Growth Performance, Milk Productivity, Productive Longevity, and Milk Composition of Mugalzhar Horses.

The Mugalzhar horse is an important native breed of Kazakhstan, valued for its adaptability, milk production, and meat productivity under extensive pasture-based management. This study evaluated the relationships among first-lactation milk yield, productive longevity, lifetime milk production, growth, morphometric traits, and milk composition in the Mugalzhar horse breed using data from 100 mares with complete lifetime production records, 108 adult mares and 12 stallions for morphometric evaluation, and 68 foals (35 colts and 33 fillies) for growth analysis. The study was conducted on Mugalzhar horses maintained at the "Senim" farm in the Turkestan region of Kazakhstan. Morphometric measurements of adult horses, growth dynamics of foals from birth to 18 months of age, milk productivity records, productive longevity indicators, and milk composition data were analyzed. PCA, Pearson correlation analysis, and HCA were applied to assess relationships among productive and compositional traits. Adult stallions and mares closely conformed to breed standards, with stallions exceeding mares by approximately 7.8% in live weight. Foals exhibited intensive postnatal growth, with live weight increasing 6.8-6.9-fold between birth and 18 months of age. Milk yield increased from 722.3 kg during the first lactation to 986.9 kg during the fifth lactation before declining in later lactations, whereas milk composition remained relatively stable. Moderate-to-high first-lactation milk yield groups (G3-G4) achieved the greatest lifetime milk production (24,961-25,997 kg), longest productive life (7.9-8.5 years), and highest number of completed lactations (6.6-6.9). PCA revealed two major biological gradients representing productivity and longevity, while HCA identified G3-G4 as the most favorable production cluster. Milk fat and protein concentrations were negatively associated with milk yield traits. Moderate first-lactation milk yield was associated with the greatest lifetime productivity and longest productive herd life, whereas extremely high first-lactation milk yield was not associated with superior lifetime performance. These findings suggest that first-lactation milk yield may be considered together with longevity and other functional traits in future breeding programs for Mugalzhar horses; however, the results should be interpreted in light of the observational design of the present study.