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A comparison between Polish Holstein-Friesian and F1 hybrid Polish Holstein Friesian × Swedish Red cows in terms of milk yield traits

The intensive breeding work of Holstein-Friesian cattle has led to the decrease in the diversity within the population and to inbreeding depression, which may impair its functional traits. In addition, as shown by the research, production traits are negatively correlated with functional traits such as reproduction, health, and longevity, which have a very strong impact on the profit of dairy farms. The aim of this study was to compare milk yield traits of hybrids obtained by crossbreeding of Polish Holstein Friesian (PHF) cows and Swedish Red (SRB) bulls with values obtained for pure PHF cows. For the study, 100 primiparous cows were selected and divided into two groups. The experimental group consisted of 50 crossbreds (PHF×SRB), while the control group consisted of 50 purebred PHF cows. The study showed a higher content of milk components (fat by 11.78 %, protein by 9.06 %, dry matter by 5.75 %) in PHF×SRB, as compared to PHF. A lower level of SCC (by 38.94 %) has also been shown in hybrids, which indicates their higher resistance to udder diseases. The experiment demonstrated a highly significant impact of heterosis on performance parameters and technological quality of milk in F1 generation obtained as a result of crossbreeding between PHF cows and SRB bulls.


Introduction
According to the commonly accepted definition, heterosis is a phenomenon consisting in increasing the phenotypic value of quantitative traits of the first generation of hybrids with respect to homozygous parents. The effects of heterosis are opposite to the effects of inbreeding depression, if the breeds are properly selected for breeding. Therefore, any crossbreeding program should start from an appropriate selection of breeds, which should be complementary.
Dairy farming around the world is based on the most efficient Holstein Friesian (HF) breed of two color varieties: black and white (HO), and red and white (RW). In Poland, the local variety is Polish Holstein Friesian (PHF) breed, which was created as a result of crossing the Polish Black -White and Polish Red -White cows with HF bulls (Nowicki, 2011). According to the data of the Polish Federation of Cattle Breeders and Dairy Farmers (PFCBDF), in 2019 the PHF breed accounted for almost 88.61 % of the active population (HO -84.87 %, RW -3.81 %). In 2018, the average life expectancy of the PHF breed of the HO variety was 5.41 years, whereas of the RW was 5.37 years, which indicates that cows have been in use for almost 3 years (HO -2.93 years, RW -2.79 years) (PFHBiPM, 2020), but the length of use has been shortened in relation to the results obtained in 2016 (Adamczyk et al., 2017). The earlier deficiency is associated with cow's health problems (82.84 %) (Adamczyk et al., 2017), which very often starts with high nutritional requirements, especially with regard to energy demands in the early stages of lactation, leading to problems with metabolic disorders ( Pryce et al. (2014), a 1 % increase in the inbreeding level was associated with a decrease in milk yield by 28 L/lactation in Holstein-Friesian cows, and by 12 L/lactation in Jersey cows. Additionally, the increase in inbreeding level was correlated with a decrease in milk, fat, and protein yields. Also, Smith et al. (1998) and Doekes et al. (2019) showed that an increase in the inbreeding level by 1 % in HF resulted in productivity reduction by 36.3 kg and 27 kg/lactation, respectively. In addition to the problems associated with increased homozygosity, there may also appear problems with an increased incidence of rare lethal or harmful recessive disorders, such as bovine leukocyte adhesion deficiency -BLAD (Kehrli et al., 1990) or uridine monophosphate synthase deficiency -DUMPS (Shanks et al., 1984).
The increasing level of inbreeding does not allow taking full advantage of the effect of the breeding progress. Therefore, crossing procedures become an excellent alternative because these extend the length of animal use, improve reproduction parameters, and reduce metabolic disorders (PFHBiPM, 2017; PFHBiPM, 2018; PFHBiPM, 2019; PFHBiPM, 2020; Sørensen et al., 2008). In addition, a steady increase in the inbreeding of the Holstein-Friesian population (around +0.2 % per year) indicates that crossings will be necessary for most milk producers in the future. Modern breeding should be focused on product quality as well as animal health and welfare, because they guarantee the profitability of production. The main goal of crossbreeding Holstein-Friesian cows with bulls of other dairy breeds is to improve performance traits. Thus, the aim of this study was to compare Polish Holstein-Friesian and F1 hybrid Polish Holstein Friesian × Swedish Red cows in terms of milk yield traits.

