW Beef, dairy X K ?X TS TS Carcase weight CW Beef, dairy X K ?X TS TS Mortality MT Beef, dairy X K ?X TB TB Weaned calf Weaning weight direct WWd Beef X K ?X TB TB Calf quality CQ Beef X K ?X TB TB Calving Calving ease direct CEd Beef, dairy X K ?X TB TB Gestation length direct GLd Beef, dairy X K ?X TB TB Carcase Carcase fat score CFS Beef, dairy X K ?X TS TS Carcase conformation score CCS Beef, dairy X K ?X TS TS Reproduction Reproductive success RS Beef, dairy – 12K ?X RA Calving ease direct CEd Beef, dairy – 12K ?X 1X RA TB Calving ease maternal CEm Beef, dairy – 12K ?2?X RA Weaning weight direct WWd Beef – 12K ?X 1X RW TB Weaning weight maternal WWm Beef – 12K ?2?X RW Mature weight annual MWA Beef, dairy – 12K ?X RA Mature weight heifer MWH Beef, dairy – 12K ?X RH Mature weight cull MWC Beef, dairy – 12K ?X RC e Gestation length R direct GLRd Beef, dairy – 12K ?X 1X RA TB e Gestation length R maternal GLRm Beef, dairy – 12K ?2?X RA a X and X are numbers of discounted expressions of a sires genes at birth and slaughter, respectively, per calf born. TB TS b X , X , X and X are numbers of discounted expressions of a sires genes in his daughters at annual calvings, weaning of their RA RW RH RC calves, replacement heifer age and at culling, respectively, per female calf born which is destined to become a replacement. c K is the average proportion of a bulls progeny not destined to be replacements and is calculated as 121 2P where P is the proportion of a sires daughters which become replacement heifers. d Feed intake aggregated assuming two summers and two winters for summer and winter feed intake, respectively. e In the reproduction sub-index, the effect of gestation length on the barren cow rate is omitted to avoid double counting of effects on reproductive success. mans, 1994 using EE values for foods presented by sexes showed only trivial effects on the economic Amer and Emmans 1998. value of winter feed intake. It was assumed that baled and wrapped silage Summer feed costs were calculated accounting for costs Irish pounds IP13.63 per tonne wet basis annual, per hectare, variable costs of fencing IP23, and that per animal feeding costs are independent of weed control IP10, fertiliser IP120 and provision the volume of feed fed on average to each animal of drinking water IP20. Opportunity cost of land Anonymous, 1998. After allowance for projected was excluded from the calculations, because of the reductions in future feed prices, weaner concentrates increasing importance of support payments linked to and feed barley were assumed to cost IP119 and upper limits on stocking rates which are applied on a IP110 per tonne wet basis, respectively Anony- per head, rather than on a per unit of feed intake, mous, 1998. Calculations allowing for differences in basis. concentrate feeds for an animal’s first and second Weaning weight WW is included as an objective winter, different breed types, finishing systems and trait based on the principle that each unit of weight 226 P .R. Amer et al. Livestock Production Science 67 2001 223 –239 gained prior to weaning is achieved using relatively result in economic values which are highly depen- cheap summer pasture feed, which would otherwise dent on the mean carcase characteristics of the group have to be made up using more expensive feeds in of commercial animals at the time when they are the first winter. In this way, benefits of increased slaughtered. Amer et al. 1998b have shown that weaning weight are calculated independently of any substantial variation can be expected in carcase trait benefits from calves heavier at weaning having economic values across breeds, sexes and finishing higher slaughter weights, and the potential for double systems for commercial calves. counting is avoided. It was calculated that approxi- There are currently only very limited price dif- mately 16 units of EE would be saved in the first ferentials in Ireland even though carcases are in- winter for every 1 kg of additional weaning weight dependently graded according to European Union using the model of Amer and Emmans 1998. standards. This is in marked contrast to the consider- able variation in value of carcases of different grades 2.4. Economic values of growth traits at market prices to processors, although these price signals are partially distorted by non-discriminatory The economic value for carcase weight at a export subsidies for carcases and meat going to non constant age was calculated directly from the carcase European Union destinations. price schedule IP1.78 per kg carcase weight, Despite these problems, efforts are under way to ignoring feed costs, which are accounted for else- ensure that a carcase pricing system reflecting ulti- where in the breeding objective. Allowance was mate consumer value does become implemented in made for an expected reduction in the beef price to the near future. Industry representatives and sci- 85 of 1998 levels due to modifications to the entists have established a payment schedule which is Common Agricultural Policy in the European Union. expected to form the basis of a future carcase pricing system and this was used in the current analysis. 