Mean HBG ET of 5.0 mm d -1 was significantly less that of 6.1 mm d -1 found in KBG during the months of July through Sep during 2000 and 2001. While measured KBG mean ET was at the upper end of the range of those previously reported (3.6-6.3 mm d -1 ), throughout the duration of this study measured KBG ET was well correlated (R 2= 0.954) to that predicted by the Northern Colorado Water Conservancy District for KBG mowed at the same height (6.4 cm) (Fig. 2.1). As evaporative demand increased, the magnitude of difference between HBG and KBG ET increased (Fig. 2.1). For example, regression analysis showed that HBG ET was 85, 82, 81, and 80% that of KBG as predicted demand increased to 4, 6, 8, and 10 mm d -1 , respectively.  Our data is similar to that of Aronson et al. (1987) and Sheffer (1979) who found changes in the relative ranking of cool-season turfgrass ET as evaporative demand increased. Under non-limiting soil moisture conditions turfgrass water use is understood to be a function of climatic conditions and stand density. However, shoot characteristics which decrease shoot surface area and increase canopy resistance have been shown to moderate transpiration and increase drought resistance in turfgrasses (Burt and Christians, 1990; Kim and Beard, 1988). Relative to KBG, HBG exhibited coarser leaf texture and slower shoot growth rate (as we will discuss below) that may have served to moderate transpiration.

Shoot Growth

At each soil depth HBG exhibited significantly greater root length density (RLD,) total root length (TRL) and total root mass (TRM)  than KBG (Table 2.1). Greatest RLD was observed within the 0-20 cm in both grasses, with HBG distributing 82.9% and KBG 90.3% of their TRL at this depth. At the 20-40 cm depth HBG produced 9.8% and KBG 7.3% of their roots. The remaining 7.3% of HBG and 2.4% of KBG TRL were distributed within the 40-60 cm soil level.  Kentucky bluegrass roots were observed to be finer and more pliable than HBG roots, which appeared thicker and coarser. Calculation of the mass to length ratio (TRM:TRL) for each grass supported this observation, with HBG exhibiting a TRM:TRL of 0.0978 mg cm -1 and KBG 0.0523 mg cm -1 .

As in the field, HBG exhibited greater RLD, TRL, and TRM than KBG. Differences were seen, however, in both grasses, in RLD distribution throughout the sand profile when compared to the field. In the greenhouse, HBG root distribution from 0-20 cm was reduced to 65% and KBG to 79.4%, indicating greater distribution of roots deeper in the profile under sandy conditions when compared to the clayey field soil. At 20-40 cm, HBG RLD was increased to 23.3% and KBG to 19%. Compared to field study, greater difference in RLD between two grasses was in the sand based soil profile in the greenhouse study where HBG distributed 11.7 % and KBG 1.6% of their RLD.  Hybrid bluegrass MRE was 14.6 cm longer that of KBG. These results support those of Carrow (1996b) who noted that under field conditions genetic rooting potential is rarely achieved due to physical and chemical stresses. Hybrid bluegrass and KBG TRM: TRL were 0.0859 and 0.0571 mg cm -1 , respectively, which were not significantly different from those under field conditions.  Fine root systems are generally believed to be more efficient investments than coarser root systems. However, as Eissenstat (1992) points out, this ignores other functions of roots that may permit coarse root to be more adaptive under adverse edaphic conditions such as greater longevity.

Depletion of water from the top 20 cm of the soil profile progressed from field capacity to approximately 23% volumetric soil water content over a 25-day period in Study I and a 30-day period in Study II (Fig. 2.2). During Study I KBG maintained leaf water content at approximately 72% for 12 days after irrigation ceased, after which time it began to decline rapidly (Fig. 2.3). Hybrid bluegrass maintained an inherently lower initial leaf water content of approximately 67% for 10 days after irrigation ceased, which

slowly declined to approximately 62% over the next 23 days, after which time it began to decline dramatically.  Similar trends in leaf water content were observed in Study II (Fig. 2.3), however, because SWD of the top 0-20 cm of soil occurred more slowly, KBG was able maintain an approximate leaf water content of 70% for approximately 19 days, until it began declining. Hybrid bluegrass leaf water content began to decline 30 days after irrigation ceased. In both grasses and in both studies, initial decline of leaf water content coincided, approximately, with depletion of available water from the 0-20 cm soil level (28%) as previously determined for this field (Dahlin, 1992).

In Study I KBG began exhibiting leaf wilt on day 12 of dry down and by day 25 showed > 45% leaf firing (Fig 2.4). In contrast, HBG did not begin to exhibit leaf wilt until day 22 and by day 28 showed only 15% leaf firing. In Study II KBG again exhibited leaf wilting earlier than HBG (at 22 vs. 33 days for KBG and HBG, respectively). After 40 days of dry down KBG exhibited > 90% leaf firing while HBG only exhibited 35% leaf firing. During both dry down cycles, HBG gradually exhibited an increasingly grayish appearance as SWD progressed, whereas KBG progressed more quickly to leaf firing.

Our results indicate that HBG exhibits a lower inherent ET rate than KBG, as well as the ability to moderate its ET as evaporative demand increases relative to KBG, thereby reducing water loss. Hybrid bluegrass exhibited lower leaf water content and slower shoot growth than KBG under non-limiting soil moisture conditions, characteristics advantageous to reduce water use. Additionally, HBG possesses a significantly deeper, more extensive root system than KBG, enabling water extraction from a greater volume of depth when surface soil water is depleted. We conclude that these factors combined contribute significantly to the reduced water use and greater drought avoidance observed in HBG relative to KBG in these studies.

Water Usage for a Traditional Colorado Lawn Using Reveille Turf

(Inches per week)

April   May    June    July     Aug     Sept.    Oct      Average

                                                                                                    .19”     .56”     .93”      .93”      .75”     .56”        .37”         .56” 
 

Daily Average for 7 months is 2.03mm per day

This information was extrapolated from  NCWCD turf water study and CSU research done on Reveille.

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