Influence of delayed milk ejection on mammary gland health and milking performance in dairy cows: A systematic review and meta-analysis

Delayed milk ejection (DME) could compromise udder health and milking performance. The objectives of this systematic review and meta-analysis were to (i) explore the current evidence, assess the risk of bias, and identify the knowledge gaps in published studies that investigated the effect of DME on udder health and milking performance in dairy cows, and (ii) quantify the magnitude of the difference in milk yield per milking session (MY), milking duration (MD), and peak milk flow rate (PFR) between cows with and without DME. We performed the current systematic review and meta-analysis according to PRISMA 2020 statement. We tracked the pertinent peer-reviewed publications from PubMed using the following syntax (delayed milk ejection OR disturbed milk ejection OR bimodal milk flow OR bimodality) AND (bovine OR cattle OR cows), and additional relevant studies from the identified articles' reference lists. We calculated the pooled-mean difference in MY (kg), MD (seconds), and PFR (kg/min) between cows with and without DME using the random-effects models. The selection process identified 15 studies as the final number of studies qualified for review, including six interventional and nine observational studies published between 1980 and 2023 conducted in seven countries, including the USA, Germany, Italy, Switzerland, Netherlands, New Zealand, and Portugal. The results indicated that while the DME had a minimal influence on the udder health, it can reduce MY by 1.55 kg per milking session (95 % CI = −2.10, −1.00). The observed evidence here suggests that DME can negatively influence the milking profitability.