2.5. Economic value of mortality Proposed price differentials for standard European Union carcase grades are shown in Table 2. For the For beef cross calves from the dairy herd, the purpose of deriving economic values, carcase con- economic value of mortality trait of calves was formation and carcase fat scores were transformed to taken as the opportunity cost of lost revenue from linear, 15-point, scales Kempster et al., 1986. not being able to sell the calf at weaning after an Multivariate distributions of carcases are specified by allowance for saved feed costs from 1 week of age breed type and sex according to parameters in Table until weaning. Weaned calf prices were assumed to 3. Fifty thousand carcases were simulated for each be IP100 and 64 per 100 kg for steers and heifers, set of parameters, and the average price for the respectively. A deduction of IP54 was made for milk replacer and weaner ration costs Anonymous, 1998. Table 2 It was assumed that mortality in calves from beef Price differentials for standard European Union carcase grades as a proposed for use in Ireland cows occurred around birth and that the calf could be replaced with a Friesian calf. From Anonymous EU carcase grades Price differential 1998, it was also assumed that the Friesian calf p kg CW would cost IP120 and that revenue at weaning would U2, U3 118 be reduced by IP15 per 100 kg. Assuming equal U4L 115 U4H 113 proportions of steer and heifer calves which would U5, R2, R3 17 have achieved 260 and 235 kg by autumn means the R4L 14 average opportunity cost of a dead calf is IP157. O3 O4L 22 2.6. Economic values of carcase quality traits R5, O4H 24 O5, P2, P3, P4L, P4H, P5 211 a Economic values for carcase traits derived using U, R, O, P range from good conformation to poor conforma- carcase grading thresholds and price differentials tion, 1, 2, 3, 4L, 4H, 5 range from low to high carcase fat. P .R. Amer et al. Livestock Production Science 67 2001 223 –239 227 Table 3 a Base carcase weights, carcase conformation scores and carcase fat scores for groups of slaughtered steers by breed type used to simulate b,c,d slaughter groups and derived percentages of each group to total male and female calves slaughtered Sire breed type Dam breed type Carcase weight Carcase conformation Carcase fat Males Females kg 15-point scale 15-point scale Traditional Holstein–Friesian 320 9.4 9.9 7 Traditional Traditional 320 10.8 12.6 11 12 Traditional Small continental 320 11.2 10.4 3 3 Traditional Large continental 330 11.2 9.0 3 3 Traditional Dairy cross 320 10.1 11.4 11 12 Small continental Holstein–Friesian 320 9.8 7.7 2 Small continental Traditional 320 11.2 10.4 7 8 Small continental Small continental 320 12.2 8.2 5 5 Small continental Large continental 330 12.2 7.5 1 1 Small continental Dairy cross 320 10.5 9.2 11 12 Large continental Holstein–Friesian 330 9.8 7.0 3 Large continental Traditional 330 11.2 9.7 11 13 Large continental Small continental 330 12.2 7.5 4 4 Large continental Large continental 350 12.2 6.7 4 4 Large continental Dairy cross 330 10.5 8.5 18 21 a Adjustments to carcase weights 270 and 220, carcase conformation scores 20.6 and 11.3, and carcase fat scores 0 and 21.5 were made to simulate groups of heifers and bulls, respectively. b Phenotypic standard deviations for carcase conformation and carcase fat were assumed to be 2.5, with a correlation between the two variables of 0.4. c Sources: Keane, 1994; Keane and Diskin, 1996; Keane and More O’Ferrall, 1992; Keane et al., 1989; More O’Ferrall and Keane, 1990. d Carcase conformation and carcase fat scores transferred to a 15-point scale based on Kempster et al., 1986. carcases computed from the simulations. Economic theoretical grounds, the importance of seasonal feed values for carcase conformation and carcase fat at a availability and management of animals to maximise constant age by breed type and sex were computed receipt of direct support payments in determining by re-simulating carcases adjusted upwards by 1 unit slaughter decisions do support such an assumption. for each, respectively, and taking the change in the Furthermore, results of Amer et al. 1998b under a average carcase price as the economic value ex- similar carcase grading system suggest that this pressed per unit on the 15-point scales. Economic assumption may only have trivial impact on the values were also calculated in a similar way for two economic values. alternative scenarios. The first scenario