Material and methods
The experiment was carried out in an experimental dairy farm of the Warsaw University of Life Sciences (WULS, Warsaw, Poland), where about 350 cows were kept in a free stall housing system with an average yield exceeding 10,000 kg of milk in lactation. For the study, 100 primiparous cows were selected and divided into two groups. The experimental group consisted of 50 crossbreds (Polish Holstein Friesian x Swedish Red; PHF×SRB), while the control group consisted of 50 purebred PHF cows. Data from milk performance evaluation were the basis for obtaining information on milk performance (milk production in individual lactation months, fat, protein, dry matter content, somatic cell count and milk performance in the whole lactation). Samples of milk were collected from the cows for laboratory analyses at monthly intervals, at ten time points. FTIR was used to analyze the composition of the milk. The cows' feeding regime was based on the total mixed ration (TMR) diet (ad libitum) ( Table 1).
The data obtained were analysed statistically using IBM SPSS 6.0 package. The distribution of the milk chemical composition was checked by the Shapiro-Wilk test. The ANOVA analysis was performed to establish influence of the genotype on milk chemical composition and somatic cell count (SCC). The changes in concentration of basic chemical components in regard of genotype and lactation stage were established by the multi-variance analysis.
The following statistical model was applied: Y = µ + A i + B j + (AxB) ij + e ijk where, µ -mean, A i -day in lactation, B j -genotype (PHF, PHF×SRB), AxB -interaction between day in lactation and genotype, e ij -random error. Only the interactions between factors whose influence was statistically significant (P≤0.01 or P≤0.05) were considered. The Pearson's correlation was used to quantify the degree of a linear relationship between two variables (x and y).
Results and discussion Figure 1 shows the lactation curves of PHF and PHF×SRB cows. The lactation curve is a reflection of the daily production of milk, which is a resultant of both genetic and environmental factors. The shape of both curves is similar, which may indicate that neither purebred cows or hybrids had feeding problems. Heins et al. (2006) also showed a similar trend. According to Lopez et al. (2015), the peak lactation in HF cows is between weeks 4 and 8 of lactation. However, as shown by results obtained, the peak lactation in cows from the PHF group fell on the 3 rd month after calving, i.e. about the 9 th week of lactation. PHF×SRB crossbreds reached their lactation peak earlier, which decreased in the second month of lactation, i.e. about 5weeks after calving. Similar tendencies were observed by Slósarz   in the purebred cows (-0.74 %) than in the hybrids (-0.37 %). The obtained trends confirm those reported earlier by Slósarz et al. (2016). After the 5 th month of lactation, the fat content increased, due to the changing physiological and metabolic state of cows, as well as to decreasing productivity, which is confirmed in Figure 1. Changes in fat content in relation to milk production were confirmed by a negative correlation of -0.245 (Table 2). in both the purebred and crossbred cows, which indicates, first of all, consistent breeding work of the whole dairy herd in which the experiment was conducted. The difference visible from the very beginning is related to the higher content of this component in hybrids, which is connected with the use of SRB breed, which according to PFCBDF data is characterized by a significantly higher average protein content in comparison with PHF cows (SRB 3.55 %, PHF HO 3.34 %, respectively) (PFHBiPM, 2020). However, Ezra et al. (2016) did not show any significant differences in protein content between HF and HF×NRF. In the present study, at the beginning of lactation, the protein content was at the highest level, which was associated to the physiology and colostrum secretion, i.e. colostrum is rich in immunoglobulins and whey proteins. The concentration of this component then decreased to the lowest level in the second month of lactation, which was probably associated with a high energy demand (Figure 3). Additionally, studies have shown that the protein content of milk is negatively correlated with milk production, which is confirmed by result -0.583 (Table 2) Changes in the dry matter content during lactation in purebreds and hybrids are presented in Figure 4. Dry matter is a very important parameter from the technological point of view, it is a resultant of components such as: protein, fat, lactose and other components defined as ash (Jaworski and Kuncewicz, 2007). Our research showed differences in the dry matter content, which may be indicative of a higher technological usefulness