Seasonal effects on nutritional value of Oklahoma prairie grass when grazed in extensive or intensive systems

Abstract The objectives of this study were to determine the effects of sampling month and grazing system on tallgrass prairie forage nutritive value. Native tallgrass prairie pastures (n = 6) were assigned to either an extensive (EXT) or intensive (INT) grazing system for 5 consecutive years. Extensive system pasture annual stocking rate was 6.1 hectares per cow-calf pair with cows remaining in their respective pastures year-round. Intensive system pasture annual stocking rate was 3.0 hectares per cow-calf pair with cows present in the pastures 160 days per year [mid-May through mid-July (58 ± 6 d) and mid-September through mid-December (102 ± 4 d)]. Forage samples were collected monthly from two to four locations within each pasture. Samples were dried, separated into subsamples of leaf material and whole plant matter, ground, and analyzed using near-infrared spectroscopy. Data were analyzed using a mixed model (SAS 9.4) with grazing system, month, sample type (leaf or whole), and all two-way interactions as fixed effects and year as a random effect. Forage crude protein (CP) was not different between leaf or whole plant samples (P = 0.97) and increased (P < 0.001) in May (10.8% CP), declined in the fall, and stayed low (2 – 4% CP) in both systems. Neutral detergent fiber (NDF) was greater during May to July (P = 0.040) in the EXT than the INT pastures. Acid detergent fiber (ADF) differed by month (P < 0.001) and was lowest between April and May. In vitro true dry matter digestibility (IVTDMD) was greater in INT (63.7%) than EXT (62.3%) pastures regardless of month (P < 0.001), and both systems were greater during May through August (65 – 75%; P < 0.001) than the winter months (56 – 60%). These data indicate that protein supplementation is likely required outside of May and June, especially for lactating cows.