considers a situation where changes to either carcase grading or 2.7. Economic value of reproductive success direct subsidies result in animals being slaughtered at lighter carcass weights e.g., 300 kg for steers, A critical cost when deriving economic values for irrespective of breed type on average than they are reproductive traits is that of a barren cow. The cost currently. The second scenario assumes that heifers of a barren cow can be used directly as an economic and steers are slaughtered at a mean carcase fat score value for reproductive rate. Alternatively, it can be of 7 points. used indirectly in more complex reproductive and All animal traits at slaughter, other than the one breeding models which assign economic values to changed genetically, are implicitly assumed to be component factors such as conception rates, length constant. This implies that a genetic change in of the post-partum anoestrous interval and gestation carcase fat or carcase conformation will not cause a length which contribute to reproductive failure complementary change in any management variable. Amer et al., 1996. While this assumption cannot be justified strictly on In this study, the barren cow cost is calculated 228 P .R. Amer et al. Livestock Production Science 67 2001 223 –239 assuming that barren cows are culled following a shorter gestation length results in a longer effective pregnancy diagnosis and replaced in the breeding breeding season and less barren cows Amer et al., herd with an in-calf heifer. It is important to recog- 1996. However, because the barren cow rate is nise that there is not a one-to-one relationship already taken into account in the reproduction index between the level of culling due to reproductive reproductive success, this component was only failure and the number of in-calf heifer replacements included in the computation of the economic value required in the herd. This is because older cows for the terminal sire index. The model of Amer et al. culled for reproductive failure have only a limited 1996 was adapted to Irish beef and dairy cow herds remaining life span in the herd and would have had based on assumptions in Table 4. Parameter sets to be replaced in due course anyway. corresponding to base herds, favourable herds where The costs of replacement heifers were derived as biological parameters are favourable but the length opportunity costs from not being able to sell equiva- of the mating period is shortened, and unfavourable lent beef animals with an addition of IP40 to cover herds where biological parameters are unfavourable breeding expenses. Average cull cow values were long mating period were considered Table 4. also calculated based on assumed prices for cows of Changes in the proportion of the herd barren over each age and the proportion of cows in the herd of five alternative gestation lengths range from base that age. Thus, the cost of a barren cow was taken as value minus 2 days to base value plus 2 days were the weighted average reduction in cost of replace- evaluated for each scenario. The linear regressions of ments minus the weighted average net revenue from the proportion barren on gestation length multiplied cull cow sales given a 1 improvement in reproduc- by the cost of a barren cow see above for the base tive success. dairy and base beef parameter sets were taken as the economic values for gestation length. 2.8. Economic value of gestation length The second component of the economic value of gestation length accounts for the longer summer Economic values for gestation length were calcu- growing season for calves born earlier in the spring. lated assuming two components to the overall bene- The value of this component was quantified as the fit. The first component is based on the rationale that average pre-weaning growth rate kg per day mul- Table 4 Assumptions used to calculate economic values for gestation length GL – reproductive component Parameter Dairy Beef Base Favourable Unfavourable Base Favourable Unfavourable Mating period days 100 85 110 115 90 125 Post partum interval mean days 45 45 55 55 50 60 Post partum interval S.D. days 5.75 5.75 5.75 5.75 5.75 5.75 Conception rate first oestrous 0.4 0.4 0.4 0.45 0.45 0.45 Conception rate subsequent 0.55 0.6 0.5 0.6 0.65 0.55 Oestrous cycle days 21 21 21 21 21 21 Gestation length mean days 283 280 286 286 283 289 Gestation length S.D. days 5 5 5 5 5 5 a Percent barren GL22 days 3.18 3.05 5.89 2.41 1.98 5.25 a Percent barren GL21 day 3.26 3.10 6.25 2.60 2.10 5.69 a Percent barren 3.34 3.16 6.65 2.81 2.16 6.40 a Percent barren GL11 day 3.51 3.23 7.11 3.07 2.30 6.92 a Percent barren GL12 days 3.61 3.37 7.61 3.36 2.38 7.79 Regression barren per GL day 0.11 0.08 0.43 0.24 0.10 0.63 a Percentage barren ignoring cows with systematic reproductive disorders such as cystic ovaries. The effects of systematic disorders on barren cow rates are assumed to be independent of gestation length. P .R. Amer et al. Livestock Production Science 67 2001 223 –239 229 tiplied by the economic value for weaning weight. A severe assistance, 4 veterinary assistance and 5 constant pre-weaning growth rate of 1 kg per day caesarean section. Calculation of total costs for each was assumed for all dairy and beef production calving type in excess of those of a ‘‘no assistance’’ systems. calving for calving categories 2 to 5, respective- ly, are shown in Table 5. 