The study was conducted in the autumn/winter period in North Kazakhstan of 2023-2024. The research aimed to assess the impact of extruded compound feeds (ECF) on the productivity and quality of milk from highly productive black-and-white cows.

Three experimental groups (EG) and a control group (CG) were formed for the experiment. Each group consisted of 10 cows, which were taken from farms located in northern Kazakhstan. These cows were selected to be analogous in terms of live weight, productivity, age, and calving date. The CG cows were fed the basic diet (BD), and the EG, in addition to the BD, received 1.0, 1.5, and 2.0 kg of ECF, which consisted of barley and corn in equal proportions. Once a month, control milking was carried out to account for milk productivity, and milk was selected for chemical analysis.

The chemical analysis of milk was carried out using a CombiFoss FT+ infrared spectrometer  to control the following indicators: mass fraction of fat, mass fraction of protein, casein, lactose, dry matter, urea, and somatic cells (according to the State Standard (GOST) 32255-2013 "Milk and dairy products. An instrumental express method for determining the physicochemical identification parameters using an infrared analyzer") [16].

Milk density and non-fat milk solids (NFMS) were determined according to Kazakh standards (ST RK) 1483-2005 "Cow milk. Test methods for determining the composition and density of milk". The analysis was performed using a Lactane 1-4 ultrasonic milk analyzer. The ultrasonic method was based on measuring the ultrasound velocity in milk [17].

The titrated acidity of milk was determined according to GOST 3624-92 "Milk and dairy products. Titrimetric methods for determining acidity". The essence of the method is the titration of milk with potassium or sodium hydroxide in the presence of a phenolphthalein indicator.

The active acidity was determined according to GOST 32892-2014 "Milk and dairy products. The method of measuring active acidity". It is a potentiometric method based on measuring the potential difference between two electrodes immersed in milk [18].

Results

The milk productivity of cows is the quantity and quality of milk received over a certain period. Milk productivity is a complex feature conditioned by the morphological structure of the udder and its functional features, which are associated with metabolism and nervous and humoral regulation. The control milking of highly productive dairy cows during the experiment is presented in Table 1. Adding the ECF increased daily milk yields by 1.96-4.24 kg. A significant increase was observed in EG II. In September, the average daily milk yield in EG II exceeded the CG by 12.9% at P<0.05 and by the end of the experiment, by 23.9% at P<0.001. Compared with the CG, milk productivity in EG I increased by 15.4%, in EG II, by 19.4%, and in EG III, by 10.0%. The lactation curves of the groups are shown in Figure 1.

In the four months of lactation, the milk yields changed significantly. By the 2nd month, cows had a peak, and then a gradual decrease in milk yields was observed. The highest milk yields by the 2nd month indicate that the cows were highly productive since this is their characteristic feature. Starting from the 2nd month, there is a gradual decrease of 4-5% for each subsequent month.

By the nature of lactation curves, highly productive black-and-white dairy cows have a high and stable lactation activity. After calving, the peak of lactation was observed at 2 months of lactation. High milk productivity persisted for a long time, that is, the lactation curve is smooth.

The mass fraction of milk fat in all groups was minimal in the 2nd month of lactation but then increased. There is a relationship between milk yield and the mass fraction of fat in milk, so if milk yield decreases, the mass fraction of fat in milk increases and vice versa. In this regard, a decrease in butter fat yielding capacity is considered acceptable, since an increase in the average daily milk yield was observed.

EG III had the highest milk fat content throughout the study, but when converted to butter fat, this group was inferior to other groups. When comparing the EG with the CG in terms of butter fat yielding capacity, it can be concluded that they exceeded the CG by 7-10% throughout the experiment.

Table 2 shows cow productivity over the entire period of the experiment. All groups had a high level of milk productivity. The maximum average daily milk yield was obtained from the EG II cows. The EG outperformed the CG by 10.0-19.4% in terms of average daily milk yield. EG III had the highest indicators of fat mass fraction and protein mass fraction. However, after the recalculation of butter fat and milk protein, this group showed lower results than EG I and EG II. In terms of milk fat, EG II exceeded the CG by 25.9% at P<0.01. As for milk protein, EG II exceeded the CG by 23.7% and EG I and EG III by 3.9 and 9.2%, respectively.

In terms of milk with basic fat content, EG II had the highest indicator, surpassing the CG by 25.6% and EG I and EG III by 5.1 and 8.4%, respectively. In the Republic of Kazakhstan, a differentiated norm of basic fat content in cow milk has been established for each region, according to Resolution No. 785 of the Cabinet of Ministers of the Republic of Kazakhstan dated September 21, 1992. For the North Kazakhstan region, the basic fat content is 3.5%. During the experiment, we studied the organoleptic parameters of milk. The following indicators were determined:

• color;
• appearance and consistency;
• smell and taste.

After the organoleptic assessment carried out by a commission, we compared the data obtained with the requirements set out in TR TS 033/2013 [21].

