Effects of stocking rate, improved pasture and weaning age on the reproductive performance of primiparous cows

Foram utilizadas 92 vacas primíparas aos três anos de idade (46 Braford – 46 Hereford), manejadas em campo nativo nas cargas animais (CA) de 240 kg/ha (T1) e 320 kg/ha (T2), de 01.09.00 a 06.03.01, ou em pastagem melhorada de azevém (Lolium multiflorum L.) na CA de 400 kg/ha (T3) por 80 dias pós-parto e, posteriormente, em CA em campo nativo igual a T2 até o desmame à idade convencional (06.03.01). A estação de acasalamento transcorreu de 23.11.00 a 15.02.01. O desmame precoce (DP) foi realizado em 46 vacas em 01.01.01. O peso ao parto (PP), ao início do acasalamento (PIA) e ao final do acasalamento (PFA), não foi influenciado pelos tratamentos. Entretanto, os tratamentos influenciaram significativamente a condição corporal ao parto (CCP), ao início do acasalamento (CIA) e ao final do acasalamento (CFA). As vacas Braford tiveram PP, PIA e PFA significativamente superiores em relação às vacas Hereford. A taxa de prenhez (TP), o intervalo de partos (IEP) e o intervalo parto-concepção (IPC) não foram influenciados pelo DP. A TP não foi influenciada pelos tratamentos (T1= 93,8%; T2= 90,6%; T3= 100%) e pelo DP (DP= 97,8%; DC= 91,3%). Contudo, aos 21 dias após o início do acasalamento, 15,6 e 17,9% das vacas do T1 e T3, respectivamente, encontravam-se prenhes, enquanto nenhuma vaca (0,0%) do T2 havia concebido. Aos 42 dias após o início do acasalamento, 46,9 e 71,4% das vacas do T1 e T3 encontravam-se prenhes, respectivamente, contra 37,5% das vacas do T2. As vacas do T3 tiveram IEP (386,8 dias) e IPC (101,8 dias) significativamente menores que as vacas do T2 (399,0 e 114,0 dias, respectivamente). O T1 teve IEP (390,9 dias) e IPC (105,9 dias) intermediários, não diferindo dos demais tratamentos.Ninety two primiparous cows at three years old (46 Braford – 46 Hereford), were assigned to stocking rates (SR) of 240 kg/ha (T1) and 320 kg/ha (T2) on natural pasture, from 09.01.00 to 03.06.01, or 400 kg/ha (T3) on improved pasture of ryegrass (Lolium multiflorum L.) during 80 days postpartum, and after, at same SR of T2 on natural pasture until the conventional weaning (CW; 03.06.01). The breeding season was from 11.23.00 until 02.15.01. Early weaning (EW) was imposed to 46 cows in 01.01.01. Live weights at calving (LWC), beginning (LWB) and at the end of the breeding season (LWE) were not affected by the treatments. Treatments affected significantly the body condition at calving (BCC), beginning (BCI) and at the end of the breeding season (BCE). The Braford cows had higher LWC, LWB and LWE than the Hereford cows. The pregnancy rate (PR), calving interval (CI) and calving-conception interval (CCI) were not affected by EW. PR was not affected by the treatments (T1= 93.8%; T2= 90.6%; T3= 100%), neither by EW (EW= 97.8%; CW= 91.3%). However, at 21 days after the beginning of the breeding season, 15.6 and 17.9% of the cows in T1 and T3, respectively, were pregnant, while none cow (0.0%) in T2 was pregnant. At 42 days after the beginning of the breeding season, 46.9 and 71.4% of the cows in T1 and T3 were pregnant, respectively, and 37.5% of the cows in T2. Cows on T3 had CI (386.8 days) and CCI (101.8 days) shorter than cows on T2 (399.0 and 114.0 days, respectively). Cows on T1 had intermediary CI (390.9 days) and CCI (105.9 days), being similar to T2 and T3