Effect of short-term grazing exclusion on herbage species composition, dry matter productivity, and chemical composition of subtropical grasslands

2.1 Study site

The experiment took place in a riverine grassland of approximately 1,000 ha at Madi (27°50′N, 84°28′E), which is next to Chitwan National Park and is situated on the bank of the Riu River, 25 km south of Bharatpur Municipality in Chitwan, Nepal. The experimental site was 228 m above sea level. For the past 30 years (1989–2018), the average annual temperature was 25°C, and the average annual precipitation was 1,051 mm, with most of the precipitation during the monsoon season (June to August). During the research year, the study site’s average annual temperature was 25.14°C, and there was 1,200 mm of total annual rainfall (Figure 2). The average organic matter content (1.262%), pH (5.2), total nitrogen content by Kjeldahl method (0.051 g kg⁻¹), and available phosphorus content (Bray and Kurz P method, 34.81 mg kg⁻¹) of the soil were all examined. The three main grazing animal species were buffaloes, cattle, and goats, and adjacent residents grazed livestock freely. The silts, sands, and clays deposited by river floods and inundation were used to form the experimental grassland.

Figure 2

Maximum, minimum, and average temperature patterns and monthly precipitation for the study area in Madi Chitwan, Nepal.

2.2 Sampling design and quadrat allocation

In the grassland, six line transects, each measuring 100 m, were randomly placed every 150 m apart. To preserve the vegetation impacted by grazing and trampling, three 1 m × 1 m quadrats each were randomly marked within each transect and fenced in May 2020 (GE plots). Additionally, following grazing, the GA plots were placed at random 20 m apart from each fenced plot. Samples of the herbage were obtained at regular intervals from the three successive harvests (15th September to 15th November). There were 108 sampling quadrats employed in total (54 GE and 54 GA, respectively).

2.3 Cover-abundance of the herbages

Prior to cutting or herbage sampling, the dominance of herbage species and botanical coverage were assessed in all sampling plots. The botanical composition i.e. grasses, forbs, legumes, graminoids, and bare soil were recorded over the grassland to determine the herbage dominance [14], and the coverage of each of the species present within the sampling quadrats (1 m × 1 m) was recorded following the Braun-Blanquet cover-abundance method [15].

2.4 Herbage sampling

The herbage samples were taken from the GA and GE plots using a random sampling procedure. Each sampling unit had three consecutive harvestings at monthly intervals. To simulate the overall removal of foliage by grazing livestock, foliage within a quadrat of 1 m × 1 m was cut 5 cm above the ground. At the same time, 18 additional samples were collected from GA plots, where livestock species were allowed to graze freely during the day. The dead matter (litter) was avoided for reducing the bulkiness of the herbage samples during transport. Before the lab analysis began, the gathered samples were packaged and labelled for respective treatments and harvest dates.

The tiller number per square meter in grasses was determined by counting every tiller per plant from each quadrat [16], recorded for each harvest. After oven-drying (60°C for 48 h) samples in the Animal Nutrition Laboratory of the Agriculture and Forestry University, Nepal, the dried samples were then manually separated into leaf and stem portions and weighed to determine the LSR [17].

2.5 Laboratory analysis

The laboratory analysis was carried out in accordance with the recommendations for determining proximate composition [18]. In the Animal Nutrition Laboratory of the Nepal Agriculture Research Council, Lalitpur, Nepal, total ash (TA), ether extract (EE), and crude protein (CP), as well as fibre content and mineral composition (Ca and P), were analysed.

Following the established procedure, the neutral detergent fibre (NDF), acid detergent fibre (ADF), and acid detergent lignin (ADL) were measured [19]. Additionally, components such as the non-structural carbohydrate (NSC) = 100 – (CP + NDF + EE + Ash), hemicellulose (HCEL) = NDF-ADF, cellulose (CEL) = ADF-ADL, and total solubles (TS = 100-NDF) were estimated [20]. Later, the percentage of body weight of cattle-based DM digestibility% (DMD% = 88.9 – 0.779 × ADF) and DM intake% (DMI = 120/NDF) were also computed [21].

The atomic absorption spectrophotometer’s (Chromtech^®, USA) accepted procedure was used to analyse the herbage’s calcium and phosphorus content.

2.6 Statistical analysis

R Statistics (version 4.2.3) was used to analyse the data.

A 2 × 3 factorial analysis of variance model was used to analyse the effects of two treatments (GA and GE) and time of harvest (harvested at three different dates) on the parameters, which is given as follows:

where µ is the constant factor, σ _i is the effect of ith level of treatment, β _j is the effect of jth level of time of harvest, ρβ is the interaction effect of treatments and time of harvests, and ∈ _ijk is the random error.

Although results and figures are given with untransformed values, all variables were subjected to normality tests for parameters except those presented in Table 1. Duncan’s multiple range test was used to compare the mean differences set at a 5% level of significance.

3.1 Abundant herbage species

The top six most dominant herbage species in the experiment are shown in detail in Table 1. At all the treatments and time of harvest, Saccharum spontaneum remained the most dominating species (Table 1). However, during the subsequent harvests in the GA plots, the grass species were found to be in a decreasing trend (Table 1).

