Growth and feed conversion of Boran (Bos indicus) and Holstein X Boran heifers during three physiological states receiving different levels of a tropical diet

Forty-eight heifers of two genotypes, Boran (Bos indicus) and Holstein×Boran, were allocated to three feeding levels equivalent to 1.0, 1.2 and 1.4 times calculated metabolisable energy requirements for maintenance (MEm). The experiment started at the onset of reproductive activity, when the Boran and crossbred heifers were 4.4 and 2.2 years old, respectively. The response to feeding level was investigated in three physiological states: (i) puberty and growth (87 days), (ii) work and growth (94 days, 51 days thereof with work), and (iii) pregnancy and recovery from work (115 days). Neither genotype nor feeding level had effects on nutrient digestibility, digesta passage rate and reproductive performance. Daily gains and feed conversion efficiency were higher in well-fed heifers (P<0.001 and P<0.01). Estimated MEm was 0.54 MJ/kg metabolic weight (W0.75) in both genotypes during puberty and growth. Imposing work required additional 0.20 and 0.10 MJ ME/kg W0.75 in Boran and crossbreds, respectively. Recovery from work, associated with early pregnancy, apparently reduced MEm to 0.30 MJ/kg W0.75. This could be explained as a result of compensatory growth. The efficiency of utilisation of metabolisable energy for growth (0.28) did not differ (P>0.1) between genotypes and physiological states.