INTRODUCTION
Efficient milk production in Irish dairy herds is dependent upon producing the maximum amount of milk from grazed grass. To co-ordinate peak milk yield with maximum grass growth, cows must calve compactly before turnout to pasture in the spring. In contrast, in industries where breeding occurs all-year-round, as in North America and most of the rest of the European Union, both the cows and the systems of management have evolved differently. This emphasis on dairy herd fertility in Ireland is only equalled in other grass-based milk production systems, such as New Zealand and parts of Australia. Irish dairy farmers aim to get the maximum number of cows pregnant in a short breeding season and in the past have done so successfully. However, in recent years changes in our farming systems have led to a decline in dairy herd fertility. Infertility remains the primary specific reason for cow disposal in dairy herds (Fig. 1).
Figure 1. Cow disposals by primary specific reason for culling from DairyMIS herds
(Source: O’Farrell et al., 1997)
This paper will outline the decline in Irish dairy herd fertility, current Irish dairy herd fertility performance, the costs associated with poor herd fertility, investigation of poor herd fertility and finally, nutritional, management and breeding responses to the decline in herd fertility.
DECLINE IN HERD FERTILITY
Irish surveys of dairy cattle showed that herd fertility was high throughout the 1960s and 1970s. Calving rates to first service by artificial insemination varied between 60 and 69 per cent, calving intervals between 357 and 380 days and infertile rates between 2 and 10 per cent. The first indication of a significant decline in Irish dairy herd fertility was detected in a retrospective analysis of data from Teagasc research herds. The highest conception rate to first service was recorded in 1980 (67 per cent) and declined to 59 per cent in 1988. Subsequent studies in commercial herds in the 1990s confirmed this phenotypic trend (Fig. 2) when calving rate to first service declined significantly by 0.7 to 0.9 per cent per year. The decline in calving rate to all services was, however, smaller (0.5 per cent per year), (Mee, 2004).
Figure 2. Calving rate to first service in DairyMIS herds between 1991 and 1998
(Source: Mee et al., 1999)
These trends recorded in research and DairyMIS herds are consistent with fertility data from cows in milk recording herds during the 1990s (Fig. 3). These data show an increase in calving interval of 0.9 days per year (1993:389 v. 1999:395 days) and a decrease in reappearance rate between first and second lactation of 1 per cent per year (80.4 per cent in1993 v. 73.2 per cent in 1999).
Figure 3. Calving interval in Irish milk recording herds between 1990 and 1999
(Source: ICBF, 2002)
Furthermore, the genetic data from Irish progeny test bulls show a similar decline in survival and calving interval proofs in the 1980s and 1990s (ICBF, 2002). Overall, these data suggest that the decline in dairy herd fertility began during the 1980s with a reduction of 0.5-0.9 per cent per year in conception or calving rate and an increase in calving interval of approximately one day per year.
These trends are not unique to Irish herds. Similar trends have been reported world-wide; in the UK, France, Germany, Spain, the Netherlands, Scandinavia, Israel, Australia and North America. Even in New Zealand, where cow fertility has been assumed to be high, recent survey data indicate conception rates are lower (first AI conception rate of 53 per cent) than in previous years (Xu and Burton, 2000). In addition, a decline in fertility has recently been detected in maiden heifers selected for milk production between 1981 and 1998 in the UK (Pryce et al., 2002).
Contrary to this profile of declining herd fertility, Whitaker (2002) suggested that declining herd fertility has been happening only on some farms in the UK but not on all farms. This may reflect significant regional or herd variation in the occurrence of risk factors for reduced fertility.
CURRENT IRISH HERD FERTILITY – “Moorepark Farm Fertility Study”
Given these trends in herd fertility, Teagasc embarked on a large-scale longitudinal study in 1999 to benchmark reproductive performance and to identify factors contributing of fertility in Irish commercial dairy herds. This ongoing study of 78 spring calving herds has collected data on approximately 6,500 cows per year. Data were collected on management, nutrition, genetics, health and fertility with all herds enrolled in DairyMIS. Results for fertility traits are shown in Table 1. Estimated 305-day milk yield averaged 6,570 kg/cow, with cows having 50 per cent Holstein-Friesian genes. On average, herd fertility was below that required to achieve compact calving. However, the top quartile of herds on conception rate to first service achieved good herd fertility (conception rate to first service 59 per cent, infertile rate 11 per cent, 14 week breeding season). In a follow up questionnaire survey, the majority of farmers replied that they followed recommended fertility management practices (Mee et al., 2002). One of the primary risk factors associated with poor fertility was BCS; at breeding and loss from calving to breeding (Buckley et al., 2003a). The conception rate described here (48 per cent) is consistent with recent reports from other dairy industries (Australia, 49 per cent, New Zealand, 53 per cent, Northern Ireland, 37 per cent, North America, 45 per cent and United Kingdom, 44 per cent) although definitions do vary.
