| Sumario: | This paper Reports on both the individual and combined effects of age at first caving (AFC) and live weight at first calving (LWFC) for crossbred heifers (Friesian x Zebu) on first lactation Milk yield. Optimal health and nutrition management was followed. Fifty one heifers were allocated to younger. (Y) and older (O) age groups. Within each AFC groups heifers were blocked according to live weight as lighter (L) and heavier (H) live weight groups. Mean AFC and LWFC of the treatment groups were 37.4 months, 45.2 months, and 320.2 kg and 419.4 kg, respectively. Heifers calving at O age produced 6% (90.4 ,t) more milk than Y age group while L live weight treatment heifers produced 29.3% less .milk than H live weight treatment animals. Heifers calving Y_H treatment group produced 17.7% (259 t.) more milk than those calving at Y_L group. Heifers calving in O_L treatment heifers had 39.4% less milk output than O_ H treatment heifers over 305days, indicating greater effect of LWFC than AFC. Milk production increased by 4.3 t. and 41.6 t. for every increase in 1 kg of LWFC and a month in AFC, respectively. The combined effects of AFC and LWFC were significant (P< 0.001) and showed that the effects of one month reduction in age at first calving can be offset by 5.3 kg increase in live weight at first calving.
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