Determination of grazing time with relationships between grass layer height and biomass change in natural pastures

This research work was conducted in Karahisar village of Tekirdağ city during the years of 2010 and 2011. The basic purpose of this study was to determine the starting dates of grazing in pastures, and the relationships between grass layer height and biomass change in protected, as well as, the grazed areas of natural pastures of the said village. In pastures, 16 different points had been marked in 4 rangelands on tetrad sampling areas for measurements. The measurements were carried out with 10day intervals between the dates of 10 th March to 20 th July. Height range of the grass layer in pastures had been changed from 5.04 to 48.73 cm in grazed pasture, while 6.47 to 56.83 cm in case of protected pasture. Grass layer height was reached to the highest range between 10 and 20 June in grazed pasture, but in 20 th June in protected pasture. A significant relationship had been found between grass layer height and sampling dates of grazed pasture (R 2 = 0.9773) and protected pasture (R 2 = 0.9808). The highest biomass change was observed in grazed pasture (242.39 kg/ha) in 10 th of June, while in protected pasture (275.59 kg/ha) in 20 th of June. The results of this evaluation have been reached to a conclusion that the appropriate grazing would be started between 20 and 30th of April when vegetation sorted to 15 cm in pastures.


INTRODUCTION
Meadow and pasture ecosystems have different potential of appearance and production from each other in different ecological conditions of the earth using solar energy with its own internal mechanisms (Altın et al., 2011a).Pastures play an indispensable role in the continuation of the food chain by meeting the needs of heterotrophic organisms, while performing this function.
Pastures occupy 25% of the world's land and 18.8% of the territory of Turkey (Anonymous, 2012a).Turkey's total livestock assets are equivalent to 14.3 million animal units (AU) (Anonymous, 2013).For healthy and economical livestock breeding, the animals need approximately 65.3 million tons of quality roughage/forage.26.8 million tons of this forage are met from forage crops (Ak, 2013), and also 11.2 million tons from meadows and pastures.After all, Turkey's need for roughage is up to 27.3 million tons (up to 27.3 million tons in Turkey are needed for quality roughage).Best way to cover the gap is to increase the production of pasture by using pasture improving techniques.For a successful pasture improvement, it is required to know the existing vegetation characteristics of pastures.
Pastures comprise several species of different ecological requirements.The development of species in different periods of growing season provides continuity of vegetation forage quality.This is defined as periodicity and it reduces competition among plant species in pastures.However, the grassland vegetation types are in competition constantly in order to maintain growth or regrowth along with vitality (Çetiner et al., 2012).The competition in meadow and pastures occurs mostly in terms of water, nutrients and light.Competition is higher for these factors in the plant communities where dense population of taller plants exist.Species forming stolon, lumpy, rhizomes, shoots and the tangles compete for more areas (Erkovan et al., 2008).If plants show a need to any of these resources simultaneously, their growth, development and product-making capacity may be reduced.In addition, the competitiveness of plants varies depending on the grazing time and intensity (Alhamad and Alrababah, 2008;Çetiner et al., 2012).The high competition in plant communities of tall plants adversely affects the growth of some species (Şahin, 2007).
The vegetation of pastures in Turkey were classified as short (<60 cm), medium (60-120 cm) and tall (> 120 cm) plants by Davis (1965Davis ( -1985) ) (Davis, 1988).Bakır (1987) reported that the grazing maturity of pastures as 7.5 to 10, 15 and 20 cm for short, medium and tall plants; respectively.Terzioğlu and Yalvaç (2004) have found the grass layer height of the vegetation as 9.86 to 10.50 cm for short tall plants; for 13.24 to 15.63 cm the medium tall plants; 17.48 to 19.50 cm for the long tall plants in his recommended period for the beginning of the grazing pasture (10 th of May ).Difante et al. (2009) reported that when the average height of grass layer is 65 cm in Brazil, pre-grazing yield is 7.130 kg ha -1 and after grazing is 3.810 kg ha -1 . The purpose of the study is to reveal the relationship between height of grass layer with biomass increase of the vegetation of protected and grazing pastures in spring green forage period (from 10 March to 20 July), and to determine the appropriate starting time of grazing. .

