Paper I reports the effect of protein levels on the reproductive performance of Kedah-Kelantan heifers. Twenty-seven pregnant heifers were allocated to 6 treatments, with 5 different levels of crude protein viz. at 12, 15, 18, 21 and 24%. The sixth treatment contains 18% crude protein but no urea and has only soybean as a protein source. The animals were fed to appetite individually a ratio of 75:25 of Napier grass (Pennisetum purpureum) and concentrate in terms of dry matter intake. Changes in liveweight, birth weight and feed conversion were measured for a total of 322 days of experimentation. During the period of gestation, no significant differences were found between treatments for any of the parameters measured. Dams fed diets containing higher than 18% crude protein gained less weight, reached lower liveweight before calving and lost more weight after parturition. There was very little difference in the gestation length and birth weight of the calves. The different components of foetal tissues and foetus also showed little difference. Significant differences (P<0.01) between treatments were found for the liveweight changes during the period of lactation. Dams fed 12% crude protein and soybean meal maintained their weight during lactation and were significantly different from dams fed 18, 21 and 24% crude protein in their liveweight at weaning, their average daily gain and their weight loss during lactation. There was no significant difference in the feed intake during either gestation or lactation. The inclusion of high levels of urea in ruminant feed and its effect on reproduction was discussed. 

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