According to Table 2, there was no significant difference (p>0.05) in the milk fat content between genetic groups, and the coefficient of variation (CV) was 18.02%. These results indicated uniform forage and concentrate intake during the experimental period, confirming the results reported by Rennó et al. (2006), who also observed no such variation when offering the same diet to cows of different genetic groups.  Grandisson and Ford   (1986) refers to the bulky as the most important food in the diet of dairy cows, which can influence the physicochemical characteristics of milk, and according to Oliveira et al. (2007), the proportion of forage offered to lactating cows as source of bulky is related to the production of acetate, which is the primary precursor of milk fat synthesis.

The average results of the fat content of milk from HB cows ranged from 3.35 to 3.62% , and are in accordance with minimum standards established by legislation (Brasil, 2011), which recommends minimum fat content of 3.0%. Mean fat values ​​higher than those observed in the present study were reported by Santos et al. (2008), who evaluated the quality of refrigerated milk and found average fat contents ranging from 3.79 to 4.07%; however, the dairy systems evaluated by these researchers had low technological level.

In assessing the composition of milk from Holstein cows kept in elephant grass pasture, Voltolini et al. (2010) reported mean fat values ​​of 3.98%; however, the milk production observed by these authors was lower (16.72 kg) than the average of 23.27 kg found in this study.

Table 2 shows that the milk produced by HB cows resulted in a significant difference (p<0.05) in the lactose contents, especially in the genetic groups 5/8 blood HB, 7/8 blood HB and 15/16 blood HB, compared to ½ blood HB and ¾ blood HB cows, which produced milk with lower lactose content. The lactose content of milk ranged from 4.51 to 4.74%, which is the parameter that less varied among chemical components in bovine milk.

Average lactose results slightly smaller than those of this study were described by Oliveira et al. (2007), who observed values ranging from 4.35 to 4.42%. Silva et al. (2010) found intermediate values ​​compared to those of the present study, with average of 4.57% in animals manually milked on dairy farms in southwestern state of Goiás.

The association of pathogens with external factors results in a wide range of damage to the animal; however, milk production may decrease and the secretory tissue can also be damaged, in addition to variations in milk components such as reduced lactose content, which can occur due to the increase in SCC, related to the scaling of the secretory tissue (Silva et al. 2000).

The DDE contents of milk from genetic groups differed significantly (p < 0.05), and the results demonstrated that 5/8 blood HB, 7/8 blood HB and 15/16 blood HB animals had, respectively, mean values of 9.06, 8.98 and 9.13%, whose results are higher than those obtained for 3/4 blood HB cows (8.79%), followed by ½ blood HB cows (8.57 %).

The mean DDE values found in this study were higher than those recommended by Brasil (2011), which establishes minimum value of 8.4%, and similar to those reported by Santos et al​ (2008), who found DDE values from 8.4 to 8.8% with 7/8 Holstein/Zebu cows. According to Reis et al. (2007), DDE is positively correlated with protein and lactose, which was also observed in the present study, explaining the superiority of 5/8 blood HB, 7/8 blood HB and 15/16 blood HB animals, which produced milk with higher DDE concentrations.

The protein contents of milk did not differ significantly (p > 0.05) among 5/8 blood HB (3.35%), 3/4 blood HB (3.29%), 7/8 blood HB (3.32%) and 15/16 blood HB (3.39%) groups, but were higher (p < 0.05) than ½ blood HB group, which showed mean protein content of 3.12%. The lowest mean protein value found in milk from ½ blood HB cows was due to the higher milk volume produced by these animals, causing dilution of this component, corroborating the study by Schutz et al. (1990), who reported that variations in protein percentages are negatively related to milk production. The mean protein values ​​of milk from different genetic groups were higher than those recommended by Brasil (2011), which is at least 2.9%.

The casein contents in milk, which are related to cheese production yield were higher (p<0.05) in milk produced by 5/8 blood HB, 7/8 blood HB and 15/16 blood HB genetic groups, with mean values of 2.62, 2.59 and 2.64%,  respectively.  ½  blood  HB  cows  produced  milk with lower casein content (2.44 %) and the casein content of milk from 3/4 blood HB cows did not differ significantly between groups. Mean casein content ​​of 2.21% was reported by Freitas Júnior et al. (2010), which is lower than results of this study.

As Hermansen et al. (1994), the casein contents have a positive correlation with the protein content of milk, since casein represents approximately 80% of total proteins of milk; therefore, in this study, the genetic groups that showed the highest protein levels consequently showed the highest casein levels. Zanela et al. (2006) reported that the percentages of casein and solids are affected mainly by nutrition; however, as there was no difference in the diet in the present survey, it could be inferred that there is a close relationship between increased blood levels and increased casein content in milk.

The urea concentrations in milk produced by 5/8 blood HB and 15/16 blood HB genetic groups were higher compared to 3/4 blood HB genetic group and no significant difference (p>0.05) in the urea levels between groups of animal previously mentioned and ½ blood HB and 7/8 blood HB groups. The urea in milk is an important source in the nutritional monitoring of the herd, as it is related to protein metabolism of dairy cows (Jonker et al., 1998). The cause of variation in the urea content of milk in the present study may be related to the metabolism of the different blood levels studied, since metabolism is related to the degree of adaptation of animals to the environment.

The average urea results in this study were below normal levels, and according to Gonzales and Campos (2003), it should be between 24.10 and 34.2 mg/dL and that the milk nitrogen urea concentration (NUL), where 1 mol NUL is equivalent to 2.14 mol of urea, is directly related to the blood nitrogen urea concentration, which can be changed depending on the levels of degradable protein in the rumen and the protein: Energy ratio of diets offered to dairy cows, and reported that urea concentrations in milk below 19 mg/dL indicate protein deficiency in the diet offered, thus explaining the low urea levels found in the present study.