MATERIALS AND METHODS
This study was conducted in Karahisar village of Tekirdağ city Gür et al. 3311 during the years of 2010 and 2011 on protected and grazing pastures.
Tekirdağ is located in the transition zones between Mediterranean, Central Anatolia continental and the Black Sea climate.The average temperature and total annual precipitation (2010( -2011( ) is 15.4°C and 766.8 mm, respectively. Long-term (1965( -2011) ) average temperature and total annual precipitation is 14.3°C and 589.8 mm, respectively (Figure 1).The average temperature and annual precipitation amounts of all months in the research years has been higher than the long term average (Anonymous, 2012b).
Soil samples were taken from the research areas and then the levels of pH, organic matter, P and K were measured.Organic matter ratio was 6.45, 2.66%, 8.50 ppm and 165.64 ppm on grazed pasture, while 7.72, 1.19%, 3.67 and 60.85 ppm on protected pasture, respectively.It was determined that grazed pasture was found slightly acidic and clayey.On the other hand, the protected pasture was slightly alkaline and loamy.
In this study, four sample parcels of 600 m 2 (30 m × 20 m) area each in grazing and protected pastures have been established.The measurements were taken from four rows in each of four designated sampling areas of the pastures.A total of 14 measurements were carried out using 10-days intervals for monitoring the grass layer height and biomass change between 10 March and 20 July.Grass layer height was measured from soil surface to the top end length of plant as "cm".To determine the biomass change, the aboveground of 50 × 50 cm = 0.25 m 2 area was harvested.Firstly, the fresh samples were dried in the shade, then dried into the oven at 70°C for 24 h until constant weight (Altın et al., 2007) and secondly, the weights were identified as kg /ha.
To assess the reliability of results, they were subjected to statistical analysis by SSPS 15.0 software package.Means were compared by using Duncan multiple comparison test.

Grass layer height
The average height of pasture vegetation consisting of herbaceous species have demonstrated significant changes in the first growing season (March 10 to July 20), where measurements were made.Also, a significant differences (P <0.01) were found between the grass layer heights of grazing and protected pastures.
According to the average data of two years, grass layer height on grazing pasture at the first sampling (March 10) was measured as 5.04 cm.With the progression of the seasons, the plants continued to grow and increase in height was continued until June 20.The grass layer height of pasture on June 20 reached an average of 48.73 cm.Then, with increasing temperature and decreasing precipitation, plant growth were stopped, even depending on drying in the existing plant components, breakage and spillage was occurred.In the last measurement date (20 July), grass layer height was reduced to 41.14 cm (Figure 2).The pasture grass layer height of protected pasture in all sampling dates was more than that of grazing pasture.While, the average height of pasture vegetation was 6.47 cm on March 10, it reached to 56.83 cm on June 20 with the progress of growth.After this date, as well as grazing on pasture, grass layer height began to decrease, and a month later was reduced to 48.69 cm (Figure 3).
In pastures dominated by cool climate species, the maximum plant growth was observed in the spring season (Opitz von Boberfeld, 1994) hence, the grass layer touched the highest point of level in the said season of the same year (Bayraktar, 2012).However, growth of the cool climate plants start with rise of air temperatures above 0°C in the spring season (Serin and Tan, 2001) and this plants draws a curve S-shaped and slanted (Tosun, 1971).The initial slow growth are due to low air temperatures and insufficient photosynthetic area of the plants.Then, both increase in temperature of the air and soil, and preferring the products of photosynthesis instead of reserve nutrients by the plants with increasing in photosynthetic tissues provides acceleration in growth (Altın et al., 2011a).In the last period, with the gradual increase in air temperature and reduction in rainfall the effects of the summer drought is seen and the growth is under pressure.The plants growth rate varies according to type, time and environmental factors (especially temperature and soil moisture) (Koç, 2001) and cool climates forage grasses make their best growth at temperatures around 20°C (Miller, 1984).In contrast, in general, the plants enter stress after 25°C, then their growth slows down and stops (Moser and Hoveland, 1996).So, the growth has slowed in the last period of developments coinciding with the beginning of summer season.Besides this unfavorable weather changes, an important part of cool climate plants begin to bloom towards the end of the spring.
In general, it was observed from this study that intense blooming range was between 10 and 30 May.With the beginning of flowering, vegetative growth was reduced, no new leaves (photosynthetic tissue) were sprouted and the movement of height increase stopped.After this period, the growth of grass layer height being decreased because of spending energy for flower, fruit and seed formation.The reduction in height of grass layer in the last period of this study was because of branches and leaves lose of the plants due to drying of plants entering drought stress and seed maturing.The height reduction after drying in plants causes more broken and spillage of generative branches.In this study, the first slow growth period in both pasture types lasted until April 30.From this date, growth has accelerated, this rapid growth has continued until June 10 on grazing pasture and until June 20 on protected pasture.The fastest growth on pastures has been in the month of May.The drought of summer season in the region under normal circumstances begins from the June.During two years (2010, 2011) of our study, the June precipitation of both years (45.6 and 101.8 mm) was higher than that of long-term average rainfall (36.6 mm) (Figure 1).This situation has both prevented early onset of drought stress and helped to continue of plant growth.Even, the presence of water and rain amount in the pasture increases the annual production and extends grazing season (Erkovan et al., 2008).Grass layer height, according to the years, showed a similar change.However, while the grass layer height of vegetation on grazing pasture in 2010 was significantly lower.The height difference of two years on protected pasture was not significant.As seen in Figures 2 and 3, grass layer height change is consistent with the two-year average.

