The Assaf sheep breed has a national breeding program based on increasing milk production per lactation and quality. This research attempts to present a simple tool to decompose the average annual value of 150d standardized milk yield in factors attributable to the environment and the genotype of the animals, using production data (512.889 lactations of 204.050 ewes), pedigree information and official genetic evaluation of the breed (209.849 animals). The genetic evaluation is performed using BLUP methodology, using an animal model with repeated measures including environmental effects controlled by the farmer (feeding, health, management, facilities), environmental effects not controlled by the farmer (lactation number, type of birth, interval between lambing and the first control), the additive genetic effect and the permanent effect of de ewe. For a study period of 19 years (1993-2011) the results show a 98% increase in milk production. This progress is attributable to the uncontrolled factors (7.7%), the controlled factors (36.5%) and the breeding program (55.8%). Farmers have made major efforts to improve animal production by improving the management of their farms and achieving a remarkable genetic progress that in 2008 exceeds for the first time to improve handling.