Table 1. Fertility traits in the Moorepark Farm Fertility Study
| Fertility trait |
Herd mean |
Herd range |
| Pre-breeding* |
|
|
| Anovulation (%) |
10 |
0-21 |
| Cystic ovaries (%) |
3 |
0-10 |
| Subclinical endometritis (%) |
24 |
8-38 |
| Pyometra (%) |
2 |
0-8 |
|
|
|
| Breeding season |
|
|
| Calving to first service (d) |
71 |
59-92 |
| Calving to conception (d) |
88 |
73-103 |
| Submission rate (21-d) (%) |
70 |
25-96 |
| Conception rate to first service (%) |
48 |
29-71 |
| Infertile rate (%) |
14 |
2-40 |
| Breeding season (weeks) |
15 |
9-25 |
* Determined by ultrasonography approximately one week before the mating start date.
These trends were confirmed by an analysis of National Farm Survey data, which showed that in the year 2000 less than ten per cent of farmers calved 90 per cent or more of their cows over a nine week period (Donnellan, et al., 2002).
HERD FERTILITY COSTS
Previous studies at Moorepark have demonstrated the economic advantages of a compact calving pattern, which allows up to eighty per cent of milk to be produced from grazed grass. For spring-calvers this means the planned start of calving should be four weeks prior to the predicted turnout date to grass (Fig. 4). Even within herds calving compactly, economic analyses have shown the benefits of cows becoming pregnant in the first three weeks of the breeding season compared to the second three weeks (Morton, 1999). In an economic analysis of the impact of different Holstein-Friesian genotypes, Dillon and Buckley (1999) showed that the highest margin/cow was achieved with medium genetic merit animals. In this herd, the reduction in yield in the medium merit cows was more than offset by savings due to lower infertile and replacement rates. Similarly, a recent economic comparison between dual-purpose and Holstein-Friesian cows showed that the latter were less profitable, in a milk quota environment, primarily because of their poorer fertility (Evans et al. 2004).
Figure 4. Effect of calving date on opportunity cost (reduction in farm margin/cow) of alternate calving dates
(Source: Crosse et al., 1994)
With the advent of the Economic Breeding Index (EBI) it has been possible to estimate the cost of infertility for Irish herds. Each day increase in calving interval costs €2.07 per cow in the herd. This reduced margin per cow may be substantially greater (up to €17.50 per cow in the herd) where the herd is dried off on a fixed date. Each one per cent increase in culling (lower survivability) costs €11.40 per cow in the herd. The economic costs associated with an extended calving interval include lower annual milk yields, fewer calves, more services, extra veterinary costs and a slip from the profitable months of calving in seasonal calving systems.
In the UK a €4:1 return on preventive veterinary routines for infertility has been reported (Esslemont, BSAS Meeting, Galway, 1999) and in the Netherlands a profit of approximately €40/cow (2003 prices) was associated with implementation of a fertility control programme (Dijkhuisen et al., 1984). In New Zealand the case for cost-effective use of hormonal treatments for anoestrus has been presented (Deadman, 1997).
INVESTIGATION OF POOR HERD FERTILITY
Approach
Although poor fertility is now common in our dairy herds, there is wide variation in performance between herds and often between years. Hence, decision rules need to be adopted as to when investigation of herd fertility problems is warranted. The investigation of problem herds can be a frustrating business, especially if undertaken superficially at the crush while individual cow examinations are being carried out. Rarely are herd problems due to single factors and rarely is there a single solution. In order to standardise the approach to such investigations, Teagasc Moorepark has developed an investigation protocol to ensure each herd is addressed in the same systematic manner (Fig. 5).
This protocol has worked best where the farmer, farm manager, vet, Teagasc adviser and other relevant parties sit down around the table and go through all aspects of the problem in a systematic manner. Any investigation should begin with what the farmer or farm manager believes is the problem and the most likely causes. Particularly relevant are pertinent changes since the previous breeding season. Good records and good record analysis will identify whether the problem is a cow or a herd problem and whether it affects predominantly one age group or time of the season. This may confirm or refute the farmer’s and vet’s views. Record analysis will allow comparison with previous years performance and realistic targets for this particular herd type. Cognisance needs to be taken of the distorting effects of small herd size on herd fertility indices (Table. 2).
