## 4 Discussion  
The experiment analyzed the first three weeks of lactation. There was a sharp increase in milk production during the early stage of lactation, which continued until the so-called peak of lactation was reached. In the results, it was possible to observe an increase in milk yield while, at the same time, seeing a decrease in the content of milk components (Table 1) [8, 28, 29, 30, 31]. [PERSON] et al. [32] reported that changes in gross composition of milk were useful risk predictors of energy balance in early lactation, e.g., fat/protein ratio > 1.4 and milk fat > 4.8%. Additionally, [PERSON] and [PERSON] [33] demonstrated that the optimum fat/protein ratio was 1.2-1.4, while values higher than 1.4 were connected to energy deficiency and subclinical ketosis. The highest fat/protein ratio (1.56) was demonstrated in the first week of lactation (Table 1), which indicated a negative energy balance during the first lactation period. Changes occurring in the basic composition of milk at the beginning of were related to several factors, the first of which, as reported by [PERSON] et al. [29], was the production of colostrum, which occurred at the beginning of lactation in all female mammals and differed significantly in composition from milk. Another factor may have been the changes in feed intake being provided to animals during the drying-out period and immediately after parturition. Yet another issue was the so-called dilution effect, which is associated with increased milk production [8]. All changes in the basic composition of milk should be analyzed in detail by farmers, because it is here that the first signals indicating an emerging problem in dairy cows that have a poorly balanced feed intake, which can lead to the occurrence of diseases of metabolic origin, can be observed [10, 29]. At the beginning of lactation, a negative energy balance (NEB) is very often observed in dairy cows. This observation is associated with the cows' energy requirements for milk production not being sufficiently covered by their feed intake [34, 35, 36]. As a result,  
Figure 4: C18:2 trans10, cis12 [g/100 g fat] content depending on the BHBA level and age of the cows. Data were presented as least squares means with the standard error of the mean. Statistical differences between groups at \(p\leq\) 0.01 and collections at \(p\leq\) 0.01. Treatment effect: 1 st week of lactation; 2 nd week of lactation; 3 rd week of lactation. Means marked with the same letters differ significantly at lowercase letters, \(p\leq\) 0.05; uppercase letters, \(p\leq\) 0.01. Groups: healthy primiparous (PK); healthy multiparous (MK); ketotic primiparous (PH); ketotic multiparous (MH). the animals mobilize the stored fat in their bodies, which is converted, during lipolysis, into fatty acids that are the precursors to non-saturated fatty acids, namely palmitic acid (C16:0), stearic acid (C18:0), and oleic acid (C18:1) [37]. The NEFAs formed during lipolysis adversely affect the metabolism of the liver, which, due to the large amount of NEFAs, uses them to produce \(\beta\)-hydroxybutyric acid, disrupting the metabolism of the compounds formed in the liver [38]. This outcome is the cause of ketosis, which, according to [PERSON] et al. [38], is one of the most commonly diagnosed conditions in dairy cattle and can affect up to 60% of the cows in a herd. One of the main markers used to diagnose this condition is BHBA in the blood [39, 40], the results of which, for our study, are shown in Table 3. The BHBA content in the blood of animals that participated in the experiment varied, though it was characterized by the first week of lactation having the highest level, followed by a decrease in content value. BHBA is also a precursor to the synthesis of milk fat in the mammary gland; this fact was also confirmed by the obtained results, in which high fat content and high BHBA in the blood were observed during the first week of lactation [10, 29, 41].