Condición corporal al parto y parición posterior en las razas Angus, Hereford, sus cruzas y retrocruzas en pastoreo de campo natural

Body condition score is a subjective visual assessment tool used to determine the nutritional status of beef cows, which is closely related to subsequent reproductive efficiency. In Uruguay, beef cattle production is carried out under natural grassland grazing, a system with marked forage seasonality that affects the body condition of the animals. Likewise, the superiority of crossbred animals over pure breeds has been demonstrated, attributed to the effect of heterosis on reproductive traits. The objective of this study was to evaluate body condition score at calving (BCS at calving) in Angus, Hereford, their F1 crosses and backcrosses under a diallel design on natural grassland grazing, as well as its relationship with the probability of calving in the subsequent cycle. The hypothesis was that there is a positive relationship between BCS at calving and the probability of calving in the subsequent cycle, and that crossbred cows require a lower BCS at calving than purebred cows to achieve similar calving levels, due to the favorable effect of heterosis. The database of the beef cattle herd at the Bernardo Rosengurtt Experimental Station (Faculty of Agronomy, Universidad de la República, Cerro Largo) was used, comprising 3,433 records from 1,345 cows and 98 bulls during 1994–2006, in six maternal genotypes (HH, AA, AH, HA, RTHH and RTAA). A linear mixed model was fitted to evaluate the effect of maternal genotype, animal category and calving year on BCS at calving, with cow included as a random effect. In addition, a logistic regression with the BCS at calving × Genotype interaction was fitted to estimate the BCS at calving threshold required to achieve an 80% calving rate for each genotype, and heterosis was calculated as the percentage difference between the crossbreds and the average of the parental pure breeds. The results showed that HH presented the highest adjusted BCS at calving (3.68), whereas AA and RTAA were significantly lower (3.55 and 3.46); the AH, HA and RTHH crossbreds did not differ from HH. Calving year and animal category also significantly affected BCS at calving. Regarding calving, the reciprocal crosses AH and HA presented the highest values (84.1% and 83.1%), followed by RTHH (79.6%) and RTAA (76.8%), whereas the pure breeds HH and AA showed the lowest values (74.3% and 70.6%). The BCS at calving threshold for an 80% calving rate was 3.22 in HA, 3.38 in RTAA, 3.64 in RTHH, 4.29 in HH and 4.48 in AA, whereas AH exceeded that percentage regardless of body condition score. All crossbred genotypes presented negative (favorable) heterosis for this threshold. It is concluded that there is a positive relationship between BCS at calving and subsequent calving, confirming the proposed hypothesis, and that F1 crossbred cows require a lower body condition score than pure breeds to achieve equivalent calving levels, an advantage that decreases as heterosis levels are reduced in the backcrosses. The determination of body condition score is reaffirmed as a practical, accessible and useful tool for decision-making in beef cattle herds in the country.