Table 2. Effect of herd size on intervention thresholds for herd fertility indices
| Herd size |
Submission rate (%) |
Conception rate (%) |
Infertile rate (%) |
|
(target >80%) |
(target >60%) |
(target <10%) |
| 20 |
<62 |
<40 |
>18 |
| 50 |
<69 |
<48 |
>16 |
| 75 |
<71 |
<50 |
>15 |
| >100 |
<73 |
<52 |
>15 |
*submission rate may be as low as 62% in a herd of 20 cows before there is a 95% chance that it genuinely lower than the target value (80%) because of the small herd size and its distorting effect on the data.
Problem areas
Where a herd problem has been identified, factors affecting the entire herd need to be examined. Hence, dry cow feeding, transition feeding, breeding season feeding and “quota feeding” need to be analysed along with supportive information from laboratory analyses of silage, grass, blood or other samples. The breeding program in the herd along with cow and sire EBI, genotype and cow size need to be taken into account. When possible, actual body condition scores should be recorded and the results examined. To facilitate this, Teagasc Moorepark in conjunction with the Irish Farmers’ Journal has recently produced a BCS chart specifically designed to guide farmers, advisers and vets through the essentials of body condition scoring, complete with targets (Buckley et al., 2002). Milk records from the current and previous lactation, including full lactation and peak yields and protein values will facilitate discussion on this often-subjective evaluation of high yielding herds. Infectious diseases are not uncommon, especially in large herds. Results from clinical examinations, laboratory results and vaccination regimes need to be drawn together to get a picture of the relevance of individual infectious agents to the problem. The presence of mineral imbalances may be identified from the clinical signs, laboratory results and the responses to supplementation. Their likely causative relationship with the problem under investigation must then be assessed.
Herd or individual cow?
Problems affecting individual animals, but not the herd, need to be identified and not confused with a herd problem. Thus, inadequate identification, individual sick cows, cows with calving problems or pre-service problems affecting less than 10% of the herd need to be teased out. These individual cows are often the reason for the investigation in the first place and may accurately reflect the herd status or merely individual cows with problems. Some factors affecting the success or failure of services are not specific to the type of service, and include; recent vaccination, changes in weather, sire semen fertility and timing of service, all of which need to be discussed. If either AI or DIYAI are practised, inseminator results, technique, semen storage and handling need to be addressed. Where the bull is under suspicion, the periods of use, bull power, libido, semen examination and veterinary examination information need to be collated. Recent Teagasc data indicate that wide variation in fertility exists between sires and both within DIY and commercial AI technicians (Buckley et al. 2003b).
At the end of this systematic process, the real problems and the likely contributory factors can be clarified. This may be at variance with initial perceptions or may confirm the farmer’s views.
RESPONSES TO THE DECLINE IN HERD FERTILITY
Given the genetic component in the herd fertility decline and the fact that the proportion of Holstein-Friesian genes in the national dairy herd has not yet reached 1, it is predicted that the decline would continue in the absence of remedial strategies. The multifactorial nature of the problem indicates that multiple approaches involving dairy herd nutrition, reproductive management, animal health management and type of cow will need to be adopted. Both short-term feeding, reproductive and animal health management strategies and long-term breeding strategies are required.
Nutritional management strategies
The depth and duration of negative energy balance (NEB) post partum is influenced by genetic merit for milk yield, dietary energy density and dry matter intake. Feeding and management strategies can be employed to ameliorate the extent of NEB and ensure cows maintain adequate body condition score (BCS). Transition cow management such as increasing the length of the dry period, increasing grass allowance at pasture, feeding good quality silage to dry cows, appropriate supplementation and differential group feeding will ensure that a target BCS at drying off (3.0) and pre-calving (3.25) are achieved (Buckley et al., 2003a). Postpartum, early introduction of grazed grass with appropriate supplementation has major benefits with regard to BCS. Regular monitoring of BCS can avert excessive losses (>0.5) and ensure target BCS pre-breeding (>2.75) (Buckley et al., 2003a).
Given the innate genetic drive towards milk output at the expense of fertility, diets that redirect nutrient partitioning away from milk production have been examined. Examples of such nutritional strategies include insulinogenic diets (Gong et al., 2001) or diets with a high energy:protein ratio. Postcalving supplementation with gluconeogenic substrates, such as propylene glycol, has also shown reproductive benefits (Miyoshi et al., 2001). Lipid supplementation has generally, but not always, resulted in positive reproductive responses (Fahey et al., 2002a, McNamara et al., 2003a).
