Muzaffarnagari sheep represent one of the major meat-type breeds in South Asia and play an important role in small ruminant production systems in semi-arid environments. This study evaluated the impact of long-term genetic upgrading, achieved through the sustained use of improved Muzaffarnagari rams over successive generations, on the growth performance of lambs derived from local non-descript sheep populations under field conditions. Live body weight records of N = 1011 lambs were collected from 120 flocks managed under traditional husbandry practices, categorized as genetically upgraded or non-upgraded based on sire usage. Growth traits were analysed using the General Linear Model in SPSS (version 26.00) to assess the effects of genetic group and sex. Least-square means for upgraded flocks at birth, 3, 6, 9, and 12 months were 3.13 +/- 0.04, 15.88 +/- 0.16, 22.53 +/- 0.21, 26.93 +/- 0.20, and 30.96 +/- 0.25 kg, while corresponding weights for non-upgraded flocks were 2.80 +/- 0.04, 13.34 +/- 0.18, 19.61 +/- 0.19, 24.49 +/- 0.21, and 27.64 +/- 0.23 kg. Males were consistently heavier than females across both genetic groups, and significant effects of flock type and sex (P < 0.01) were observed at all growth stages. Lambs from upgraded flocks consistently maintained higher body weights under farmer-managed field conditions, indicating realized on-farm performance advantages associated with sire-based genetic upgrading. The study was limited by its reliance on field-managed flocks, where environmental factors could not be fully controlled.