The dominance of S. spontaneum as compared to other species would be due to its tall stature and invasiveness and being with more regeneration capacity than the other species when they are mixed in grassland and, thus, it may dominate over other small understorey grasses [22] being an aggressive and allelopathic species in nature [23]. It has been reported that the S. spontaneum has a faster adaptation habit in unfertile, degraded, and poorly drained soil conditions too [24,25].

3.2 Herbage mass productivity and herbage composition

The DM and tiller productivity, LSR, the calculated intake, digestibility and feed quality (relative feed quality), and the botanical groups were significantly affected by the treatments and time of harvests (Table 2), respectively, and their interaction (T × H).

The GE plots always had the highest DM and tiller productivity across all harvests. However, the calculated intake and herbage digestibility were found to be highest at the GA plots in the third harvest, respectively. Repeated reports relating to GE experiments [26,27] had concluded that the higher herbage productivity in GE grasslands is often associated with the lower defoliation rate of herbage that allows higher biomass allocation. When grazing is allowed, the palatable species are often heavily defoliated by the grazing livestock or it might also be due to less or no grazing pressure as the mechanism for increased biomass after GE [28,29]. Another mechanism further suggests that GE allows the continuous increment in plant height and coverage of abundant species leading to improvement in the moisture-holding capacity of the soil and thus in return promotes plant growth [28,29,30]. The prevention of soil compaction due to trampling could also allow better growth and development of roots that in turn would allow for higher biomass productivity [30,31]. In the present study, the DM productivity was found to be highest in the first harvest than in the third harvest, which might be caused by the lower regrowth at the advancing stage of maturity [32]. It has been pointed out that the more frequent and severe the clipping, the more DM is depressed [33]. Reduction of DM yields by increased frequency and intensity of harvesting [34,35] had shown that frequent defoliation leads to a greater LSR, which was not shown in the grazed plots in the present study. The present findings are similar to the findings where LSR was also found to be higher in the un-grazed pasture land and lower yield from grazed land [17]. The higher leaf proportions in the stem might divulge due to the selection of the leaf portion of herbage than the stem by the animal, which ultimately lowers the leaf portion of the plant at the end. But, consecutive harvesting of herbage imparts a higher LSR as can be seen in third harvest compared to the first harvest of the current study. This might be due to the higher regrowth potential of the leaf than the stem [32,17]. The higher tiller number in the GE plots might be associated with the less demolition of tillers due to grazing and trampling. Correspondingly, the tiller number per square meter was found to be higher in the first harvest in contrast to the third harvest which might be due to the higher mortality of tiller in the later age of growth which could be due to other factors such as the insect kill, early winter senescence, and wildlife attack [36]. This might be because herbage had repeated defoliation which reduced the nutrient reserve and decreased the survival of the tiller. Livestock in the GA site of grassland grazed herbage, so tillers were damaged by trampling. If the tiller is repeatedly defoliated, support from physiologically integrated neighbouring tillers is cut off [37].

3.3 DM productivity and herbage botanical groups

The average vegetation coverage was recorded at about 94% in the GE and 92% in the GA site at the first harvest (data not shown in Table 1). The herbage cover was reduced continuously during the second and third harvests and finally recorded 84% in the GE plots and 80% in the GA plots at the third harvest (data not shown in Table 1). The interactive effect of treatments (T) and time of harvest (H) had a significant effect on the botanical composition of the herbages (Table 2). All the plant growth characteristics could not have been mentioned in the present study. For example, A. conyzoides might not be preferred by livestock during grazing due to its toxin content or some of them might have grown later than grasses e.g., C. rotundus might have been abundant at later harvests in the present study.

Under the GE condition, on average, 73.5, 67.5, and 63.0% of grasses were recorded in the first, second, and third harvests, respectively. Under the GA sites, on average, 63.5, 54.5, and 46.0% of grasses were recorded in the first, second, and third harvests, respectively. The coverage of other-forbs was about 16.0, 13.0, and 11.0% at first, second, and third harvests of the GA site and about 11.0, 10.0, and 8.0% at first, second, and third harvests of the GE sites, respectively. On average, legumes were the minor species. The abundance of other-graminoids was increased at the later stage of harvest in both treatments, which would have probably been due to its comparatively less aggressive growth habit than that of grass species. The details of the herbage cover and their abundance are presented in Table 2.

Grazing reduces the vigour capacity and reproduction ability of plant, so the grass coverage was decreased within the GA area [38]. There was a higher density of grasses and broadleaf forbs within the protected area than in open areas [39], and this statement is supportive of the present research findings well. Grazing causes a reduction in the proportion of grass species [40] and a similar trend was observed in the present study. It is usual that the continuous trampling of graziers on grassland decreased the green grass cover at the end of a grazing period [41] due to the cessation of certain herbaceous species growth but increased the content of broadleaf herbs and this statement supported the current study. The other-forbs were higher in the grazed condition as compared to the non-grazed condition because of their grazing tolerant habit [42]. The total other-graminoids in the grazed area were more dominant than in the un-grazed area and the amount increased extremely in the grazed area at the final harvest than in the first harvest. It might be because the other-graminoids are less preferable to livestock than grasses and have more grazing resistance [43]. It would also be likely that the other-graminoids might not tolerate the shade of the grass canopies at earlier stage.

