EFFECTS OF GRASS-WEED INTERACTIONS ON MORPHO-ECOLOGICAL TRAITS AND WEEDING BENEFITS OF RANGE GRASSES IN SOUTHERN KENYA RANGELANDS

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INTRODUCTION
Livestock production is the predominant economic activity practiced by communities residing in rangelands often free-ranging on open grasslands grazing system.However, due to poor grazing management as evidenced by extinction of vital indigenous grasses such as Chloris roxburghiana (CHLORIS), Cenchrus ciliaris (CECI), Enteropogon macrostachyus (ENMA) and Eragrostis superba (ERASU) among others, the region continues to be degraded rampantly.Range land degradation poses a critical threat to the community livelihoods in the rangelands (Mganga et al., 2015).In addition, degraded grasslands promote the proliferation of unwanted weed species (Ouko et al., 2020).ASALs landscapes of Kenya have been overrun with noxious weeds such as Ipomoea kituiensis Weeds pose a serious problem especially in newly established pastures if weed control is not taken seriously during pasture establishment for reseeding rangelands (Mganga et al., 2010).
The pasture-weed critical period of competition is experienced during the early stages of pasture establishment (About 60 days post emergence).Intense weed control is required, thereafter the weeds that emerge will not have impact on the pastures yields (Mahmoodi et al., 2016).Weeds are fierce competitors, often possessing attributes of high seedling vigor and short life cycles (Badhai et al., 2021).Generally, pasture weeds negatively impact livestock industry through; reducing quality of forage and quantity of yields, depletion of soil nutrients, grazing interference, poisoning of animals, and increased land management costs (O'Connor et al., 2020).They slow pasture production, utilization and longevity (Ghanizadeh et al., 2019).For an effective weed management strategy farmers ought to acquire knowledge on weeds identification, weeds biology, their ecological effects, values and harm to the rangeland pastures (Lemus et al., 2010).This study sought to investigate the competitive interactions of three range grasses; CECI, CHLORIS and ERASU that are being promoted for adoption by Agro-pastoral communities for improved rangelands productivity in Southern Kenya.The grasses are suitable as livestock forage and as rangeland species and have been successfully used to rehabilitate degraded semiarid rangelands in Kenya (Mganga et al., 2015).

Study Site
The study was carried out in the marginal areas of Makueni County.The County lies between Latitude 1º 35' south and Longitude 37º 10' and 38º 30' east.The regions are characterized by low rainfall, less than 500mm annually (Gichuki, 2000), and are prone to drought events.Bimodal rainfall season is experienced, with long rains occurring between March and May and short rains between October and December.The short rains usually are more consistent and accounted for 60% of the annual rainfall, with the long rains contributing only about 37% (Gichuki, 2000).This region is hot and dry with a mean annual temperature of 22.6°C, a mean annual maximum temperature of 28.6°C, and an annual minimum temperature of 16.5°C (CYMMIT, 2013).The county marginal area is low-lying grassland and has a high potential for ranching.These regions have limited and seasonal water sources, and therefore agricultural production is mainly undertaken under rain-fed conditions.

Study description
A randomized complete block design with split-plot arrangement was used for this study.A total of 36 treatments were evaluated.Four (4) weed management regimes and three (3) grass species constituted the treatments with field experiment of three replicates.The weed control regimes applied were: continuous/ frequent weeding (T1), weeding on the 8 th weeks (T2), weeding on 10 th week (T3) and none weeding/ control (T4) while the grass species used were CHLORIS, CECI, and ERASU.The trial was established during the long rain season, at sub plots (3 m x 3 m) with a 1 m boundary.The grass seeds were sown by hand along the furrows at a seeding rate of 5 kg ha -1 recommended for pasture grasses indigenous to semiarid areas of Kenya (Mganga et al., 2021).Data on the morpho ecological performance on plant parameters including data on tiller density, height, and biomass were collected at the bloom stage.Plant morpho-ecological indices have been used in earlier studies as a measure of successful ecological rehabilitation (Scotton, 2019).The ability of grasses to outcompete and suppress weeds is determined by their growth and morphological traits.
The number tillers were counted and height was measured from the crown to the base.The grass height was measured from randomly selected plants in each quadrat using a meter ruler from the grass crown to the tip of the grass spike.A 1 m 2 metal frame quadrat was placed in each plot three times, and the above-ground biomass at the stable height of 5cm clipped and the fresh weights measured using an electronic weighing scale.A sample of the harvested herbage was placed in labeled sample bags then oven-dried for 48 hours at 60 0 C and the dry matter (DM) weights taken.The sample fresh weight-dry matter conversion factor was used to inferentially estimate the DM weight of the fresh biomass harvested in the quadrat.These weights were then extrapolated to production in kilograms per hectare.The morpho-ecological data, One-way statistical analysis (ANOVA) was done to test for significant differences between the treatments.LSD significance difference post hoc test was used to separate significant differences (P < 0.05) between the weeding regimes.All the results indicated arithmetic means of replicates.Pearson correlation analysis was used to examine the relationship between the measured morpho-ecological traits using SPSS software version 22.

