Variability of crude protein in crested wheatgrass at defined stages of phenology

Abstract

Variability of crude protein concentration in crested wheatgrass (Agropyron desertorum (Fisch. ex Link) Schult) is an important consideration in the development of grazing programs. Crude protein (CP) concentration in crested wheatgrass was monitored at specific stages of phenology for 5 years. During that time September through August precipitation varied from 68 to 142% of the 37-year mean. Vegetation was clipped once at 10 phenological stages beginning in April. At the last clipping date, in mid-August, regrowth accumulated after prior clippings was collected. Over the 5-year period, CP of vegetation clipped during mid vegetative growth in late April varied 35%, relatively, from 14.7% in 1983 to 9.5% during 1985. Even though 1984 crop year precipitation exceeded 1983 by 84 mm, maximum topgrowth biomass was 449 kg/ha lower in 1984. Also, CP percentage of vegetative growth in April was 1.8% lower in 1984 than in 1983. Regrowth CP was positively correlated (r=0.98) with June precipitation, and with the number of rain events in July (r=0.97). Plants clipped in the boot stage had greater forage CP in August than plants clipped prior to boot stage. However, regrowth biomass was affected by soil water availability and was highly variable. Crude protein in vegetative growth was marginal for growing steers in 1985. Although growing stock are often supplemented in late summer, these data are interpreted to show that spring supplementation may be needed some years.