Irrigation levels affects biomass yields and morphometric characteristics of range grasses in arid rangelands of Kenya

Moisture regimes and soil moisture changes within the experiment

Above ground dry mater (AGDM) production

At the 30 % FC moisture profile, CG was the most prolific (>9.0 t ha⁻¹) followed by SB (7.2 t ha⁻¹). At the 14 weeks and 80 % FC, CG and SB were still the most prolific (>9.0 and 12.0 t ha⁻¹) compared to CC and CR which yielded slightly about 2.0 t ha⁻¹. Around 50 % FC moisture, EM’s performance dropped significantly to 6.7 t ha⁻¹ compared to CC and CG (>8.0 and 9.0 t ha⁻¹). Within 30 % FC moisture regime, CG and SB species were the most prolific (9.4 and 7.1 t ha⁻¹). At about 16 weeks old (maturity) and 80 % FC, SB and CG exhibited higher performance (>10.0 and 13.0 t ha⁻¹, respectively) than the rest. CC was the poorest yielding about 2.5 t ha⁻¹. Within 50 % FC moisture level, EM, CC, CG and SB out-performed the other two (9.0–12.0 vs 1.5–2.5 t ha⁻¹). Under 30 % FC moisture treatment, CG performed much better than all the other species (>10.0 vs <7.0 t ha⁻¹). Under the rainfed conditions CR, CG and SB were consistently the most productive species although they produced <10 % what any of the irrigation treatments produced. For the mixed species treatments, the five-species treatment combination performed consistently higher (p ≤ 0.05) than the two-, three- or four- species combinations. However, there were unexplained variations among the two and three mixed species in terms of yields declining or increasing at higher soil moisture levels. This could have been a result of complex inter-specific completion responses, which was not keenly studied in this study, and may need to be further evaluated with this respect. Wetzel and Van Der Valk (1998) also reported plant response to environmental factors, intra-specific competition, nutrient levels and inter-specific competition to be important factors affecting productivity. But notably also, the study reveals some species like C. ciliaris and C. gayana were performing poorer under high (80 % FC) moisture content.

The high biomass yields of E. macrostachyus and S. sudanense could be attributed to the faster germination rates observed in this study that gave them a head start in accumulating biomass at early growth stages (Koech et al. 2014). The faster germination rate of seeds allows the plant to have a competitive advantage over the slow germinating plants due to the enhanced competitive advantage for resources (Kadmon and Schimida 1990). The differences in biomass production for the species at the different growth stages could be attributed to the morphological differences of the grasses. S. sudanense tent to be stemmier and was able to maintain higher vegetative productivity, which accounts for the higher AGDM yields recorded at all the three soil moisture content. The same species had higher tiller numbers and tiller height compared to the other species which also explains the outperformance of the species amongst the rest. S. sudanense had deeper roots than all the species and this may have allowed the plant to utilize deeper soil moisture even under lower soil moisture content as reported in the same study under a different objective (Koech et al. 2014). S. sudanense has also been observed to have higher WUE, higher production capacity per unit area and tolerance to drought and high temperatures (Uzun et al. 2009).

The higher AGDM yields of >10 t ha⁻¹ observed in S. sudanense at all the three soil moisture content concur with those of Ferat et al. (2009), who reported the lowest yields for S. sudanense variety named Chopper to be 11.68 t ha⁻¹ under irrigation.

The lower AGDM production exhibited by C. ciliaris at 80 % FC can be attributed to the species low tolerance for high soil moisture and water logging conditions (Jacobs et al. 2004). This was observed from the slow percolation of water at this moisture level which took 30–45 min to clear from the soil surface after irrigation. The slow infiltration in the area may be a result of a hard pan below the soil profile, even though this was not tested, but the area was previously cultivated using tractors and irrigated which are known to cause sub-surface hardpans. However, other factors like the high clay content in the study are vertisols could also have contributed to the low infiltration (Yaalon and Kalmar 1978; Waiser et al. 2007). The phenomenal increase in AGDM for C. ciliaris at lower soil moisture content can be attributed to the species drought tolerance and hence the resultant higher AGDM (Heuzé et al. 2013). Akram et al. (2008) identified C. ciliaris as drought tolerant and having ability to accumulate N, P, K⁺ and Ca²⁺ during growth which enhanced its productivity at lower moisture levels. The species has also been reported to produce higher biomass (14.6 t ha⁻¹) under irrigation and high WUE (0.7 kg ha⁻¹ m⁻³) in the United Arab Emirates in sandy loam soils which was similar to this study (Osman et al. 2008). However, the yields were higher than what was realized in the present study, which could be due to the difference irrigation methods used.