An alternative approach to reducing negative energy balance by nutritional strategies, is to reduce milking frequency from twice to once daily for a short period post partum (McNamara et al., 2003b). However, while body condition improves, milk production drops by up to 25% while on once daily milking and by 10% across the complete lactation. Studies in New Zealand have shown that once daily milking around the time of breeding can increase submission rate by 10% (Rhodes et al. 1998). Once daily milking is most applicable to cows that calve down in poor body condition, for cows not seen in heat, in herds over quota and to reduce labour inputs at a busy time of the year in seasonal calving herds.
Reproductive management
Calving assistance
Management of calving can have a substantial effect on subsequent reproductive performance. In general, premature intervention during calving is not recommended to avoid both trauma and perinatal mortality (Mee, 1999) and when assistance is necessary it should be carried out as hygienically as possible. Results from the Moorepark Farm Fertility Study indicate that both conception rate and pregnancy rate can be adversely affected by calving difficulty (Buckley et al. 2003a).
Heat detection aids
While the critical importance of good heat detection efficiency in increasing pregnancy rates is not disputed, results from commercial dairy farms indicate room for improvement. Submission rates are, on average, 70% but vary between 25 and 96% across herds. Critical factors influencing submission rate are previous calving pattern, heat detection efficiency and anoestrous cows. Despite high usage rates of oestrus detection aids on dairy discussion group farms (Mee et al., 2002), anecdotal evidence from observing herds during the breeding season indicates such usage is much lower in general in Irish dairy herds. Alternative labour efficient methods of oestrus detection need to be explored including vasectomized bulls and, in future automated oestrus detection or ovulation prediction. Given the apparent reduction in oestrous behaviour in high yielding cows, synchronisation of ovulation rather than synchronisation of oestrus, with timed AI, may become more important in the future. Such protocols are commonly used in North American dairies where oestrus detection rates are poor.
Pre-breeding examinations
Traditionally, “problem cows” were presented to the veterinarian either post partum or prebreeding for diagnosis and therapy as appropriate. In small herds with good records selection of cows for examination was not difficult. With increasing herd size and the advent of cheaper ultrasound equipment, pre-breeding examinations of cows with or without problems has become more common.. Pre-service examinations may be a useful aid to confirm if cows not observed in heat are anovulatory, and for what reason e.g. have not resumed cyclicity at all post calving, are cystic or have a persistent corpus luteum (CL) which can contribute to severe uterine infection. Examinations can also be used to identify the presence and extent of uterine infection or other abnormalities. Pre-breeding scanning or handling may be particularly useful where calving records or records of pre-service heats are not available. Pre-service scan data from the “Moorepark Farm Fertility Study" where almost all cows in each herd were scanned (7455 pre-service scans across 61 herds over 2 years) showed that almost 14% of all cows scanned were not cycling at the start of the breeding season (Table 1). This ranged from 2% to 28% across the herds. On average, a very small proportion of cows (2.4%) had severe uterine infection (pyometra). However, in individual herds this was as high as 9%. Almost a quarter of all cows had mild or moderate endometritis (or had not completed uterine cleansing after calving). In total almost two-thirds of all cows were cycling with normal uterine involution and no infection. Early detection and precise diagnosis aid your vet’s decision as to the appropriate treatment regime.
Veterinary therapies
Good calving and pre-breeding records provide a valuable source of information for decision making regarding use of veterinary therapies. The use of tail paint with regular observations and recording of pre-service heats aid the identification of anoestrous cows. Cows should be observed closely for signs of uterine infection (foul smell, off form, dirty discharge) which require veterinary treatment. Veterinary herd fertility investigation combined with judicious use of reproductive therapies can detect and alleviate impending fertility problems, respectively. While the future use of controlled breeding technologies within the EU is unclear they do offer an immediate strategy to deal with fertility problems, particularly in large herds.
Results from the “Moorepark Farm Fertility Study” show that over 22% of cows received at least one “fertility treatment”. However, the results to-date indicate that less than 10% of all cows justified therapeutic intervention. With the large number of cows monitored and the excellent fertility treatment records available, it was possible to assess the effectiveness of the various therapies commonly used in Irish commercial dairy herds. The efficacy of treatment was determined according to which category the cow was assigned following the pre-breeding examination. Thus in some cases the farmer decided to treat a cow based on the scan results but in other cases cows with the same scan result were not treated. This allowed for evaluations of the efficacy of treatments after adjusting for relevant cow and herd factors.