2.9. Economic value of calving ease Probabilities of assistance types are calculated within sex of calf by age of dam first, second and For the purpose of the breeding objective, the greater than second parity combinations. Threshold economic value of calving ease was defined here on differences were used which were derived from the an underlying liability scale with animals, within a international literature and which are also consistent sex and age of dam sub-class, assumed to have with incidences of calving difficulty in the UK phenotypic values distributed on a standard normal reported by Allen 1988 and McGuirk et al. distribution Meijering, 1980. Such an approach is 1998a. Percentages of specific assistance types fully compatible with a genetic evaluation system for were computed assuming threshold differences of calving ease using a threshold model. There are 0.6, 0.3 and 0.3 between slight assistance, severe many advantages of such an approach McGuirk et assistance, veterinary assistance and caesarean sec- al., 1998b. tion sequentially. The category of calving assistance required by an Following Meijering 1980, let pu denote i 5 1 i animal depends on where its phenotypic value lies, to t probabilities of a normally distributed calving on the underlying scale, relative to thresholds which liability falling within a pair of thresholds T and i partition this scale into calving assistance categories. T given a sub-population mean u, and let a i 11 i Following Amer et al. 1998a, the categories consid- denote calving costs associated with calving ered were 1 no assistance, 2 slight assistance, 3 liabilities between T and T . The average cost of a i i 11 Table 5 Calculation of costs for calving assistance categories Caesarean section Veterinary assistance Severe assistance Slight assistance Stockman hours 3 3 4 1 a Cost per hour IP 7 7 7 7 Veterinary cost IP 120 30 Probability of dead calf 0.25 0.10 0.05 b Cost of dead calf 210 210 210 210 Probability of a dead cow 0.05 c Cost of a dead cow – dairy 359 359 359 359 c Cost of a dead cow – beef 409 409 409 409 Reduction in reproductive success 0.35 0.25 0.15 0.05 d Barren cow cost – dairy 134 134 134 134 d Barren cow cost – beef 149 149 149 149 Lost milk production litres 600 300 300 100 e Cost of milk IP litre 0.13 0.13 0.13 0.13 Total cost for dairy cow 336.35 183.50 97.60 26.70 Total cost for beef cow 266.10 109.25 71.35 35.45 a Based on statutory minimum of IP3.91 per hour Anonymous, 1998 but revised upwards to account for overtime rates, requirement for experienced stockman and or opportunity cost of owner operators time. b Based on Anonymous 1998. c Based on the expected cost of a replacement per infertile animal. d Based on the expected cost of a replacement minus the cull value per infertile animal. e Based on milk price of IP0.19 litre in Anonymous 1998 after deduction of IP0.06 litre variable costs mainly feed. 230 P .R. Amer et al. Livestock Production Science 67 2001 223 –239 calving SC in a sub-population with mean liability 1.7 McGuirk et al., 1998b and that calves are of u is calculated as equally distributed across categories. A mean weaned calf price function was first derived assum- t ing an underlying normal distribution for calf quality SCu 5 O a pu i i i 51 score with an IP0.25 kg liveweight premium for each successive increase in calf quality score and a By setting a to zero, the proportion of unassisted 1 mean liveweight of 240 kg equates to IP60 per calf. calvings is ignored, and so this equation gives The economic value was then computed as the average expected costs in excess of those from an marginal change in mean weaned calf price per unit unassisted calving. change in mean calf quality score using numerical If we treat cows of different age groups carrying differentiation. calves of different sex as sub-populations, average expected calving costs for the total population can be 2.11. Economic values of cow mature weight traits calculated as 6 Changing cow mature weight is expected to result ECu 5 O SCu q j j in changes to feed requirements for replacement j 51 heifers growth and maintenance and cow feed where u is the mean of sub-population j and q is the requirements for maintenance. Feed requirement j j proportion of animals in the total population which prediction equations based on the inter-species come from sub-population j defined according to six growth model of Emmans 1988 were used to age of dam by sex of calf combinations. For dairy predict the size of these changes. Mature weight is a cows, the proportions q were assumed to be 0.33, primary driving variable in the model. 