3.4 Fibre composition of herbage in grassland

The overall interactive effect of treatment and harvesting time (T × H) remained significant to NDF, ADF, TS, and NSC content. However, there was no effect of the fixed factors on CEL but only the harvesting time was found significant to HCEL content (Table 3).

The highest content of NDF (about 71.0%), ADF (about 61.0%), and ADL (30.0%) were found on the first date of harvest at the GE sites. The fibre content was found to be higher in the GE plots rather than GA plots and more on the first date of harvest than in subsequent harvests due to the growth of forbs and graminoids. Such trend of the fibre composition had also been reported [44] that explained that the herbage canopies clipped at short intervals reduced the NDF content but those clipped at longer intervals increased the NDF content due to the progression of the growing season which induced more fibre content, and this confirms the findings of the present study. It is likely that herbage nutritive value (NDF, ADF, and ADL) reduces as the vegetative period progressed [45], due to the advancing maturity of the herbage harvested [46].

Higher NSC content of herbage was found at the GA plots at the same harvest date rather than that in the GE sites, and the NSC content was found in increasing trend at the subsequent date of harvests towards maturity (Table 3).

The prevailing climatic condition might have also influenced the cell contents in herbage. For example, rainfall and temperature were found to be higher during the first harvest rather than in subsequent harvests, so that promoted herbage growth and NSC content [47]. It is likely that the NSC content might be higher during the late vegetative phase to mitigate the energy required to produce the inflorescence. The accumulation of NSC in the herbage occurs in general when carbohydrate production from photosynthesis is greater than the amount required for herbage growth and development [48]. For instance, the photosynthesis capacity of the plant is higher during the active growth stage, and re-growth of herbage has more NSC content. The NSC content remained the highest during the third harvest (November), which is the starting phase of the inflorescence, and the NSC content would be higher towards the cool season than the warm season in grass and legumes because the plants do not produce fructans, so use starch as majorly reserve carbohydrate [49]. The NSC content was reduced under a high rainfall duration that promotes grass growth through the utilization of reserve carbohydrates [50].

3.5 Proximate and mineral composition of herbage in grassland

The treatments (T) and time of harvest (H) and their interaction had an effect on the CP, EE, TA, calcium, and phosphorus content of the herbage (Table 4).

The proximate fractions i.e. CP, EE, and TA, and the Ca and P content were influenced by both the treatments (T) and time of harvests. The interactive effects were significant only to the Ca and Mg content. At GA plots, CP and EE remained the highest during third harvest. The CP content remained the lowest at the first harvest and in the GE plots (Table 4). The TA content also remained the highest at the first harvest and the least in the third harvest at both treatments.

The higher content of CP and EE had been found in the GA plots rather than in the GE plots, which might be due to the influence of grazing on the herbage’s nutritive value because of the removal of young and protein-rich tissues and the need of the plant to replace the older senescent parts of herbage [51]. Furthermore, grazing could raise the nitrogen availability in the soil through the deposition of animal excreta and increase the rate of N-mineralization in soil, which in turn would have enhanced the CP content in the herbage [52]. The CP and EE contents were found to be lower at the first harvest date and higher at the third harvest, which might be because the concentration increases in the re-growth of new tissues after defoliation by grazing, which further results in a delayed herbage maturation and tissue lignification [53].

The higher TA concentration at an earlier stage of harvest might be associated with the plant maturity [54]. However, it is not well understood from the present study that the herbage had a higher Ca and P content at the later stage of harvest in the GE plots [55]. However, it could be speculated that the tiny numbers of legumes in the un-grazed plots might have incorporated the mineral content. The higher LSR might have further contributed to higher Ca and P content, which might even be possible at the later stage of growth [56]. It is speculated that the herbage with increasing presence of grass and with the reduced proportions of legumes at later harvest in grazed plots might have lowered phosphorus and calcium concentration [57]. The processes affecting soil properties during floods and inundation, dung deposition, and the decomposition of herbage residues, among others, may well represent the cause of the Ca and P content of the harvested herbage, even though not measured in the present investigation.

However, the present study was confined with a small-scale herbage sampling and reports the first insight of the series of experiments intended in the same grasslands in multiple years. The investigation of other edaphic aspects, such as stocking rate and animal species effects, along with data on other anthropogenic incidences like flooding, dung deposition, and burning, would provide long-term support the findings of the experiment.

The majority of the parameters in the current study (Tables 2–4) showed a significant interactive effect of the treatments (T) and time of harvest (H), indicating that the fixed factors and their inter-relationships in the subtropical grasslands could have an impact on the productivity and quality components of the herbage. There are a number of additional aspects to be taken into account, for example, the plant’s growth and development stage (plant phenological stage), soil properties, the external environment (such as temperature and precipitation), the abundance of the herbage species, and so forth, in order to decide the suitable time for harvesting or grazing by defoliation.