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The effect of preand post-puberty feeding of rations diversified in terms of energy and protein content on the growth rate, body condition, fatness, milk yield and reproduction efficiency of Simmental heifers was investigated. The experiment was carried out on 40 Simmental heifers from 6 months of age to day 100 of lactation, allotted to 5 groups according to age and body weight. During pre(from 6 to 11 months of age) and post-puberty (from 12 to 13 months of age) the animals were fed rations of different or the same levels of energy (UFL) and protein (PDI) as the control Group K, which was fed according to IZ-INRA (2001). The respective levels of UFL and PDJ in the rations for the experimental groups in both periods were, % of control Group K : Group A, 85/85 and 115/115; Group B, 85/115 and 100/100; Group C, 115/85 and 100/100; Group D, 115/115 and 115/ 115. From 14 months of age to day 100 of lactation the animals of all groups were fed the same as the control group. Heifers that received higher UFL and lower BTJ (Group C) or increased levels of energy and protein (Group D) in their pre-puberty rations than the control group, at 11-18 months of age and in the period from 6 months of age to calving, achieved higher body weight gain than the animals of the remaining groups (P<0.05). Heifers fed rations with high pre-puberty energy levels (Groups C and D) and lower protein levels (Group C) calved about 40 days earlier and had a worse conception rate and insemination index (P>0.05) than animals in other groups. Total milk production per cow in Groups A and B was about 11% higher than in Groups K and D, and about 6% higher than in Group C, but the differences were not statistically significant (P>0.05). Independently 156 FEEDING LEVEL OF SIMMENTAL DAIRY HEIFERS of the protein level, feeding Simmental heifers pre-puberty period ration with a low energy level followed by high-energy ration caused somewhat slower BWG from 6 months to calving, increased the age of first mating and calving but stimulated milk production in primiparous cows.

The effect of pre- and post-pubertal feeding levels on the growth, reproduction, and future milk production of Simmental dairy heifers