Biomass change in the vegetation
Biomass, a total weight of green parts of the plant communities per unit area, is one of the most important features to determine the yield and capacity of grazing in pastures (Ünal and Aydoğdu, 2012).The weight of the history the highest above-ground biomass in pastures shows hay yield during growth (Bayraktar, 2012).
According to average data of two-years, biomass change of pasture vegetation was significant in the first growing season covering the spring and early summer seasons (Figure 4).Plant biomass in early March was determined by 852.9 kg ha -1 on grazing pasture.As in parallel with the growth and development of plants, the amount of organic biomass on pasture was continuously   ) was determined as close to the grazing pasture in the beginning of sampling.The biomass increase continued until June 20.However, the next increase after June 10 was not significant, because the biomass which was identified as 2744.6 kg ha -1 on June 10 has been identified as 2755.9kg ha -1 on June 20.From this date until 20 July, the plant biomass has continuously decreased.The biomass increase in the protected pasture has been a bit more after April 30 (Figure 5).
Since temperature and humidity are the most important factors in plants' organic material production, the maximum biomass increase in pastures was occurred in April and May when the temperature and soil moisture is optimum.In the protected pasture there was a significant increase every 10-day measurement range from March 20 until June 10, because plants started over wintering with sufficient reserves of nutrients from previous growing season for many years.This situation occurred in the protected pasture.In this way, plants entering winter shows earlier and strong growth in the spring season.Since the effect of protection on plants in protected pasture, each sampling time in the growing season was observed to create more biomass.So, the highest plant biomass (yield of pasture) on grazing pasture 2423.9 kg ha -1 , has been identified with more than 14% in the protected pasture as 2755.9kg ha -1 .Because temperature and precipitation values affect both of vegetative growth and development, the effects of years on plant biomass were found significant (P <0.05) on both pastures.But, an adverse situation was occurred between two types of pasture.In fact, while average more plant biomass on grazing pasture was more in 2010, the plant biomass produced was higher in protected pasture in 2011 (Figures 3 and 4).Since the reasons of plant biomass change during growth period in pasture is similar to those of the change of grass layer, it is not discussed here separately.Furthermore, the data of this study is higher than hay yield in grazing and protected pastures in the same region in studies conducted by Altın and Tuna (1991) and Tuna (2000) which showed similarities with the detected values of Altın et al. (2007) and Gür and Altın (2011).

Relations between grass layer height and biomass
The relations between grass layer height and biomass of the pastures are discussed over a two-year averages.The relationship between grass layer height and biomass in both pasture types (R 2 = 0.9862 on grazing, R 2 = 0.9762 on protected) as found significant and positive (Figures 6 and 7).Depending on the horizontal and vertical development in plants, height and diameter width were increased and as a result, increase in biomass occurs.It has been demonstrated in a similar manner that there is a linear relationship between biomass and grass layer height (Anderson and Kothmann, 1982;Bayraktar, 2012;Gökbulak, 1997;Koç and Gökkuş, 1994;Nesheim, 1990) too.Cook and Stunbbendieck (1986) reported that the existence of a strong relationship between weight with sprout diameter and length according to regression analysis.As plant height increases, biomass is increasing.As vegetation reaches a height of 10 cm around, the biomass increase is accelerating, a height of 35 to 45 cm around the stands.Based on these bilateral relations, biomass of vegetation can be estimated according to height of pasture grass.Pasture yield can be estimated as 10-12 cm is 1200 to 1350 kg ha -1 year, 15-17 cm is 1600 to 1850 kg kg ha -1 year, 28-30 cm is 2100-2150 kg kg ha -1 year and exceeds of 50 cm is 2450 to Gür et al. 3315 2750 kg kg ha -1 year.With these values, estimated rates of the covered regions of pastures with plants should be higher.The plant covered area ratios of pastures ranged from 79.1 to 82.5%.Sims and Singh (1978) reported that the annual aboveground net primary production of grazing of tall pastures is 28% more production than nongrazing pastures.Loeser et al. (2004) also reported that the grazing rangelands in semi-arid region provides 27 to 31% more production than in non-grazing pastures.

