Genetic and environmental effects on the productivity of Holstein-Friesian cattle under the climatic conditions of central Sudan.

A herd of 370 Holstein-Friesian cows were maintained in Central Sudan, Khartoum for intensive dairy production during the period 1990-1996. The area is characterized by high temperatures during the day and cools down at night, with an average thermal-humidity index (THI) of 74.8. The average adjusted lactation milk yield, milk yield per day, milk yield per day of calving interval and lactation length of the animals in the herd were 5117 +/- 123 (SE) kg, 14.7 +/- 0.25 kg, 11.3 +/- 0.36 kg and 350 +/- 8.0 days, respectively. Heifers calved at 25.2 +/- 2.3 (SD) months of age. The variation was large, which suggested large environmental fluctuations. Sire variance contributed 1.35% and 19.6% to the total variance in cows and heifers, respectively, while cow within sire contributed 19%. The heritabilities of total lactation yield, 305-day yield, milk per day of lactation, milk per day of calving interval, and lactation length in heifers were 0.78 +/- 0.24, 0.36 +/- 0.34, 0.39 +/- 0.24, 0.23 +/- 0.23, and 0.23 +/- 0.22, respectively. In cows, the heritability estimates were 0.05 +/- 0.24, 0.06 +/- 0.24, 0.08 +/- 0.24, 0.00 and 0.00 for the same traits, respectively. The repeatabilities of total lactation yield, 305-day yield, milk per day, milk per day of calving interval and lactation length were 0.02 +/- 0.03, 0.22 +/- 0.03, 0.17 +/- 0.02, 0.05 +/- 0.02 and 0.00, respectively. Regression analysis showed that, for each unit increase in THI, milk yield decreased by 0.29 +/- 0.04 kg. The stress usually caused by the combined effects of temperature and humidity was not severe in this area.