The terms “Beef-on-dairy” (BoD) and “dairy beef” have gained significant interest in the literature recently. Dairy beef is an inclusive form that describes all kinds of meat produced from the dairy system, including meat from surplus dairy calves and culled cows [1], while BoD refers to the type of breeding system describing the use of beef semen in dairy herds with an aim to produce calves with higher economic value for the meat. Due to undesired meat characteristics and a lower meat-to-bone ratio [2], surplus male dairy calves usually are unwanted products of the dairy system. This results in lower market value as compared to purebred beef calves and beef crossbred calves. There are possibilities for improving beef production from dairy systems by inseminating dairy cows with fast-growing beef breeds resulting in additional economic gain [3]. Due to these benefits increased utilization of beef semen in dairy herds across the globe has been observed. The insemination data records from England regarding the sales of beef semen document increases in the sales of beef semen from 25% in 2013 to 47.6% in 2020 [4]. Similarly in North European countries such as Denmark, Sweden, and Netherlands, the proportion of beef semen used in dairy herds ranges between 20 and 25% [5]. The insemination data reported from the United States depicts a similar picture, where beef semen counted for almost 20% of inseminations in 2019 [6]. This increase in the sale of beef semen in the US market, accompanied by a decline in the sale of dairy breed semen from 23.2 million doses in 2017 to 18.3 million doses in 2020, indicates a significant increase in the utilization of beef semen in dairy herds ^[7][8][7,8]. Similar results have been shown by a study from the United States [9] where an eight percent increase (18.2 vs. 26.1%) in the use of beef semen on Holstein dairy herds from 2019 to 2021 was reported. Trends regarding sales of beef semen are summarized in Table 1.

Prices of beef products are increasing [10] and are anticipated to keep increasing in the near future [11]. Combined with volatile milk market prices and higher revenues from the BoD calves, a further increase in the use of beef semen in dairy herds is expected by many farms. The economic gain from BoD crossbreeding is not only limited to higher revenue for the farms, but also can create higher consumer acceptance, which is an important aspect of BoD. The studies comparing meat from pure dairy breeds with BoD crossbreds have suggested that meat from BoD crossbreds can be marketed along with meat from traditional beef breeds due to similar aesthetic and eating qualities [12]. However, the use of beef semen in dairy herds may lead to an unwanted increase in the stillbirth rate and calving difficulty, relinquishing economic advantages due to higher veterinary costs and lower animal productivity. Furthermore, the welfare aspect of animal farming has emerged as a matter of paramount importance within the livestock industry. This increased attention can be attributed to growing ethical concerns and the escalating demand from consumers for more humane farming practices. Implementing a BoD breeding strategy potentially offers remedies to several issues but requires a careful evaluation of the farm-specific situation regarding configuration, income sources and herd fertility. The use of advanced breeding techniques such as genomic selection and appropriate BoD selection indices combined with a required level of vigilance, can effectively mitigate potential difficulties such as elevated calving difficulty.

2. Implications of Beef-on-Dairy at the Animal Level

2.1. Calving Difficulty and Stillbirth

One of the significantly important aspects of the BoD breeding strategy is animal welfare that is compromised by an increase in calving difficulty and stillbirth. The use of beef semen on dairy cows, especially from late-maturing beef breeds, can lead to a significant increase in the cases of calving difficulty in the herd. This increase can lead to negative consequences including a decline in farm revenue due to higher veterinary costs, loss in milk yield, and increased calving interval along with compromised animal welfare. A study on 1,598,363 calving records from Swedish dairy farms revealed a significantly higher risk of calving difficulty in dairy herds when cows were inseminated with the semen of fast-growing, late-maturing beef breeds such as Charolais and Limousin [12]. The higher probability of problematic calving when using beef semen in dairy herds might be associated with higher birth weight, the body structure of the calf, and its muscle development rate [13]. These traits differ significantly across breeds. For instance, Eriksson et al. [12] found that the difference in muscle development rates across various beef breeds significantly influences calving difficulty. Similarly, the highest growth rate along with the highest birth weight were reported for Brahman-sired crossbred calves, followed by Belgian Blue and Angus crossbreds [14]. In the same study, the crossbreds from the Brahman breed had a significantly higher birth weight accompanied by the highest incidence rate of calving difficulty. Breed differences for problematic calving in BoD calves have also been reported by a Spanish study on 525,535 calving records [15]. The highest rate of calving difficulty was found for Holstein × Belgian Blue crossbreds followed by crossbreds of the Limousin breed. In literature, other factors including the sex of the calf and gestation length have been linked with calving difficulty in crossbreds ^[15][16][15,16]. Another important trait that might influence calving difficulty in the animals is the cow’s parity number as found by a study on Irish dairy farms using 2,733,524 insemination records [17]. The calving difficulty was found to be 1.39% for dairy sires and 1.35% for beef sires in first-parity animals. With increasing parity number, however, calving difficulty rose significantly reaching up to 4.11% when inseminated with beef sires in contrast to 1.83% when inseminated with dairy sires, along with an increase in gestation length [17]. No explanation was available for these results, but extended gestation length along with the selection of beef sires might have played some role in higher incidences of calving difficulty. As suggested by Berry et al. [18], variation among the beef breed sires exist, and identification of the sires which can meet the requirements for the individual farmers is of critical importance for the success of the BoD breeding strategy. Moreover, the calving difficulty seems to vary across different breeds, and within the breed, individual sires seem to have a significant influence on the trait. Most studies agree on the difference between early-maturing and late-maturing breeds, as calving difficulty is typically more often reported in late-maturing breeds. Causes of stillbirth are a little more complicated to explain in comparison to difficult calving, where feto-pelvic incompatibility is the most important factor [19]. The study by Eriksson et al. [12] found a significantly lower stillbirth rate for crossbred calves from Hereford, Limousin, Simmental, and Charolais sires compared to purebred Holstein calves. According to the authors’ opinion, it might be related to the use of early-maturing beef breed sires like Angus in dairy herds. The stillbirth rate varies significantly across various breeds; however, little information is available regarding stillbirth rates in crossbred calves. In a study on Irish herds [20], the problem of stillbirth events was found to be significantly localized on some farms, while most of the farms did not have any problems with stillbirth. Farmers with dairy breed semen reported 0.78% higher calf mortality as compared to beef semen. This might be due to the use of semen from a particular beef breed but might be also due to management, e.g., insemination of second or third parity animals and calving management. The stillbirth rate does not appear to relate to calving difficulty. This might be due to the lack of proper reporting mechanisms and optimum vigilance at the farm level. Lack of accurate data recording combined with the distribution of data due to the frequent use of selected breeds, make the comparison a challenging task. Conclusions regarding stillbirth rate becomes even more difficult by the limitation of selective sires within the breed. Traditionally, breeding goals for the selection of sires for dairy herds have been focused on milk production and functionality traits [21], but with increasing beef and dairy crossbreeding, the development of a BoD index for the selection of beef sires to be used in the dairy herd has gained significant interest. In Ireland, BoD index ranks breeding bulls based on economic output from the calves, with the highest relative emphasis on calving difficulty and carcass characteristics ^[21][22][21,22]. Similarly, the Scandinavian countries Denmark, Sweden and Finland have introduced the Nordic beef-on-dairy Index (NBDI), which includes seven traits such as calving difficulty, stillbirth and carcass traits [23]. Likewise, the highest economic impact in the NBDI is attributed to calving difficulty due to its adverse impact on the dam, resulting in reduced milk yield and health complications due to dystocia, along with potential loss of revenue from the calf ^[24][25][24,25]. Another approach that can assist in the BoD breeding strategy is genomic selection. By using genomic selection [26], dense marker maps can be used for the prediction of breeding values [27] for beef sires and desired traits. By doing so, the selection of beef sires that cause lower calving difficulty or superior carcass quality is possible. Compared to traditional pedigree indexes, genomic selection has higher accuracy for most of the traits [28]. However, major hurdles in the implementation of genomic selection for BoD are the availability of adequate phenotypes and overall lower prediction accuracy with a combined crossbred reference population ^[29][30][29,30]. The latter fact is particularly important in the BoD context, as significant differences in the performance of purebred and crossbred animals exist [31]. Moreover, estimated SNP effects are usually breed-specific, which means that SNP effects predicted for one breed cannot be transferred to another breed. At this point, however, appropriate statistical methods such as the consideration of breed-specific effects in the prediction models can help [32].

