Nutritive Value of Nitrogen‐Fertilized Orchardgrass Pasture at Different Periods of the Year1

Abstract A study was made of the nutritive quality of orchardgrass fertilized at four nitrogen levels (0, 56, 168, and 504 kg/ha) and either free‐grazed or zero‐grazed by sheep at four periods of the year (May, June, September, March). Effects of fertilization and growth period on nitrogen, fiber, and mineral components of the herbage were noted. In trials with cut herbage fed ad libitum to sheep indoors, level of fertilization had a significant effect on protein digestibility but little effect on dry matter and cellulose digestibility or on intake. Consumption levels of the harvested grass were higher in June than in May or September, and there was a marked decline in both the digestibility and intake of frost‐killed herbage in March. A hay cutting made from first growth grass at the end of the grazing trials in May showed decreases of approximately 15% in digestible dry matter and 40 to 45% in intake when compared to cut herbage. Regressions calculated between dry matter digestibility and percentage of fecal nitrogen in the indoor trials were found to have a seasonal bias. The differences between digestibility coefficients as measured directly with cut herbage and as estimated by regression for grazing animals were most marked in the May and March trials, with approximate increases of 8 to 15 percentage units due to selection. Differences with regrowth herbage in June and in September were minor. The estimated consumption of orchardgrass by grazing sheep tended to decline from period to period, and there were significant effects of level of nitrogen fertilization on intake. An increase in intake associated with nitrogen fertilization was most evident in the May and March growth periods. Nitrogen fertilization also improved the relative acceptance of herbage as measured in palatability experiments with grazing sheep. A comparison of the intake of digestible dry matter by sheep under the free‐grazing or the zero‐grazing systems indicated significantly higher values for the grazing animals in the May, September, and March growth periods. No differences were apparent on regrowth herbage in June.