Interaction of Simulated Drought and Grazing on Rangeland

Paul Nyren and Bob Patton, NDSU Central Grasslands Research Extension Center, Streeter, ND

The growing season of 2003 was the first of a two-year study examining the impacts of both drought and grazing intensity on mixed grass prairie in the Missouri Coteau region of North Dakota. The study is being conducted at the Central Grasslands Research Extension Center (CGREC), approximately 7 miles northwest of Streeter. Study locations are within section 14 (T138N R70W) as a part of a grazing intensity study which began in 1988. The objectives of the present study are to evaluate herbage production, basal cover, and forb frequency and composition on moderate and extreme (very heavy) grazed prairie subjected to simulated drought, average precipitation and naturally occurring precipitation.

The grazing intensity trial, now in its 14^th year, has grazing treatments of light, moderate, heavy and extreme. Grazing treatments are defined by the amount of standing biomass at the end of each growing season, with averages of 65, 50, 35 and 20% remaining for the light, moderate, heavy and extreme treatments, respectively. Automated rainout shelters (Figure 1) were installed to control the amount of precipitation on rainfall treatments within two grazing intensities. In each pasture, rainfall treatments consisted of 75% of the long-term average (drought), 100% of the long-term average (average), and the naturally occurring amount (natural) received during the growing season (May – October). Two replicate shelters, 10' x 20', were randomly assigned to silty range sites for the drought and average rainfall treatments. Each site was replicated three times on both moderate and extreme grazed pastures. To prevent lateral water movement, each rainout shelter boundary was trenched to a depth of 2' lined with plastic and refilled with soil. To ensure that proper moisture was received on drought and average rainfall treatments, biweekly sprinkler irrigation was employed.

Prior to the growing season, portable enclosures were located within each rainfall treatment to prevent livestock grazing. In mid-June basal cover estimates were taken using the ten-point frame technique. A total of 500 points, recording grass species, grass-like, forbs, litter and bare ground, were taken at each site. In late July, herbaceous production was determined by clipping six 1 ft² plots within these enclosures. Forbs and grass-like species were clipped as well as individual grass species. At mid-season, forb frequency was determined using nested frequency quadrats. A total of 20 quadrats were counted in each plot. Biweekly soil moisture measurements were taken using a radioactive soil moisture probe.

As all who have lived in the northern Great Plains know, monthly and annual rainfall is highly variable. In 2003 at the CGREC, over 9 inches of rainfall was received in May and June, then only 2.2 inches fell between July and October for a total of just over 11.5 inches for the growing season (Table 1). The long-term seasonal average for May to October is approximately 14 inches, which we applied to the “average” rainfall treatment. We restricted moisture to 75% on the “drought” treatment, which meant we applied 10.7 inches to these plots.

Table 1. Monthly growing season precipitation received on the research area in 2003.
	May	June	July	August	September	October	Total
Natural	6.17	2.87	0.72	0.45	0.83	0.2	11.24
Average	2.58	3.00	2.96	2.19	1.86	1.4	13.99
Drought	2.25	2.08	2.22	1.64	1.43	1.06	10.68
All precipitation data are reported in inches

Considering herbaceous production, there did not appear to be a correlation between rainfall treatment or grazing intensity and herbage yield (Table 2). The “natural” rainfall treatment produced more grass and total yields than did the other rainfall treatments on both grazing intensities, despite having a seasonal total of rainfall less than the “average” and similar to the “drought” rainfall treatments. Poor timing of rainfall can lead to crop, hay and rangeland forage failures despite a favorable amount of seasonal or annual rainfall. There may be a corollary to this axiom that favorable timing of rainfall can lead to satisfactory herbaceous yields despite less than favorable amounts of seasonal or annual rainfall. In this case, the 9 plus inches of rainfall received in May and June on the “natural” treatment appears to have been received at a favorable time to promote more cool-season grass growth on this treatment compared to the other rainfall treatments. The other two rainfall treatments received only 60% (average) and 50% (drought) of this early growing season moisture. Cool-season grasses comprise the bulk of herbaceous yields from the mixed grass prairie.

Table 2. Herbaceous production 2003
	Grasses and Grass-like Plants	Forbs	Total Production
Natural rainfall
Moderate grazing	2246	426	2672
Heavy grazing	2189	322	2511
Average rainfall
Moderate grazing	1538	411	1949
Heavy grazing	1457	364	1821
Drought
Moderate grazing	1279	502	1781
Heavy grazing	1797	266	2063
All production data are reported in lbs/acre

It is also important to note that significant rainfall was received in the fall of 2002, prior to the start of this study. This moisture, if stored in the soil, could have acted as a soil moisture reserve allowing greater herbaceous yields on all the rainfall treatments in this study than would have otherwise been expected, especially on the “drought” treatment. Soil moisture levels were monitored on the rainfall treatments throughout the growing season and, when analyzed, may help explain these early study results. The rainfall treatments will be continued in 2004 with the vegetation responses again monitored. Following 2004, the recovery of grazed mixed grass prairie following a severe drought will also be evaluated.

In an effort to evaluate the impact of both the level of grazing and the effect of drought, a means of controlling the amount of precipitation on the pastures was needed. For this purpose the Center designed and built small, portable, automated, low cost shelters that could be placed on similar range sites on the normal and extremely heavy grazed pastures. These shelters were designed to cover a 10 x 20 ft plot. When not in use the shelters retract 20 ft from the plot area. Each shelter is equipped with a 12 volt electric gearmotor which moves the shelter when rainfall closes the contacts on the sensor and retracts it when the sensor dries again. The shelters are designed so that normal grazing can occur on the study plots and they can be moved to different pastures at the end of the study.