The higher yields of C. gayana at the three soil moisture contents compared to C. ciliaris contradict the observations of Asadullah and Ahmed (2010), who reported the latter to perform better than the former in water utilization. However, their study focused on moisture extraction capacities and not the biomass yields. All the same, C. gayana have performed better than C. ciliaris maybe due to the higher germination rates which might have given it a competitive advantage (Koech et al. 2014). The ability of the species to germinate faster allows for quicker and faster penetration of roots to deeper soil profiles and thus increasing the volume of soil from which water is extracted and hence drought tolerance (Bibi et al. 2010). Similar results of higher yield in C. gayana than C. ciliaris and E. superba with fast germination were reported by Bulle et al. (2010) in study done Northern Kenya under rainfed conditions. Ontitism et al. (2000) also reported potential of C. gayana to be higher for livestock feed in dry rangelands. Likewise, the higher AGDM biomass yields depicted by C. gayana of over 10 t ha⁻¹across all the three soil moisture content can be attributed to its adaptability to both low and high soil moisture content similarly to S. sudanense. This gives them an advantage over many range grass species in the semi-arid rangelands and hence greater potential for pasture establishment and reseeding denuded rangelands.

The extremely low productivity of all the six grass species under rainfed treatment (control) was attributed to the low and poor distribution of precipitation received in the study area during the 4 months growing season (297.9 mm with 149.8 mm received within 3 days) (Table 1). The slightly higher yields shown by S. sudanense under rainfed over the other five species is attributed to its faster germination rate, deep rooting which averaged about 43.7 cm compared to the other species that ranged between 16 and 38 cm. This trait may have enabled S. sudanense to utilize water on the deeper subsoil profile which is known to dry at a slower rate during the dry seasons. Snyman and Joubert (1996), also reported S. Sudanense to have deeper rooting system that enhances water absorption.

The high AGDM in the five grass species mixtures than monocultures can be attributed to the functional diversity effect (Berdahl et al. 2001; Thompson 2013). This is basically the contribution of each species to the total biomass yield based on their growth characteristic and morphological traits which play a major role in determining the total yields from the varied adaptability capacities. These findings concur with the study by Mganga et al. (2010a) working with C. ciliaris, E. superba, and E. machrostachyus grown in pure stands and in mixtures. They reported mixture stands to have higher yields compared to pure stands and attributed this to growth characteristics and morphological traits of the individual species under mixtures. Similar observations were also reported by Thompson (2013), working with grass-alfalfa mixtures and orchardgrass and fescue mixtures and observed mixtures to have outperformed alfalfa, orchardgrass, and tall fescue monocultures by 12 %, which was related to functional diversity effects. The fast growing species like E. macrostachyus and C. gayana contributes to faster buildup of biomass at the early growing stages in mixtures. When grass species are grown in mixtures in the natural environment, some species dominate others due to competitive abilities for water and nutrients but that is not certain since climatic and environmental conditions always come into play (Bergh 1968). This study observation was done in a short period and we cannot confidently conclude that five species mixture level has higher yields and therefore there is need for further studies to observe the responses over longer periods. Ideally, it was expected that mixed stands would have more competition and hence lower yields, but the contrary was observed. There is need for further research on what really happens under mixtures in influencing the grass biomass yields.

Effect of soil moisture on the morphometric parameters of the grasses

The observed differences in tiller numbers and heights can be attributed to the morphological growth differences associated with genetic constitution of the grass species. The outperformance of S. sudanense in terms heights could be attributed to the adaptability of the species by faster germination and adaptation to dry conditions (Grismer 2001).