of hybrid milk. Studies have shown that the appearance of the curve is similar to that obtained in Figures 2 and 3, confirming the dilution effect, and a significant correlation between fat (0.945), protein (0.647), and dry matter ( Table 2). A similar relationship was established by Jaworski and Kuncewicz (2007). The obtained results demonstrate a significantly higher content of particular milk components (fat by 11.78 %, protein by 9.06 %, dry matter by 5.75 %) in PHF×SRB in comparison to the milk of PHF cows. Similar trend was observed by Swalve (2007) Figure 3 shows changes in the protein content of milk. The high technological quality of milk is associated to the concentration of total proteins; in particular to the casein content, which is responsible for both, the rate of formation of the curd and its compactness, making it one of the most important components of the raw material. This ingredient is of a great interest not only to dairies, but also to farmers, who perceive it as a factor affecting the higher price of raw material (Król et al., 2011). Protein is one of the milk constituents that is difficult to modify, since its content can mainly be obtained by working in this direction selection of genetics. This component is a highly inhe- indicates healthy udder, while that above 200,000 /mL in milk indicates the occurrence of subclinical mastitis. In the present study, in the first month of lactation of purebred cows and hybrids we found a higher number of somatic cells in milk than in the following months, which is due to the beginning of lactation and changes related to the production and secretion of milk with lower efficiency. It is worth noting that the content of somatic cells in the purebred cows exceeded 200,000 /mL, which according to Juozaitiene and Juozaitis (2005) proves the occurrence of subclinical mastitis. In hybrids, however, the content of somatic cells did not exceed this value, which may indicate their higher resistance to mastitis. In the final stage of lactation, the number of somatic cells increased in both analysed groups, which was caused by the lower milk production and ongoing lactation. A similar trend, in which at the beginning and in the end of lactation the content of somatic cells in milk is higher, and at the peak of lactation a lower number of somatic cells is maintained, was shown by Jakiel et al. (2011). In our study, crossbreds were characterized by a lower level of somatic cells (38.94 %) in comparison to PHF, which indicates their greater resistance to udder disease, which was also confirmed by Hansen (2012a, 2012b). In the studies by Swalve  2017), the crosses of HF×SRB and HF×SR were characterized by a higher SCC than the purebred HF. Table 3 presents the average production values for purebred PHF and crossbreds. Daily milk production in the first lactation was significantly higher by 14.61 % in the purebreds than in the crossbreds. On the basis of the results obtained for the fat-to-protein ratio (FPR), it can be concluded that the feeding dose for both groups of animals was composed correctly and the animals did not show any metabolic disorders; due to the average value of this parameter (1.26 for PHF and 1.29 for PHF×SRB). According to Haas and Hofirek (2004), the appropriate FPR in milk should be from 1.2 to 1.4. A ratio below 1.1 indicates the occurrence of a metabolic disorder such as acidosis. However, the ratio above 1.5 may indicates the occurrence of ketosis (Riehardt, 2004;Puppel and Kuczyńska, 2016), which was also confirmed by Ranaraja et al. (2018).      Table 4 presents the results concerning milk performance results for full lactation of purebred PHF and PHF×SRB cows. The longer lactation (by 63.47 days) was shown in the PHF cows in comparison to the crossbreds; which may indicate problems with reproduction of the purebred cows. During the prolonged lactation, milk production by purebred cows is significantly higher (by 2919.4 kg), and the milk is characterised by lower contents of individual components (fat -0.50 %, protein -0.28 %, dry matter -0.74 %) that determine its technological quality, in comparison to milk from the hybrids. However, the amount of raw material obtained, despite the lower concentration of the above mentioned components, results in a higher milk yield from the PHF cows (fat +76.11 kg, protein +73.7 kg, dry weight +313.97 kg).

Conclusion
Crossbreeding between the PHF and SRB breeds has a positive effect on the content of fat, protein and dry matter in milk, as well as on udder health. Additionally, the crossbreds were characterized by shorter lactation than the purebred PHF, which may be indicative of their smaller reproductive problems. However, more study is needed on the effects of crossing between breeds on dairy cattle.