The effect of supplementation with three levels of Leucaena leucocephala leaf meal (LLM) added to a diet composed of cotton seed hulls (CSH) and maize bran (MB) on milk yield, milk composition and body weight changes of grazing lactating cows was studied in a completely randomised design. Marginal product analysis for the different levels of LLM as a substitute to cotton seed cake (CSC) was also calculated. The following treatment diets were used in the experiment: Diet 1, (control) composed of 1.8 kg DM CSH and 1.8 kg DM MB. Diet 2, 3 and 4 had an addition of 1.2, 2.0 and 2.6 kg DM of LLM respectively. Diet 5 was composed of 1.8 kg DM of CSC and 1.8 kg DM of MB. Treatments, number of lactations, farm and initial milk yields significantly (P < 0.01) influenced milk production. LLM supplementation to grazing cows significantly increased milk production, weight gain and milk composition (P > 0.05). Cows on treatment 1 achieved no significant (P < 0.05) increase in milk yield compared to cows on treatment 2, 3, 4 and 5 which attained net milk increase of 3.6, 4.1, 6.7 and 4.4 litres/cow/day respectively. There was no significant (P > 0.05) difference in net milk produced by cows on treatment 2, 3 and 5 compared to treatment 1. LLM supplementation significantly (P < 0.05) increased total solids (TS) in milk compared to that of cows under control, but there was no significant (P > 0.05) influence on milk crude protein (CP), butter fat (BF), solids not fat (SNF) and ash in all the diets. Body weight was significantly (P < 0.05) increased by LLM supplementation. Degradation coefficients of the treatment diets were high and varied significantly (P < 0.01) and had an influence on the metabolizable energy value of the treatment diets. Cows on diet 2 to 4 had a weight gain of 190.9, 101.8 and 80.9 g/cow/day respectively and cows on diet 5 attained the highest weight gain (234.5 g/cow/day). Diet 4 had the highest marginal productivity of 4.3 followed by diets 2, 3 and 1 with marginal productivities of 2.8, 0.6 and 0 respectively. It was therefore, concluded that the optimal supplementation level using LLM as a substitute for the more expensive CSC was diet 4.
