Performance Efficiency of Pasture-raised Primiparous Beef Cows of Three Different Biotypes and Two Milk Production Levels

Abstract

Background: Productivity and efficiency of beef production systems should be estimated based on cow size, calf growth rate and cow reproduction rate, which are critical factors for the evaluation of production characteristics of beef cattle of different biological types. One of the ways to measure beef cow productivity is based on the ratio of calf weaning weight to cow weight. The purpose of this study was to evaluate the performance and reproductive efficiency of primiparous beef cows of three different biotypes and two milk production levels, from calving to weaning, in pasture-raised beef systems. Materials, Methods & Results: Forty-two primiparous Angus cows were classified according to body weight at calving, as follows: Heavy (431 kg); Medium (388 kg); and Light (348 kg). Cow efficiency as a function of biotype was evaluated at two different moments: calving and weaning, based on the ratio of calf weight to cow weight*100 (in kilograms). Calf production efficiency was determined based on the ratio of calf weaning weight (CWW) to cow pregnancy rate (PR), resulting in the calf production index = kg of weaning calves per cow (CWW*PR/100). The experimental design was a completely randomized factorial 3 x 2 x 2 design (three biotypes x two calf sexes x two milk production levels). The results were subjected to ANOVA and F-test. The Light and Medium cow groups produced more (P < 0.05) kilograms of calf per kilogram of cow (16.0 and 15.1 kg, respectively) than the Heavy group (14.0 kg). Light cows showed lower (P < 0.05) milk production and, as a result, lighter (P < 0.05) calves at weaning (151.9 ± 4.3 kg) than Medium and Heavy groups, 166.0 ± 3.7; 166.5 ± 4.0 kg, respectively. At beginning of the breeding season, cows of the Lower milk production group were on average 27.49 kg heavier than cows in the Higher milk production group. There was an interaction (P < 0.05) between total milk production and calf sex on characteristics of performance in calves and performance efficiencies of the system. Light and Medium cows showed 51 and 25 kg total weight gain from calving to weaning, corresponding to 14.7 and 6.4% of body weight, respectively. The Heavy group, however, showed a 3 kg loss (0.5% of body weight) during the same period. Discussion: A higher growth rate was observed in the pre-weaning period of the calves of heavy cows; as a result, these calves were heavier at weaning. To achieve this result, heavier cows may compensate this higher nutritional requirement using body reserves. This biological adjustment may not be economically efficient, since the subsequent reproduction of these cows could be impaired by increases in milk production. Therefore, the target cow in a pasture-raised beef system is one whose low nutritional requirements enable her to produce milk, resulting in heavier calves, and whose physiological conditions enable her to conceive again during the breed season. Light cows can be considered an efficient group, since their pregnancy rate enabled them to create a positive difference for the group, notwithstanding their lower milk production and lighter calves (P < 0.05) at weaning. This tendency is expected, since increased growth rates are associated with a decrease in puberty and early finishing age. It is essential to select animals according to their efficiency in order to reduce the cow’s requirements, since this favors the increased productivity of cows of lower maintenance cost in relation to their body weight, leading to higher biological and economic efficiency in the beef production system. In conclusion, because of their lower nutritional requirements, cows of small and moderate biotypes and lower total milk production are more efficient than cows of larger biotypes and higher milk production in pasture-raised beef systems.