Milkability in dam-calf contact systems.

Dam-calf contact (DCC) rearing is a form of cow-calf contact (CCC) in which dairy cows nurse their calves for some months until weaning while additionally being milked throughout lactation. Growing implementation, consumer demands, and research interest indicate that CCC and DCC are viable systems for the dairy industry. However, machine milking in these systems presents notable challenges with multifactorial drivers. Those are discussed in this review based on current scientific knowledge regarding milking in nursing dams: Machine milk yield (MMY), flow and fat content of the harvested milk are reduced compared to non-nursing controls while protein and lactose levels vary and udder health is unaffected or improved. In systems with full-time DCC, relative MMY reduction can reach 60-70% during the nursing period whereas highly restrictive suckling regimes cause smaller MMY losses but may not meet the calf's nutritional needs. Post-weaning and lactation MMY can be reduced or unaffected. Low udder fills at the start of milking delay milk let-down potentially conflicting with standardized milking procedures and machine settings. A 0.5-1.5% decline in milk fat concentration indicates incomplete milk ejection and incomplete udder emptying which can further reduce MMY. These effects likely reflect a disturbance of the milk ejection reflex, as in nursing dams, suckling is a more potential stimulus for oxytocin (OT) release than milking. Underlying central mechanisms of inhibited OT release at milking in nursing dams may be related to the cow-calf bond affecting the dams' oxytocinergic system but the complexity of required experimental set-ups compromises research in cattle. In bonded dams, multisensory stimulation by the calf may be a stronger trigger for OT release than the milking procedure. Further, separation from the calf during milking and dam-individual variation regarding the ability to release milk in response to machine stimulation may play a role. Zooming out, social scientific studies reveal that DCC farmers see poor milkability as an unsolved challenge. However, it is not limited to nursing dams, as stress affects milk ejection. To quantify incomplete udder emptying, research should refine strip milk fat analysis for on-farm use. Cow-individual milk ejection dynamics must be better understood to assess if breeding for better milk let-down in response to machine milking while maintaining good maternity traits can improve milkability in DCC systems.