0.25 and 0.42 for heifers, first calving and older Heifers were assumed to be fed pasture in the dams, respectively. Corresponding proportions for summer and a silage barley or concentrate diet in beef cows were assumed to be 0.2, 0.18 and 0.62. A their first two winters at a rate assuming that they calf sex ratio of 1:1 was also assumed. had to reach 85 of mature weight at first calving in The economic value, EVu of a unit change in order to maintain optimal economic performance. calving difficulty on the underlying normal scale for Effective energy contents and prices of summer and a population is calculated as the partial derivative of winter feeds were assumed to be equivalent to those the equation for expected calving costs. Because of used for finishing heifers. Calving was assumed to be the complexity of the equation, the partial derivative at the beginning of spring turnout. The economic was taken numerically, rather than algebraically. value for the mature weight effects on replacement Finally, the values of u for which economic values heifer feed requirements was calculated as the in- were derived for the selection indexes were obtained crease in feed costs for growth and maintenance by back-solving to obtain proportions of an average from birth to two years of age and for growth only herd of cows requiring any assistance which corre- from 2 to 3 years of age with a 1 kg increase in spond to proportions observed in practice. The mature weight. Additional maintenance requirements proportions were taken as 0.08 for herds with from 2 to 3 years of age are assumed to be accounted medium to small beef cows, 0.06 for herds with large for in the cow maintenance economic value ex- beef cows, 0.20 for dairy herds mated to a large pressed annually by cows of 2 years of age and continental breed, 0.18 for dairy herds mated to a older. Feed costs were discounted to their present small continental breed and 0.12 for dairy herds value equivalents at 2 years of age assuming all mated to a traditional beef breed. expense is incurred prior to the season in which it is to be fed. The cow maintenance economic value was calcu- 2.10. Economic value of calf quality lated as the change in daily cow maintenance requirements EE units with a 1 kg change in The economic value of calf quality scored as 1 for mature weight multiplied by 155 and 210 days of poor, 2 for average and 3 for excellent was calcu- winter and summer feed costs, respectively. Cows lated assuming a phenotypic standard deviation of P .R. Amer et al. Livestock Production Science 67 2001 223 –239 231 were assumed to be fed pasture in summer and a here is adapted from Everett 1975 and Van Vleck silage diet costing IP0.0068 per EE unit in winter. and Everett 1976 who addressed the value of The price differential for cull cows based on genetically superior semen in dairy cattle. Amer weight was used as the economic value for cull value 1999 has also applied the methodology with as influenced by mature weight. The 1997 price of modification to sheep. It is assumed that individual IP148.30 per 100 kg carcase weight was multiplied estimated breeding values not EPDs or estimated by 0.85 to account for projected beef price reduc- progeny differences are used for individual traits, tions and adjusted to a live weight equivalent but that indexes are expressed as the contribution of assuming a killing out proportion of 0.5. a parent to the profitability of each progeny born. The equations used to calculate the discounted 2.12. Aggregation of economic values genetic expressions are in Appendix A. It is impractical to have a separate selection index for each breed type and sex of calf combination. For 3. Results example, a single bull is expected to produce calves of both sexes and is likely to be mated to more than 3.1. Feed intake and weaning weight one cow breed type. Full aggregation of the econ- omic values for use in a single industry index is Economic values for FIW were robust to different possible, but must take account of the distribution of steer and heifer production systems. They were also sexes and breed types of slaughtered animals. Index- robust across breed types where from 15 to 20 of es containing economic values with intermediate total winter EE intake is supplied from concentrate levels of aggregation can also be derived to suit or barley. Base values were calculated to be 0.008 specific applications, for