Cow age, resilience and productivity of cow–calf systems

Context The energy availability for cow–calf systems is related to climate disturbances, and energy use is influenced by the age of the cows. Both factors determine the productivity and resilience of herds after climate disruption. Aims Identify the time needed for the herds composed of cows of a varying age to achieve resilience after energy restriction in the pre- and postpartum. Methods The resilience of systems was compared by deterministic dynamic simulation and herds were considered initially stable but different in (1) energy availability for all herd in the first year of production, namely at 50% (low, L), 75% (medium, M) of 100% (high, H) of the requirements according to the NRC (2000, 2016), with these energy levels being restricted to 60 days before and 60 days after the birth, and (2) the maximum age of the cull cow (lifetime, LT), namely 4 (LT4L, LT4M, LT4H), 6 (LT6L, LT6M, LT6H) of 11 (LT11L, LT11M, LT11H) years. From the second year, availability returned to meet the energy requirements of all animals. The availability of energy for the herd was simulated in natural and cultivated pasture. The productivity was determined as the relation between the kilograms of animals sold and the area used for production over a productive cycle. The system model was considered resilient when 95% of the standard productivity was reached (prior to energy restriction). Key results The time required to return to stability was 2 years in the LT6 and LT11, whereas LT4 required 3 years. Furthermore, LT6 had a higher productivity than did the other herds. The older the culled cow was, the greater was the change in the composition of the product commercialised. Conclusions Cows with an intermediate culling age allowed a sustainable intensification of the system, because they showed better productivity than did older cull cows. Furthermore, after climate disturbance, they presented more resilience than did younger cull cows. The level of energy restriction did not directly influence the time to resilience within each evaluated discard-age group. Implications Herds that culled females before reaching adult age are less resilient after disturbances, such as changes in climate or changes of any nature that interfere with reproductive rates.