The average urea levels ranged from 11.79 to 13.69 mg/dL, and these values ​​are smaller than those found by Oliveira et al. (2001), which ranged from 47.41 to 58.82 mg/dL, when evaluating the increasing levels of non-protein nitrogen in the diet of Holstein cows.

The statistical analysis of SCC (Table 2) in this study was performed as log¹⁰, but the discussion will be based on the unit recommended by Brazilian legislation for milk quality, which establishes maximum count of 600,000 SC/mL for the period 01 January 2012 to June 30, 2014, for the region comprised in this study, located in central Brazil.

The SCC values was significant influenced (p<0.05) by the blood level, where 3/4 blood HB cows produced milk with  higher  SCC  (585,000 SC/mL)  compared  to   other blood levels, which showed no significant difference (p>0.05) from one another. The mean SCC value of 229,000 SC/mL and maximum value of 585,000 SC/mL reported in this study were similar to SCC values found by Moreira et al. (2003), of 289,000 SC/mL and 465,000 SC/mL when compared to the SCC value found in the milk of dairy cows fed high forage percentages. According to Souza et al. (2009), variations in SCC have been assigned to the pathogen species present in the mammary gland, and Streptococcus agalactiae is the main responsible for higher SCC in the milk of dairy cows. Although the present study has shown variations in SCC values, the mean values ​​were below 600,000 SC/mL, corresponding to the maximum allowed by IN 62/2011, which from 2014 will be reduced to 500,000 SC/mL.

The numerical SCC values ​​of 5/8, 3/4 and 7/8 blood HB cows were higher (403,000 SC/mL to 585,000 SC/mL) when compared to the average values of ½ and 15/16 blood HB cows, respectively, resulting in SCC values of 229,000 and 236,000 SC/mL; however, the SCC results are in accordance with Brazilian legislation for milk quality.

 

 

 

 

Variations in milk production (Table 3) were significantly influenced (p < 0.05) by genetic groups, and the group of ½ blood HB animals was more productive (28.95 kg) compared to 5/8 blood HB (23.15 kg), 3/4 blood HB (22.79 kg), 7/8 blood HB (23.87 kg) and 15/16 blood HB (20.30 kg) animals. The average milk production results of multiparous Holstein cows reported by Silva et al. (2011) were higher (30.56 to 31.07 kg / milk / day) compared to the average of 23.27 of the different genetic groups of this study.

The higher milk production attributed to 1/2 blood HB genetic group can be explained by the greater capacity to adapt to the milk production system under study. Similar to that observed in the present study, Facó et al. (2002) reported that there was no benefit in increasing the proportion of Holstein genes in the most hostile environmental conditions; however, for more favorable conditions, such increase seemed to be interesting to increase milk production.

The HB blood levels were negatively correlated (Table 4) with milk volume (r = -0.1184), indicating that the higher the Holstein/Braunvieh blood level, the lower the milk production. With increasing Holstein blood level, there was a decrease in milk production, indicating lower adaptability of these animals the study conditions. Cows with higher Holstein blood level showed lower milk production capacity in environments that do not meet minimum comfort conditions required by these animals.

 

 

 

 

There was a negative correlation (r = -0.2741) between genetic composition and the number of deliveries, that is, the greater the HB blood level, the lower the number of deliveries. Thus, it appears that animals with highly specialized genetic composition for milk production have difficulty to impregnate.

Blood level was positively correlated (r = 0.1382) with the defatted dry extract content, demonstrating that the higher the HB blood level, the greater the DDE content in milk. There was no significant correlation (p>0.05) between blood level, days in lactation, SCC and fat, protein, lactose, urea and casein contents.

The correlation between milk volume and days in lactation (r = -0.5184), fat (r = -0.3727), protein (r = -0.4952), lactose (r = -0.1185), DDE (r = -0.5275), urea (r = -0.2272) and casein (r = -0.3874) contents was negative. The higher the milk volume produced by HB crossbred cows, the lower the days in lactation, so that cows in the early stage of lactation produced higher milk volume.

In the conditions of the present study, the higher the volume of milk produced, the lower the fat, protein, lactose, DDE, urea and casein contents, indicating that there is a dilution of milk components with the increase in the volume of milk produced by cows, which agrees with the results by Trevaskis and Fulkerson (1999), who observed that high milk production cows fed with forage produced milk with lower urea concentration, possibly due to the dilution effect of increased production.

Oltner et al. (1985) observed a positive correlation between milk production and urea concentration in milk. Kaufmann (1982) found a similar relationship, which was attributed to increases in protein content in the diet and in milk production, and not only to the milk production effect. The volume of milk produced was positively correlated (p<0.01) with the number of deliveries (r = 0.4590), therefore, it is desirable that lactating cows are kept for longer time along with the herd in order to significantly express the productive potential.

There was no significant correlation (p>0.05) between milk volume and SCC and therefore the incidence of mastitis would be related to handling and environment and not to productive factors.

Days in lactation resulted in negative correlation (r = -0.3036) with the number of deliveries, indicating that cows with long periods of lactation had fewer offspring. The explanation is the involuntary persistence of the exploitation of cows with high milk production potential.

Days in lactation were positively correlated with body fat (r = 0.3018), protein (r = 0.5944), DDE (r = 0.5553), urea (r = 0.3261) and casein (r = 0.5096) contents. Cows with longer lactation produced milk with higher fat, protein, DDE, urea and casein contents. This fact is due to a reduction in the volume of milk produced by animals at the final stage of lactation and corresponding increased concentration of milk chemical components.