example, for beef bulls of and 0.0025 IP per EE feed intake unit for winter and any breed type mated to dairy cows. summer, respectively. For intensively finished bulls An intermediate level of aggregation was achieved assumed to be fed a diet with 60 of EE supplied here by building a demographic model of slaughtered from concentrate, the cost per unit of EE was cattle in Ireland. The model was driven by numbers IP0.011. of beef breed inseminations by AI centres in Ireland Under the assumption that 1 kg extra weaning along with percentages of total inseminations to beef weight results in a reduction by 16 units of EE and dairy cows. Slaughtered animals excluding cull requirements in the first winter, the economic value cows sired by natural mating in both dairy and beef for weaning weight in heifers and steers was IP0.09 herds were back calculated using the population sizes per kg. of dairy and beef cows, assuming that 80 of these animals calve each year. It was also assumed that 3.2. Mortality and carcase weight most surplus heifers from dairy cows were sired by beef breeds and consequently used as replacements The economic value for calf mortality was IP2 in the beef herd. Bulls were ignored in the aggrega- 1.51 in beef cross dairy calves and IP21.57 in beef tion steps as there are currently very few slaughtered calves for each percent increase in mortality. For in Ireland. The derived proportions of heifers and carcase weight, the economic value was IP1.51 per steers slaughtered by sire and dam breed type are kg increase with no account taken for differences in presented in Table 3. carcase quality for different breeds because the effects on resulting indices would be trivial. 2.13. Discounted genetic expressions 3.3. Carcase quality Application of beef selection indices outlined in Table 1 requires estimation of discounted genetic Economic values for carcase fat score and carcase expressions for calves from a terminal sire, and also conformation score calculated within breed type and for maternal traits expressed by self replacing sex combinations were aggregated according to their females and their female descendants. The approach expected frequency from Table 3. Bulls were 232 P .R. Amer et al. Livestock Production Science 67 2001 223 –239 Table 6 Carcase trait economic values aggregated over heifers and steers resulting from matings with weighted proportions of beef and dairy cows aggregate and resulting from matings with dairy cows only dairy cross, ranges for steers, and weighted average deviations to economic values for bulls and heifers a b Base scenario Reduction in CW Reduction in fatness Carcase conformation c Aggregate EV IP unit 5.1 5.5 5.0 c Dairy cross EV IP unit 7.1 7.4 6.1 Range for steers IP unit 2.8 to 7.6 3.0 to 7.6 2.3 to 7.6 Heifer deviation IP unit 20.4 10.4 10.5 Bull deviation IP unit 22.2 21.7 21.4 Carcase fat c Aggregate EV IP unit 24.0 24.1 22.0 c Dairy cross EV IP unit 24.5 24.3 22.2 Range for steers IP unit 22.7 to 25.0 21.7 to 25.0 20.9 to 23.8 Heifer deviation IP unit 11.2 10.5 10.3 Bull deviation IP unit 10.7 10.1 11.0 a Mean carcase weights CWs reduced to 320, 300 and 340 kg for steers, heifers and bulls, respectively. Carcase conformation and carcase fat were reduced by 20.015 and 20.025 points per kg for every 1 kg reduction in carcase weight relative to values in Table 4. b Mean carcase fat reduced to 7, 7 and 5.5 points for steers, heifers and bulls, respectively. Carcase weight and conformation were also adjusted using the constants specified in footnote a. c EV denotes economic value. ignored in the aggregation steps as there are current- dairy cows were IP149.11 and 134.45, respectively. ly very few slaughtered in Ireland. These costs are relevant as components for economic Table 6 shows the overall aggregate economic values of reproductive success, gestation length and values along with the range observed within steers, calving ease. The differences in costs of replacement and the weighted average effects of sex heifer or heifers and cull cow revenues expressed for a 1 bull relative to steers on carcase conformation and change gave economic values for reproductive suc- carcase fat economic values for current, and two cess of IP1.49 and 1.34 for beef and dairy cow alternative scenarios. Both the reduced average car- matings, respectively. The economic value for re- case weight, and reduced average fat score scenarios productive success in dairy cows is only relevant to resulted in a small increase in the economic value for the breeding objective of a beef breed when it is carcase conformation Table 6. However, when affected by the fertility of the beef bull. animals were assumed to be much leaner at slaughter Mature weight economic values linked to feed than in the base situation, the negative economic requirements showed a modest dependency on the value for carcase fat score was almost halved. expected mature weight to be achieved by the Aggregate economic values encompassing all mat- replacement heifer resulting from a specific mating. ings to dairy cows are also shown in Table 6. For example, the economic value of mature weight Economic values for carcase conformation and car- for heifer feed requirements ranged from IP0.242 to case fat are higher and similar, respectively, for the 0.230 per heifer and for cow maintenance ranged dairy situation relative to the overall aggregate. from IP0.103 to 0.093 per year for expected mature weights of 450 and 650 kg, respectively. 