Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 7, 2006
Publication Date: July 1, 2006
Citation: Heitschmidt, R.K., Vermeire, L.T. 2006. Can abundant summer precipitation counter losses in herbage production caused by spring drought?. Rangeland Ecology and Management 59:392-399. Interpretive Summary: Why can’t we grow forage in July and August? The reason we seldom grow forage in July and August is because we seldom receive a significant amount of rain during this time. But if it were to rain a substantial amount in July and August, what and how much forage could we grow? To answer this question, we irrigated some study plots in both July and August with 3” of water and learned that: 1) we can grow 400+ lbs of forage per acre in July and August; 2) that most of that forage will be blue grama grass; and 3) that increases the total forage production for a “normal” about 25%. We also know that the probability of receiving 6” of water in this region in July and August is less than 1%. Thus, it is very risky to assume that we are going to grow 400 lbs of forage per acre in July and August. Development of a Proactive Drought Management Tool Previous research at this location has shown that, on average, about 90% of our perennial grass production is completed by July 1, but we can grow considerable forage after July 1 with ample rainfall. So the question becomes, what is the probability that we are going to receive substantial rainfall in Miles City, MT in July and August? The answer to that can be obtained by looking at long-term weather records and calculating probabilities for obtaining varying amounts of precipitation over a 30 day period. This information is available on the internet and it shows that the probability of receiving a total of 2” of precipitation in July is about 20% and in August it is about 17%. Therefore, it is concluded that ranchers can make proactive drought decisions in early July with some certainty because most of their annual forage production will be completed by then.
Technical Abstract: Drought is an inherent trait of most rangelands, and although drought is expected to occur on a fairly regular basis, managers are often ill prepared for drought. This is in part because rangeland managers are eternal optimists when it comes to expected precipitation. But reality necessitates they address 2 fundamental questions relative to drought management. 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 on current year’s herbage production? 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., 2X in July and August) will be limited in the Northern Great Plains because of limited response capacity. Study plots were 5 x 10-m non-weighing lysimeters. Treatments were established using a combination of simulated droughts using an automated rainout shelter, and irrigation. Treatments were: 1) spring drought (i.e., 1 May – 1 July) followed by ambient precipitation thereafter; 2) 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. Study revealed that substantial herbage production can be expected in this region during summer if substantial precipitation is received. This is primarily because of the positive growth response of blue grama (Bouteloua gracilis (H.B.K.) Lag. Ex Griffiths). However, long-term weather data shows the probability of receiving 2X normal precipitation in both July and August (i.e., our irrigation treatments) is <0.01 (1%). Thus, although these rangelands possess the capacity to respond favorably to “dry season” precipitation, the probability of receiving said precipitation insures levels of ecological and economic risk remain high.