During the organoleptic evaluation, we found that the milk samples had a clean smell and taste, without extraneous tastes and odors, a homogeneous non-viscous consistency without protein flakes and fat lumps, and a white color. The EG III milk had a light cream color. Adding the ECF to the BD contributed to a significant improvement in milk quality. The indicators of the physicochemical composition of milk are presented in Table 3. The casein and lactose content in the raw milk in the EG was higher than in the CG. The maximum content of casein and lactose was noted in EG II. The CG II casein content was higher than that of the CG by 6.4%, EG I by 1.1%, and EG III by 0.3%. The milk from the EG contained a higher number of solids.

This indicates that milk has more nutritious and technological properties. Thus, in the EG, the number of solids in milk was 3.8-5.7% higher than in the CG.

The NFMS is what remains of milk if all water and fat are removed. The NFMS content in the EG was higher than in the CG by 0.93-4.2%. EG II had the highest NFMS index of 8.87±0.20%.

The titrated and active acidity in the milk from the groups was within acceptable limits, which indicates the good quality of the obtained raw materials. The urea content should be given special attention, since in EG III, by the end of the experiment, the urea content in milk was 27.96 mg, which indicates an excess of metabolic energy and crude protein. In the other groups, this indicator was within the normal range. The content of somatic cells during the experiment is shown in Table 4.

The content of somatic cells in cow milk indicates under health [19]. According to GOST 31449-2013, "Raw cow's milk. Technical specifications", the number of somatic cells should not exceed 400 thousand/cm³ [20]. In our studies, the number of somatic cells in the tested milk ranged from 203.40 to 215.90 thousand/cm³, which meets the requirements. The CG throughout the experiment had the smallest number of somatic cells in milk (203.40-205.9 thousand/cm³). During the experiment, the number of somatic cells increased in all groups by the 2nd month of lactation and decreased by the end of the experiment. It can be concluded that adding the ECF to the main diet did not affect the number of somatic cells in milk. The values were within acceptable norms, which indicates that the farm complied with the technology of keeping and milking cows.

Figures & Tables

Discussion

It was concluded that the optimal amount of the ECF is 1.5 kg. Milk productivity in the EG increased by 10-20% compared to the CG. The introduction of the ECF into the diet had a positive effect on the physicochemical parameters of milk, since the mass fraction of fat in the milk from the EG was higher than in the CG by 7-10%, the mass fraction of protein by 5.1-6.5%, solids by 3.8-5.7%, and lactose by 1.1-4.3%. The studied milk samples had a clean smell and taste, without extraneous tastes and odors, a homogeneous non-viscous consistency without protein flakes and fat lumps, and a white or light cream color. In cows overfed with the ECF, urea levels in milk increased. This indicates an excessive amount of crude protein and metabolizable energy in the diet.

Using the ECF in the feeding of dairy cows does not affect the content of somatic cells in milk. The titrated and active acidity in the milk of all experimental groups was within acceptable limits, which indicates the good quality of the raw materials obtained.

Since ECF formulations have to combine several parameters including component kind and shape, extrusion temperature, ECF concentration, and dairy cow characteristics, the conclusions of previous research are highly heterogeneous.

Risyahadi et al., reviewed existing data on the effects of extrusion and found that, compared to non-extruded feed, such heat processing increased milk yield, dry matter, and lactose content in milk, which is consistent with our findings [22]. However, milk fat and/or protein content was reduced or remained unchanged in most studies [22-26]

The most significant gain after feed extrusion occurs in milk productivity [26, 27]. Increased milk productivity correlates with acetate/propionate molar ratio in faba bean and lupin fed experiments, therefore, it was suggested that a higher proportion of acetate compared to propionate results in enhanced milk productivity because acetate serves as a precursor to milk yield [23].

High-temperature processing of grain changes the molecular structure of the proteins it contains, which in turn changed the digestibility of the protein. The proteins were broken down into peptides, and this form was more accessible to hydrolyzing digestive enzymes.  Furthermore, extrusion reduced the levels of anti-nutritional factors in the grain, such as protease inhibitors, so protein biodegradability was intensified. [28]. The increase in milk protein in our case can be associated with the above-mentioned processes. Studies that obtain opposite results claim that excessive processing leads to protein denaturation and most likely converts proteins into a more stable indigestible form. Also, despite the increase in the digestibility of crude protein, this does not lead to an increase in the concentration of milk protein, which suggests that greater digestibility of crude protein does not necessarily lead to increased flow of amino acids into the mammary gland and the production of casein [29].

According to other studies, the greatest difference between extruded and non-extruded samples is in the total levels of butyrate, iso-butyrate and valerate. Untreated samples have higher levels of these fatty acids, and they accelerate blood flow in the cow's rumen, which improves the absorption of other fatty acids [30].

The sensory properties of milk were also assessed by other authors: ECF containing faba beans does not lead to a specific aroma, taste or texture of milk, which corresponds to our data [27].

Authors note that by demonstrating the positive impact of ECF on milk yield and quality, the findings provide a practical solution for enhancing dairy productivity in regions similar to North Kazakhstan. This can help meet the increasing demand for milk and dairy products, contributing to better food security and economic stability for dairy farmers.

Author Contributions

All authors developed the concept, designed the methodology, collected and analyzed the data, and prepared the original draft of the manuscript.

Conflict of Interest

The author declare that there is no conflict of interest regarding the publication of this paper.

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