Diurnal effects on nutritive value of alley-cropped orchardgrass herbage

Nonstructural carbohydrates, crude protein, and in vitro dry matter digestibility (IVDMD) are important measures of herbage nutritive value and can vary at seasonal and diurnal time scales in conventional agronomic systems. Our objective was to determine diurnal trends of nutritive value components in orchardgrass (Dactylis glomerata L.) herbage harvested daily on 1 through 15 June 2001 and 2002 from three microsites: unshaded control, loblolly pine (Pinus taeda L.) alleys, and shortleaf pine ((P. echinata Mill.) alleys at three time intervals [0500, 1100, and 1700 h, Central Standard Time (CST)]}. Diurnal responses were defined as a regression response or change in mean (P less than or equal to 0.10) with time. Orchardgrass herbage exhibited temporal change in total nonstructural carbohydrate (TNC), water-soluble carbohydrate (WSC), starch, and IVDMD. At any given sampling time, TNC was greater (P less than or equal to 0.05) in control than in pine microsites. Diurnal response of TNC differed among microsites (P = 0.10), with more rapid TNC change in the control and shortleaf pine than loblolly pine (P < 0.01). The differential response of TNC in pine microsites compared with the control was attributed to an altered amount and temporal distribution of solar irradiance. Diurnal responses of WSC, starch, and IVDMD were unaffected by microsite. Herbage levels of crude protein and IVDMD were greater, but yield and nonstructural carbohydrates were lower in pine microsites compared with the unshaded control. We conclude that diurnal change in herbage nutritive value tends to be buffered against irradiance constraints in alley crop environments.