Seasonal changes in DM, CP, NDF, and NDF digestibility of pasture forage in grazing production systems

Abstract Text: Nine grazing dairy farms were utilized in a study to measure monthly changes in forage quality of pastures over a two-year period. Farms were from a wide geographical area across Minnesota representing a range in soil type and annual rainfall amounts as well as herd size, pasture size and pasture management. Pasture forage was sampled every two weeks during the growing season and analyzed for DM, CP, NDF, and NDF digestibility concentrations. Data were analyzed using PROC MIXED of SAS. Independent variables for analyses were the fixed effects of farm (n=9), season (spring, summer or fall), year (1 or 2) and their interactions.  Across the 9 farms, spring pasture DM (23.96%) was higher (P < 0.05) than summer (23.52%) and fall (19.76%) pasture DM.  Average DM for each year was 22.71% and 22.12% for 2004 and 2005, respectively and they were not different. There were (P< 0.05) differences in CP concentrations between farms and across seasons on all farms and averaged 21.52% in year 1 and 21.85% in year 2. Seasonal average CP concentrations were 21.01%, 20.11% and 23.93% for spring, summer, and fall respectively. NDF concentration in the pasture forage was different (P < 0.05) across the 9 farms, as well as different (P < 0.05) for spring, summer, and fall grazing. However, there were no differences within farm and season or across year for NDF; averaging 46.91% in year 1 and 47.53% in year 2. Seasonal NDF concentrations were 46.63%, 49.25%, and 45.97% for spring, summer, and fall, respectively. There was a difference (P < 0.05) across farms for NDF digestibility-30 h and within farm and year. Average NDF digestibility-30 h values for year 1 were 46.33% and for year 2 were 46.55%, and 46.64%, 44.71% and 47.98% for spring, summer, and fall, respectively. In summary, fall pasture growth was higher in CP and NDF digestibility when compared to spring and summer growth across all farms. However, NDF concentrations were highest in summer but CP and NDF digestibility were lowest in summer. There are significant seasonal effects on forage quality.

Keywords: Dairy, grazing, forage quality.