Grazing maturity
Grass layer height (Altın et al., 2011b) and the change in biomass of the plant provides information about grazing maturity.To reach to grazing maturity of plants, they must grow with products of photosynthesis instead of growing with reserving nutrients.Real growth of plants carries out with producing photosynthesis.Thus, the period of accelerated growth of plants in the spring is recognized as maturity grazing (Altın et al., 2011b;Ogden, 1980).
In this study, the height increase of the vegetation on grazing and protected pastures has accelerated on April 30 and on May 10, respectively (Figure 1).On the other hand, biomass increase on grazing and protected pastures is tended to increase more rapidly from April 20 (Figure 2).According to these results, it is appropriate to initiate the grazing of the pastures after April 20 in the pasture where the experiments were conducted.However, the dependence of plant growth to climatic factors and the fluctuations of weather conditions through years affect plant growth.Therefore, the researchers has expressed that the grass layer height would be more accurate handling in determining the maturity grazing.It is recommended to start grazing when average height of vegetation becomes 7.5 to 10.0 cm in pastures where short height species have been diffused (Altın et al., 2011b;Bakır, 1987).
In this study, grass layer height ranged from 15 to 17 cm according to the types of pasture in 30 th of April.In contrast, the vegetation height has been reached to 10 cm on 10 th of April.Here, because the physiological strengthening of plant is more important than the suggested height values in the literature, there are benefits to wait until the middle of April for grazing starting.Related to this subject, Koç and Gökkuş (1995) suggested that bolting together with symptoms of yellowing in the bottom leaves may be indicator for grazing maturity.
In decision-making for maturity grazing, the moisture content of the soil is also important in addition to the current state of the vegetation.In fact, during wet periods of pasture lands, the soils under grazing animal's hooves is compressed, consequently soil volume weight increases, infiltration rates are falling and young plants is broken off with roots (Mikhailova et al., 2000).Damage  caused due to chewing from grazing in early spring, water permeability of pasture land and plant covered area decreases and the species composition is changing (Bakoğlu and Koç, 2002).Thus, Babalık and Sönmez (2009) stated that initiation of grazing without endangering the continuation of the pasture plants life and without causing compaction of pasture soils, grasslands may also begin grazing when important pasture plants in spring could be grazing.It is recommended in other studies conducted in our country that beginning grazing in Erzurum is at the earliest on 20 May (Koç and Gökkuş, 1995) and in early May in Isparta (Babalık and Sönmez, 2009).

Conclusion
The following conclusions are being considered with the help of a survey carried out for two years on grazing as well as protected pastures in Thrace region.
1. Dominant plants of pastures was short.2. The grass layer height and biomass during plant growth was changed significantly.The high grass layer height and plant biomass (herb yield) in both pastures have been achieved between the dates of 10 to 20 June.3. Appropriate time of grazing in pastures will be suggested between the dates of 20 to 30 April when the height of vegetation becomes about 15 cm. 4. Yield should be predicted according to the grass layer height.In pastures where plant cover occupies 80% of the region, 170 kg of forage is produced at the beginning of grazing season when an average of 15 cm of vegetation height has been obtained.

Figure 1 .
Figure 1.Data regarding to the average temperature ( o C) and the total rainfall (mm) of Tekirdağ province

Figure 2 .
Figure 2. Variation occurred in grass layer height of grazing pasture in 2010 and 2011(P<0.01).

Figure 3 .
Figure 3. Variation occurred in grass layer height of protected pasture in 2010 and 2011(P<0.01).
being increased and reached to 2423.9 kg ha -1 on June 10.Then, with continuous reduction of the plant biomass, it has fallen to 1786.0 kg ha -1 on July 20 th .The biomass production in the spring accelerated after April 10.Similar changes were observed in the protected pasture.Here the plant biomass (859.8 kg ha -1

Figure 6 .
Figure 6.The relationship between grass layer height and plant biomass in grazing pasture.

Figure 7 .
Figure 7.The relationship between grass layer height and plant biomass in protected pasture.