Can Abundant Summer Precipitation Counter Losses in Herbage Production Caused by Spring Drought?

Drought is an inherent trait of most rangelands and sound management necessitates managers address two fundamental questions when facing a drought situation. The first question is, ‘‘what is the probability that a useful amount of precipitation will be received over the period of concern?’’ and the second question is, ‘‘if it does rain, what will the impact be in terms of quantity and quality of herbage produced?’’ The objective of this study was to address the second question. Our hypothesis was that herbage growth response to above normal summer precipitation (i.e., 23 in July and August) would be limited in the northern Great Plains because of a general absence of productive warm-season species. Study plots were twelve 5 3 10-m nonweighing lysimeters. Treatments were: 1) simulated (i.e., rainout shelter imposed), severe spring drought (i.e., 1 May – 1 July) followed by ambient precipitation thereafter; 2) simulated, severe spring drought followed by ambient precipitation thereafter plus summer irrigation (i.e., July and August); 3) ambient precipitation only; and 4) ambient precipitation plus summer irrigation. Results indicated substantial herbage production can be expected in this region during summer when precipitation is well above average because of the positive growth response of blue grama (Bouteloua gracilis [H.B.K.] Lag. ex Griffiths), the dominant warm-season grass growing in this region. However, results also showed that level of production in the study situation (i.e., spring drought, wet summer) was only about 50% of that attained in a normal (i.e., wet spring/dry summer) year. Moreover, long-term weather data shows the probability of receiving 23 normal precipitation in both July and August (i.e., our irrigation treatments) is , 1%. Thus, although these rangelands possess the capacity to respond favorably to summer precipitation, the low probability of receiving substantial levels of summer precipitation ensures levels of ecological and economic risk remain high.