The reduction in height in all the grass species under rainfed conditions was also observed by Jeremiah et al. (2013), working with switch grass and miscanthus species under irrigation and rainfed conditions in California, USA. Their finding on the variation responses of individual species under rainfed conditions and irrigation concurs with the findings in this study where a reduction in shoot growth was noted. They observed that switchgrass under rainfed to have outperformed miscanthus both shoot and root growth. These findings explain the variations in shoot heights amongst the six species under rainfed condition. S. sudanense and E. macrostachyus have vertical growth habits with strong and thicker culms and hence have advantage over the semi-erect species like C. gayana and E. superba. Grass heights have been shown to give grasses as a competitive advantage in resource utilization and survival. Kanak et al. (2013) reported heights to enhance grass performance in mixtures by the ability to shade other plants and reduce competition for resources. This might have been the case under the five species mixture where C. gayana and E. macrostachyus that had greater height was present and contributed more to the total AGDM yields.

The observed increase in tiller lengths and shoot height for all the grass species was expected since the tillers develop and increase in length with plant maturity just like the shoot growth. Contradicting results were observed by Ogillo et al. (2010), who reported no significant difference in tillers among range grass species at all growth stages, this may be attributed to the differences in soil types and soil moisture levels in the two study sites. Site conditions, soil type and rainfall amounts have also been reported to determine the success of establishment and hence growth responses (Opiyo 2007). Tiller numbers are important for grass plants adaptability and survival under grazing pressure since they determine photosynthetic rates and ultimately the food reserves (Laidlaw 2005). The number of tillers and leaves in a grass plant determines the rate of biomass accumulation and the quality of forage (Skinner and Moore 2007). Tillers contain leaves which are easily digestible and more preferred due to less structural carbohydrates, especially the newly developed tillers with young leaves (Wilson et al. 1991). However, a tiller will have both old leaves from previous growth season and young leaves from current season. The old leaves are lower in quality but contribute to the total biomass yields (Soininen et al. 2010), while the young ones improve on the quality. The lower tiller numbers under rainfed treatment can be attributed to acute water deficit during the growing season and hence leaf firing. On the other hand, the increase in tiller numbers for irrigated treatments can be attributed to adequate water supply that enhanced tiller recruitment. These findings are contrary to those of Teague and Dowhower (2002) working on Bothriochloa ischaemum under rainfed and rainfed plus 25 mm/week of supplementary irrigation. They reported soil moisture content had no effects on tiller numbers. This observation could have been from the variation in climatic conditions since they were working in a humid climate unlike the semi-arid environment in the present study. The differences in the genetic could also be the reason. However, the findings of Mganga et al. (2010b) concurred with the results of this study where tiller numbers varied with species and growth stages. Water plays an important role in nutrient absorption and translocation by plants as well as maintaining plant temperature through transpiration, and this could have been a challenge under rainfed condition hence affecting the grasses leaf and tiller development.

Genetic differences contribute to differences in tiller and leaf numbers and this could probably be the reason in this study where grass species were not exposed to other factors that are known to influence tillering such as grazing responses by grasses as an adaptation strategy. Sarukhan et al. (1984) pointed out that grass species with higher number of tillers make greater contribution to the next generation of grass species in the sward. S. sudanense, C. ciliaris and C. gayana had higher advantage amongst the six species under the three soil moisture content in terms of tiller production and this could be attributed to their genetic constitution and the favourable environment under irrigation. Other studies have also reported tillers to increase chances of plant survival and productivity (Skerman and Riveros 1990; Laidlaw 2005). Opiyo et al. (2011) working with E. superba, also observed an increase in leaf numbers with growth. Leaf dynamics in grasses are a function of new leaf development and death of leaves which are determined by the environment and genetics of the grasses (Harper 1989; Huibert and Jan 1998). However, there is a close relationship between tiller numbers and leaf recruitment per tiller (Matthew et al. 2000). Plant leaves determine the quality of forage for livestock with higher young and green leaves contributing much to the increase in crude protein (Michel and Helene 2000; Arzani et al. 2001). The leaf:stem ratio becomes a parameter of concern when analyzing for pasture quality, hence grass species with higher proportion of leaves per tiller and less stems tend to be of better quality (Ball et al. 2001; Rad et al. 2013). Therefore the higher leaf numbers in C. gayana and C. ciliaris suggest better quality forage grasses.