Morpho-ecological performance
Continuous weed management and weeding at 8 th week of establishment, respectively show the highest grass cover, grass density (individual plants per square meter), grass height and tiller density.None-weeding management demonstrate the least performances in all the parameters measured.Biomass performances is highest for continuous and weeding at 8 th , 10 th week and none weeded in that order for all the treatments as shown in table 1.
Grass density and cover declined with prolonged noneweeded periods as shown in table 1. Grass Cover (%), Grass Density, Grass Height (cm), Tiller Density, Biomass DM (gm -2 ) tend to increase across the weeding period.The study results (Table 1) show that CECI has the highest grass density, grass cover, tiller density, biomass throughout the weeding regimes as compared to CHLORIS and ERASU.The study results compliment Koech et al., (2016), who found highest plant densities and plant cover in CECI as compared to ERASU and CHLORIS.This can also be associated to CECI having a strong spread root system that enables it to tap nutrients and water from deep soils and outcompete weeds (Heuzé et al., 2016).Grasses with higher tiller density have the capacity for restoration improved resilience after defoliation of the above ground biomass and sustenance of food reserves compared to plants with low tiller density (Mganga et al., 2021).While, Tillers supports establishment and development of seedlings in herbaceous plants.
ERASU has the highest grass height in all the weeding regimes compared to the other species.According to Ghajar et al., (2021).Plants height gives the plants a competitive advantage for light with other plants.Grass height greatly contribute to low soil erosion, maintaining vegetation cover and also contributed to reduced grazing pressure (Mganga et al., 2021).While, CHLORIS has the lowest grass density and grass cover as observed in the study.The results can be attributed to seed dormancy as a survival mechanism displayed by most range grasses.According to Baskin and Baskin (2021), 80% of ASALs angiosperms produce seeds that are in a dormant state.Seed dormancy inhibits germination therefore preventing total failure during unfavorable periods for germination.The results are illustrated in figure 1.

Table 1. Behavior of morpho-ecological characteristics with and without the presence of weeds. Species Weeding Period
Grass Density Tiller Density Grass Height (cm)

Grass weed interaction
Continuous weed management and weeding at 8 th week of establishment, respectively show the lowest weed cover and weed density.None-weeding management demonstrate the least units in all the parameters measured.The study results displayed lowest weed cover in CECI and corresponding highest plant and tiller densities, grass cover and biomass compared to CHLORIS and ERASU plots.This result in line with earlier findings by Marshall et al., (2012), that areas dominated with CECI had fewer weeds compared to areas with other grasses.This suggests that CECI outcompete and suppress weeds.This can be related to toxic allelopathic trait of CECI toward other plants (Friedel et al., 2006).Additionally, according to Heuzé et al., (2016), CECI has deep, tough roots that can go as deep as 2m and its culms are erect reaching up to 2m high.This gives the grass a competitive advantage over weeds and the rest of the other grass species.On average, the most weeds on all weeding regimes were herb, legume and woody weeds respectively (Table 3).The Pearson correlation show a positive intercorrelation between grass cover, grass density, grass height, tiller density and grass biomass (DM) parameters; a positive correlation between weed cover and weed density.However, there is a negative intercorrelation between weed cover and weed density with grass cover, grass density, grass height, tiller density and DM.There is a positive correlation between tiller density and grass density from the study findings (Table 3).The study findings compared well with research of Marshall et al., (2012); Bebawi et al., (2013);de Albuquerque et al., (2019) who found a negative interaction between weeds and performance of grass species.However, the results contrasted with Mganga et al., (2021), who observed that weeds interacted positively with ERASU grass species.
According to his study, the established plots, both grass and weeds competed to grow higher.Consequently, higher tiller densities in CECI as found in this study, suggests the species have quality forage.

CONCLUSIONS
CECI, CHLORIS and ERASU grass species, continuous and periodic weeding at 8 weeks and 10 weeks demonstrated an increase in dry matter biomass respectively.None-weeding had the least pasture performance.There was a positive inter-correlation between grass morpho-ecological parameters (grass cover, grass density, plant height, tiller density and grass biomass) and a negative inter-correlation between weed plant parameters and grass plant parameters.