The results are shown in Table 3. For example, cows which had normal scan results (uterine score 1), achieved a first service pregnancy rate of 58% with no treatment intervention. However, where hormonal intervention (mainly prostaglandin) was practised the first service pregnancy rate dropped to 51%, a reduction of 7%. A similar effect was seen in the pregnancy rate over six weeks of breeding. For cows identified with mild endometritis (uterine score 2) the first service pregnancy rate was 51% without treatment. Where veterinary therapies were administered (generally prostaglandin or washout regimes) first service pregnancy rate was the same (47%).
In contrast, cows cycling normally but with moderate endometritis (uterine score 3) had a first service pregnancy rate of 34% without treatment. Therapeutic intervention (generally involving a washout and or prostaglandin regime) resulted in substantial improvements in the reproductive performance of these cows (54%). This however was not the case in cows diagnosed as not cycling (anoestrus) but with moderate endometritis (uterine score 4). These cows had similar fertility whether treated or not. (generally with a washout plus a CIDR or coil).
As expected, the presence of severe uterine infection (pyometra) (uterine score 5) substantially reduced fertility performance. Cows with pyometra may have a discharge and can be picked up by handling the uterus with or without scanning. Cows with pyometra, which were not treated, had a first service pregnancy rate of just 17%. Veterinary therapeutic intervention (involving a hormonal regime with a washout) generally improved reproductive performance. The pregnancy rate to first service improved by 24%. On the other hand, anoestrous cows with a normal uterus and no hormonal intervention (uterine score 8) had a first service pregnancy rate of 46%. Hormonal intervention (CIDR or coil regime) resulted in a first service pregnancy rate of 34%, a reduction of 12%.
Table 3. Pregnancy rates of cows that were left untreated or treated following pre-service scanning.
| Uterine score |
Conception rate to 1st service (%) |
Pregnant after 6 wks breeding (%) |
Pregnancy rate overall (%) |
| |
Untreated |
Treated |
Untreated |
Treated |
Untreated |
Treated |
| 1 |
58 |
51 |
73 |
65 |
92 |
89 |
| 2 |
51 |
47 |
69 |
58 |
89 |
90 |
| 3 |
34 |
54 |
43 |
56 |
90 |
84 |
| 4 |
48 |
41 |
55 |
48 |
88 |
83 |
| 5 |
17 |
41 |
24 |
50 |
70 |
69 |
| 8 |
46 |
34 |
59 |
55 |
90 |
83 |
The results to date from these preliminary analyses suggest that therapeutic intervention with "normal" cows (including those with "mild endometritis") offers little benefit by way of improved reproductive efficiency. However, cows with moderate to severe levels of uterine infection respond well to treatments. Perhaps surprisingly, non-cycling cows allowed to start cycling naturally appear to perform substantially better than cows artificially induced to resume cyclicity. While these results are based on cows scanned before the start of breeding, similar trends were found in late calving cows (or fresh cows). However, in all categories pregnancy rates were lower and the differences observed between treated and non-treated cows were larger.
When treating "problem" cows it is imperative that an appropriate treatment to suit the problem is administered, based on the advice of your local veterinary practitioner. Such treatments are recorded in the Animals Remedies Register. Data from the “Moorepark Farm Fertility Study” indicate that a substantial proportion of treatments used were not appropriate at all for the problem being treated. Hence, veterinary advice should be sought when administering therapeutics to cows with reproductive problems.
Animal health management
Increased herd size has been shown to be a significant risk factor for poor fertility in Irish dairy herds (Fahey et al. 2002b). With herd expansion comes an increased risk from transmission and maintenance of infectious diseases. Hence, expanding and large herds need to specifically target biosecurity as a priority in maintenance of both herd health and herd fertility. The details of systematic preventive herd health programmes against leptospirosis, bovine virus diarrhoea virus, Johne’s disease, and infectious bovine rhinotracheitis are presented in this proceedings by Caldow et al. (2003).
Breeding strategies
Breeding strategies to reverse the decline in herd fertility include development of the economic breeding index (EBI) to replace single trait selection and crossbreeding and breed substitution to address the ongoing strain substitution. In the future marker-assisted selection may also be used.
