The evaluation of the relationship of lactose to production and reproduction traits in different breeding conditions of the Slovak Spotted dairy cows

The lactose content makes up the highest percentage portion in milk and is not part of the creation of breeding programs for national breeds for milk production, as they state Sneddon et al. (2015) and Haile-Marian, Pryce (2017). In general, the lactose and other milk components in milk may be used indirect traits related to production, fertility in conjunction with health traits that are important for the economics of farming dairy cows as reported Miglior et al. (2007).


Introduction
The lactose content makes up the highest percentage portion in milk and is not part of the creation of breeding programs for national breeds for milk production, as they state Sneddon et al. (2015) and Haile-Marian, Pryce (2017). In general, the lactose and other milk components in milk may be used indirect traits related to production, fertility in conjunction with health traits that are important for the economics of farming dairy cows as reported Miglior et al. (2007). These traits are characterized by a low value of heredity and thus largely influenced by environmental traits, especially nutrition as they state in their publications Karcol et al. (2017), Juraček et al. (2020 and other authors. Costa et al. (2019) found that the lactose percentage has potential to asses health and thus the milk quality in Fleckvieh dairy cows.

The evaluation of the relationship of lactose to production and reproduction traits in different breeding conditions of the Slovak Spotted dairy cows
The relation between percentage of lactose and milk production indicators was the subject of the research of many authors, such as Ptak et al. (2012) and Sneddon et al. (2015).
In this work to presents results to evaluate of lactose percentage in relation to the milk production traits a calving interval in selected population of the Slovak Spotted dairy cows.

Data
Data for evaluation of relationship of lactose to milk components and reproduction traits in dairy cows of the Slovak Spotted cattle was provided from by the database of Breeding Services of the Slovak Republic from 2015 to 2019.

Coding of breeding type, milk yield and calving interval
Dairy cows were divided according to the Herd Book classification to subsequent breed-type groups: S 0 -purebred Slovak Spotted with the proportion of foreign breeds less than 12.5%; S 1 -cows with the genetic proportion of the Slovak Spotted from 75% to 87.4%; S 2 -cows with the genetic proportion of the Slovak Spotted from 50% to 74.9% (Slovak Simmental Breeders Association, 2020).

Statistical analyses
For statistical processing of data using the Statistical Analysis System (SAS) version 9.4 (TS1M2), procedures MEANS, CORR and GLM (SAS, 2016). Correlation between lactose with milk components and CI were calculated as Pearson's correlations. Differences between estimated variables were tested at the levels of significance: + P <0.05, ++ P <0.01, +++P <0.001 or -P >0.05.
Linear model with fixed effects was used:

Results and discussion
The basic statistical parameters evaluated traits are presented in Expressing the correlation among milk production characteristics and the lactose in milk, for example for lactose in kg with milk, fat, proteins in kg, lactose and calving interval, were found as follows r = 0.8835, r = 0.8094, r = 0.9726, r = 0.3157 and r = -0.0132. These correlation coefficients were statistically highly significant P <0.0001. The values of correlation dependence are comparable to the values they report Haile-Marian, Pryce (2017)  Based on the results of a linear model of the effect of all effects, we found the following value of the determination coefficient for the lactose percentage R 2 = 0.3150. These effects in the linear model were statistically highly significant (P <.0001). When analysed according to the effects of individual factors on lactose percentage, we found that effect of HYS (R 2 = 0.2057) than effect of sire (R 2 = 0.1266) as shows Table 3. These effects were statistically highly significant