Abstract
Abstract Extensive horse breeding is an important sub-branch of animal husbandry in Kazakhstan, where the Mughalzhar horse breed is of high value due to its adaptation to pasture-based management and high productivity. However, the effectiveness of classical selection methods under the conditions of regional populations in the southern part of the country has been insufficiently studied, which necessitates assessing their impact on the growth and development of young stock. The study was conducted in 2024–2026 at the peasant farm “Senim” (Turkestan Region). From 285 mares of reproductive age, a selection group (n = 233) was formed. For selective mating, 18 mares of the “elite” and Class I categories and one elite-class breeding stallion were selected. Growth and development were evaluated in 10 fillies (aged 3 days to 24 months) based on live weight, four main body measurements, and absolute gain. The milk productivity of the dams was determined in the third month of lactation using the test milking method. The data were processed using methods of variation statistics. The results showed the selection group was characterized by high uniformity in conformation (Cv = 3.21% for live weight) with moderate variability in milk productivity (Cv = 13.93%). The average daily milk yield of the mares was 4.17 ± 0.28 kg. Offspring from selective mating demonstrated high growth intensity: during the first 6 months of life, live weight increased 3.09 times (from 39.0 to 159.5 kg). By 24 months of age, the fillies reached a live weight of 303.9 ± 11.06 kg and a height at the withers of 140.3 ± 4.84 cm, corresponding to the breed standard. The coefficient of variation for live weight in the offspring group decreased to 5.46% by the age of 2 years, indicating high herd uniformity in productive traits. The highest intensity of live weight gain was observed during the period from 3 days to 3 months of age, after which relative and average daily gain gradually decreased, while the positive dynamics of absolute gain were maintained up to 24 months of age. Directed selection of parental pairs using elite and Class I animals ensures the stable production of uniform offspring with growth intensity exceeding average population indicators. The milk productivity of mares (4.17 kg/day) can serve as an indirect predictor of offspring growth rate. The results confirm the effectiveness of classical selection methods in extensive horse breeding in the southern region of Kazakhstan and can be used to optimize breeding programs.