EBI
Long-term, the greatest impact of breeding strategies on herd fertility will be through genetic selection. The challenge is to select for economically important milk production traits while concurrently selecting for fertility traits both within the Holstein-Friesian breed and across other dairy and dual-purpose breeds. Despite the low heritability for fertility traits (typically <0.03), they have large coefficients of variation, not dissimilar from those of production traits (Evans et al., 2002). The response to selection is determined by the amount of genetic variation and not the heritability per se. Hence, genetic selection for better daughter fertility is possible. Experience from Scandinavian breeding programmes shows that the decline in fertility can be reversed although progress in milk production is reduced. However, better economic returns are achieved.
Using the current economic breeding index (EBI), a sire that produces more fertile daughters will have positive proofs for survival and negative proofs for calving interval. This means that a higher proportion of his daughters will survive from one lactation to the next and their calving interval will be shorter. A one-day decrease in calving interval will increase farm profit by an estimated €2.07 for each cow in the herd. An EBI of 25 for a sire means that, on average, his daughters will be €25 more profitable per lactation than the daughters of a sire with an EBI of €0.
Alternative breeds/crossbreeding
Interest is increasing in the potential use of other breeds to replace the Holstein-Friesian, or crossbreeding. Crossbreeding is the only way to rapidly introduce a new breed into the Irish dairy cow population given that over 95% of dairy cows are Holstein-Friesian. The milk production potential of a crossbred cow may be less (depends on system of milk production and choice of alternative breed) than that of a Holstein-Friesian. However, other attributes such as health, fertility and welfare may be improved due to the introduction of another breed selected more strongly for these traits. A major benefit of crossbreeding is the introduction of heterosis (hybrid vigour) which has important effects on non-production traits that contribute to improved overall fitness and longevity. Heterosis is defined as the advantage in performance of crossbred animals above the mid-parent mean of the two parent breeds, as shown in Fig. 6.
Figure 6. Heterosis or hybrid vigour is defined as the advantage in performance of crossbred animals above the mid-parent mean of the two parent breeds
In New Zealand, crossbreeding has been shown to have considerable merits, particularly in terms of fertility and survival (Harris et al., 2001). Heterosis values of 5-6% for production traits and up to over 18% for reproduction and health traits were observed. In New Zealand 20% more crossbred cows survived to fifth lactation than do Holstein-Friesians.
At present dairy farmers within Ireland are ‘experimenting’ with alternative breeds in a “look see” type approach. However, in general, they have no means of comparing the genetic level of the sires of these alternative breeds with the Holstein-Friesian. Therefore there is a danger that, in the longer term, wrong decisions are being made, particularly in relation to genetic improvement. Equally, there is large genetic variation within any breed and using sires of an alternative breed, albeit a potentially suitable breed, without genetic evaluation is not a sensible strategy.
Teagasc has three alternative breed/strain comparison studies currently underway. At Moorepark, New Zealand Holstein-Friesians are being compared with high durability and high production Holstein-Friesians (Horan et al. 2003). Rotational crossbreeding studies are being carried out with the Montbeliarde and Normande breeds (Dillon et al., 2001). A previous five-year Teagasc breed comparison study found that while Holstein-Friesians produced more milk, they were less fertile, and hence, less profitable in a quota environment than a dual-purpose breed, the Montbeliarde (Dillon et al., 2003).
In addition, a large scale Norwegian Red (NRF) crossbreeding study is underway involving over 50 Irish commercial dairy herds. At present the EBI index only provides genetic evaluations for one breed, namely the Holstein-Friesian. Therefore it is not possible to compare sires of different breeds on the one index. The objective of this field study is to generate sufficient data to provide ICBF with genetic parameters that will allow them provide genetic evaluations (also Interbull conversion factors), for one alternative breed. The outcome of this study will allow farmers to select the top sires within the EBI across these two breeds, and also provide information on the potential advantages that may be gained from crossbreeding (heterosis). In other words, additive genetic progress will continue to be the priority within both the Holstein-Friesian and the NRF breed using the EBI, but the potential advantages of heterosis can also be utilised. For over 30 years the breeding program of the Norwegian Red has successfully incorporated selection for improved female fertility and general health, including traits such as mastitis resistance, calving ease and reduced incidence of ketosis and retained afterbirths.
CONCLUSIONS
Strategies are required to improve, or halt the decline in, reproductive performance as production systems rapidly evolve. These approaches must include feeding systems to reduce negative energy balance and maintain body condition, management systems to improve oestrus detection efficiency with less labour and herd health and adoption of a total merit-breeding index to select for genetically more fertile cattle. In the absence of research in these areas recent Moorepark results indicate that the response to traditional herd fertility therapies may become increasingly diminished.
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