2.2. Growth Traits and Meat Quality

The performance of crossbred animals is usually considered better as compared to the purebred parental generation. An Irish study on 48 male calves, fattened under three different finishing strategies, found a slightly higher growth rate of Belgian Blue crossbred calves compared to Limousin crossbreds (1.076 vs. 1.009 kg/d), but the role of finishing strategy with highest concentrate in feed was more evident [33]. These results vary slightly from those reported in the United States, where in an effort to determine the growth curve of 516 animals kept under similar nutritional management, the weight of Belgian Blue crossbreds at the age of 48 weeks was found to be significantly lower as compared to Angus and Hereford crossbreds [34]. It is difficult to conclude differences exist in the growth rate of crossbreds across different studies due to the variation in the age of slaughter and rearing of animals under different production systems. Generally, it is assumed that crossbreds of BoD animals can perform better in terms of average daily gain and final slaughter weight, but studies are inconclusive in this regard. For instance, a recent comparison of Spanish beef production by Sánchez et al. [35] on 120 animals, kept under three production systems, found no difference in the production traits of Limousin, Charolais and Holstein crossbreds. Similar results were reported from New Zealand, where the comparison of 326 BoD crossbred calves under the grazing system did not reveal any significant impact of individual sires on beef quality traits of the calves [36]. The role of production system under which calves are fattened is also of significant interest as suggested by Bittante et al. [37]. In a study on 231 calves, fattened under three different production systems, researchers found a significant influence of the production system on the carcass quality, where meat produced from BoD calves born and fattened at dairy farms had better quality in terms of tenderness and cooking losses when compared to specialized fattening farms [37]. The increasing demand for high-quality beef products has put significant pressure on farmers to produce meat with higher quality. The price determination system for beef relies on the quality aspects including fat contents of meat, muscle-to-bone ratio, and color of the product [33]. Along with that, the texture of the meat and flavor significantly influence the acceptance of the product by the consumers. The flavor from the BoD crossbreds was more butter and fat-like as compared to beef breeds [38]. The steak quality of BoD crossbreds was also found to be intermediate between dairy (lowest) and beef (highest) breeds. As major grading criteria for meat are based on fat and muscle contents, BoD animals were found to have more muscle as compared to dairy animals and less fat as compared to traditional beef breeds. In summary, BoD animals produced a slightly less marketable meat quantity as compared to beef breeds but were significantly higher in comparison to dairy animals [38]. Selection of the sire with a focus on marketable traits is of significant importance in this regard as found by Martín et al. [39]. A significant influence of beef sire on rib fat depth was found, and the authors concluded that the use of beef breeds on dairy animals can significantly improve the quality of desired cuts ultimately resulting in more economic gains. The heritability of meat quality traits differs significantly across various breeds. During a crossbred study on 766 Hereford cows, sired with seven different beef breed sires, it was estimated to be low for marbling (18 ± 7)% and moderate for carcass weight (36 ± 8)% and fat color (33 ± 8)%. After the exclusion of Belgian Blue and Limousin breeds from the study, heritability for the marbling trait increased significantly [40].