3.4. Reproductive traits The economic value of cull cow value as in- fluenced by mature weight was IP2.52 per kg of Tables 7 and 8 show changes in replacement liveweight at slaughter. requirements and cull cow revenues from a 1 Barren cow rates were found to increase by 0.24 improvement in fertility of beef cows and dairy and 0.11 per additional day of gestation for base cows, respectively. The costs of barren beef and beef and dairy herds, respectively Table 4. An P .R. Amer et al. Livestock Production Science 67 2001 223 –239 233 Table 7 a Changes in replacement requirements and cull cow revenues from an improvement in fertility of beef cows of all ages Age of cow Base level Effects per barren cow years Survival rate Herd proportion Extra replacements Savings Reduced cull cow revenue b c IP IP 2 0.95 0.195 1.000 105.30 66.89 3 0.94 0.183 0.902 89.14 56.62 4 0.93 0.170 0.758 69.58 44.20 5 0.88 0.150 0.661 53.54 34.01 6 0.83 0.124 0.564 37.77 23.99 7 0.70 0.087 0.562 26.40 16.77 8 0.60 0.052 0.570 16.01 10.17 9 0.50 0.026 0.587 8.24 5.23 10 0.50 0.013 0.394 2.77 1.76 Totals 1.000 408.74 259.63 a Assuming that improving fertility by 1 at each age increases cow survival by 1 at each age and that the cost of a replacement beef heifer is IP540 opportunity cost of an 18 month old heifer in good condition plus breeding expenses of IP40 and IP40 for additional feed and management expenses. b Savings from reduced replacements calculated as the herd proportion multiplied by the replacements per barren cow for each age group multiplied by the cost of a replacement heifer. The total at the bottom of the column gives the savings per average cow. c Costs from reduced cull beef cow revenue calculated as the herd proportion multiplied by the cull cow value for each age group. Table 8 a Changes in replacement requirements and cull cow revenues from an improvement in fertility of dairy cows of all ages Age of cow Base level Effects per barren cow years Survival rate Herd proportion Extra replacements Savings Reduced cull cow revenue b c IP IP 2 0.85 0.33 1.000 93.60 58.50 3 0.75 0.25 1.179 103.56 64.73 4 0.75 0.18 0.997 81.36 50.85 5 0.75 0.14 0.755 54.36 33.98 6 0.75 0.11 0.431 25.65 16.03 Totals 1.00 358.53 224.08 a Assuming that improving fertility by 1 at each age increases cow survival by 1 at each age and that the cost of a replacement dairy heifer is IP440 opportunity cost of a 2 year old dairy cross heifer sold for beef, plus breeding expenses of IP40. b Savings from reduced replacements calculated as the herd proportion multiplied by the replacements per barren cow for each age group multiplied by the cost of a replacement heifer. The total at the bottom of the column gives the savings per average cow. c Costs from reduced cull dairy cow revenue calculated as the herd proportion multiplied by the cull cow value for each age group. additional allowance for being 1 kg heavier at 3.5. Calving ease weaning of IP20.08 was also accounted for giving economic values for gestation length in the calving Separate economic values for calving ease in dairy sub-indexes of 20.24 and 20.45 with dairy and and beef herds beef breed sires are shown in Fig. 1. beef dams, respectively. When the effects of gesta- The positions of economic values for typical mating tion length on barren cow rates were ignored ap- types are marked on Fig. 1 in relation to the expected plied in the reproduction sub-index to avoid double incidence of assisted calvings. Economic values for counting with reproductive success, the economic calving ease in dairy cows ranged from IP19.06 to values for gestation length was IP20.08. 32.65 per liability unit per calving for traditional and 234 P .R. Amer et al. Livestock Production Science 67 2001 223 –239 3.7. Discounted genetic expressions Table 9 shows values derived for discounted genetic expressions coefficients. Expressions at slaughter are lower than expressions at birth because not all calves survived, and because of discounted effects over 2.5 years. Discounted expressions by replacement females at later ages i.e., as cows are higher because of multiple calvings per lifetime.

4. Discussion