Meta-analysis of the effect of pregrazing pasture mass on pasture intake, milk production, and grazing behavior of dairy cows strip-grazing temperate grasslands

Grazing management is a key factor in pasture-based dairy systems, which can be improved given advanced knowledge of the effects of pregrazing pasture mass (PM) on the performance of dairy cows. The aim of this study was to quantify the effects of PM on the pasture intake, milk production, milk composition, and grazing behavior of strip- or rotational-grazing dairy cows, based on a meta-analysis of published research papers. A database was created that included experiments in which the effects of PM on pasture intake and milk production of dairy cows were studied. Papers were selected only if at least 2 PM were compared under similar experimental conditions, particularly the same pasture allowance (SPA). The final database included 15 papers with 27 PM comparisons. For analytical purposes, the database was subdivided into 3 subsets that varied according to the estimation height at which pasture allowance was determined; that is, where PM were compared at the SPA above ground level (SPA₀ subset), above 2 to 3cm (SPA₃ subset), and above 4 to 5cm (SPA₅ subset). Statistical analyses were conducted on the entire database (global analysis) and within each subset using linear model procedures. An interaction between PM and estimation height was found for pasture intake and milk production in the global analysis. On the basis of the predictive equations, pasture intake increased by 1.58kg of dry matter/d per tonne increase in PM when PM were compared at SPA₀, was not affected by PM when PM were compared at SPA₃, and decreased by 0.65kg of dry matter/d per tonne increase in PM when PM were compared at SPA₅. This is consistent with the effect of PM on milk production, which was positive and negative (1.04 and −0.79kg/t of PM, respectively) when PM were compared at SPA₀ and SPA₅, respectively. Grazing time was only slightly affected by PM, irrespective of estimation height, because the effect of PM on pasture intake was mainly dependent on the variation in pasture intake rate. Pasture intake rate increased with increasing PM at SPA₀ but decreased with increasing PM at SPA₅. This meta-analysis clearly demonstrates that the effects of PM on pasture intake, milk production, and behavior of strip-grazing dairy cows depend largely on the height at which the PM and pasture allowance are measured. These results have methodological implications for future grazing research because it can be recommended that PM be compared at similar levels of pasture availability (i.e., at the same pasture allowance above 2 to 3 cm) to avoid possible misinterpretations of results. They also reveal the benefits of improving grazing management and intake prediction through modeling in pasture-based dairy systems.