An analysis of Northern Ireland farmers' experiences of using a target-driven beef heifer growth management plan and development of an empirical model leading to the launch of a decision support tool to promote first calving of beef heifers at 24 months

A decision-support tool that automatically selects an age-specific target weight for beef heifers and specifies the daily liveweight gain (DLWG) required to achieve this target weight within 3 months was developed as an aid for farmers to use to promote calving at 24 months of age in the Northern Ireland suckler herd. The ability of Northern Ireland beef producers to follow this target-driven growth management plan and calve heifers at 24 months of age was appraised in a study involving a total of 459 heifers of 10 breeds on six suckler farms. A unique growth curve was generated for each farm, with the aim of achieving 60% and 90% of the estimated mature weight of their cows at 14 months and 24 months respectively. Mature weight was estimated using the 6.7% truncated mean weight of mature cows on each farm. At intervals of 3 months, heifer weight, body condition score (BCS) and diet quality and quantity were assessed on each farm and a target weight for the next visit was specified, along with the DLWG required to achieve that target weight. Typically, heifers were significantly above their target weight prior to 9 months of age (P < 0.05), but farmers were able to manage heifers so as to keep them within ±5% of their individual growth targets after the age of 9 months. Heifer breed and season of birth did not influence adherence to target weight, while individual animal and farm of origin accounted for 51.3% and 24.7%, respectively, of the variation. Adherence to the target growth curve was not influenced by whether or not a heifer produced a live calf, nor by that calf's vitality, but it was associated with the heifer's calving temperament, calving difficulty and mothering ability. Heifers that met target weight between 22 and 24 months of age had significantly heavier calves than those that were below their target weight at this age (P < 0.05). The mean age at first calving was reduced by an average of 3.74 months across the six farms (P < 0.001) as a result of applying the growth management plan. Experiences of the producers have helped to shape the development of an on-line decision support tool and to promote its use. It is anticipated that this support tool will assist producers adopt target-driven growth monitoring for replacement beef heifers, resulting in a higher prevalence of optimal 24-month first calving in Northern Ireland.