In our studies with Marandu palisadegrass, a grazing management strategy with continuous stocking where 95% of the light is intercepted by the canopy resulted in forage at a height of 25 cm, a high green leaf proportion, and low amounts of dead material during the growing season23,24. The use of N fertilization4, different stocking rates, and supplementation25,26,27 are crucial for obtaining forage with a high nutritional value, resulting in a high weight gain per animal and area, and a reduction in slaughter age and greenhouse gas emissions. In the present study we did not find any interection N doses with years. Therefore, only the significative effects of N fertilization or variation within year are presented and discussed.
Total and non-fibrous carbohydrates and total digestible nutrients
Total carbohydrate concentrations decreased linearly with increasing N levels (P < 0.01; Table 1). Nitrogen fertilization increases cell content concentrations (soluble fractions) and changed the sugar composition and bonds established between them in the cell wall28. Therefore, the reduction in TC represents the reductions observed in the NDF and ADF fractions (Table 1). The fibrous compounds in forage decrease with increasing N levels because this nutrient stimulate the growth of new tissues29. In the present study, we managed the pasture to reduce stem growth. A sward height of 25 cm stimulates tillering and growth of new tissues24.
NFC varied according to the experimental year (P < 0.01; Table 2). The NFC fraction can be rapidly degraded in the rumen, and is necessary to maintain adequate carbohydrate and protein degradation synchrony, and promote adequate microbial growth17. Variations in climatic conditions, such as precipitation, alter the production of leaves and stems, and, consequently, cause changes in the concentrations of soluble sugars, starches, and pectins22. Our results are in agreeance with those of Santos et al.22, who observed high NFC when the highest precipitation occurred. Another climatic variable related to NFC is the sunlight, which affects the amount of glucose formed during photosynthesis30. Only the year affected NFC (Table 2). The highest NFC occurred in 2016, when the highest precipitation and lowest sunlight hours were recorded (Fig. 1 and Table 2). Greater sunlight increases the photosynthetic rate, which stimulates stem elongation by promoting cell growth. Cloudy and warm days stimulate the growth of new tissues29.
Precipitation, temperature, and sunlight from 2015 to 2019 at the experimental site at the São Paulo State University Jaboticabal, São Paulo, Brazil.
Increasing N fertilization increased the TDN concentration (P < 0.01; Table 1). The TDN fraction is similar to OM in terms of digestibility and is usually found at a concentration of 60% in tropical grasses28. However, it increased on average from 62 to 65% in our study as N fertilization increased from 0 to 270 kg N ha−1. Increases in variables related to forage digestibility in response to N fertilization were also observed by several authors7,21,22. We found that TDN was strongly associated with NFC, NDFpd, and soluble protein (Tables 1 and 2).
Neutral detergent fiber, indigestible detergent fiber, and potentially degradable detergent fiber
The concentration of NDF decreased linearly with increasing N levels (P < 0.001; Table 1). Conversely, there was a cubic relationship between NDF and year (P < 0.05; Table 2). Our results corroborate other studies that found a reduction in NDF with increasing N fertilization4,7,21. Nitrogen fertilization can increase cell content and decrease the cell wall concentration28. However, this effect is climate-dependent, as observed in our study. Lower NDF concentrations occurred in 2016, when higher precipitation was observed (Fig. 1 and Table 2). Water availability in the soil is essential for N uptake by the plant. In the years that ocorred greater precipitation the conponents of cell wall descreased probably due increases in N recovery by the marandu palisade grass.
The iNDF concentration decreased linearly with increasing N fertilization (P < 0.01; Table 1). Few publications have reported iNDF concentrations. We presented this data because fiber is the most common variable used to predict the feed energy concentration, and there is a negative relationship between fiber concentration and available energy31. The reduction in iNDF concentration due to increasing N fertilization suggests that high N fertilization is a potental strategy to increase available energy in tropical grasses under grazing. However, this reduction was observed when the pasture was managed under a light interceptation level of 95%, which limited stem elogantion. Different results may occur in other forage management strategies26.
The N fertilization had a quadratic effect on NDFpd (P < 0.05; Table 2). The reduction of NDFpd with the application of N occurred up to a dose of 180 kg N ha−1. NDF provides energy for ruminal microbial syntheses, but also improves rumen function by adding structural carbohydrates to the ruminant diet32. The stabilization of the degradable fraction of NDF with increasing N fertilization suggests that the structural value, that is, the passage rate of ruminant diet, was maintained.
Acid detergent fiber
Increasing N fertilization had a negative linear effect on ADF concentration (P < 0.01; Table 1). Previous research found that increasing N fertilization had little or no effect on ADF7,29. There was a quadratic relationship between ADF concentration and experimental year (P < 0.05; Table 2). The plant cell wall concentration can vary due to variations in precipitation and temperature. Rainfall and temperature are the major factors that affect plant maturity. The combination of relatively cooler temperatures and the absence of rain well into the growing season can result in a forage reproductive state, which could increase the fiber fractions2 and might explain these changes in ADF concentrations. The highest average ADF concentration (36.15%) was observed during the growing season of 2018 when lower precipitation occurred (Fig. 1).
Total protein
A linear increase in CP concentration was observed with increasing N levels, and a linear relationship between CP and experimental year was observed (P < 0.001; Tables 1 and 2). Increases in CP concentrations of forage due to N fertilization are known. For example, Prine and Burton (1956) observed increases in CP as annual N fertilization levels increased in warm grasses. Similar results have been observed in other studies4,7,8,29. Additionally, we observed a cumulative effect of N fertilization on CP concentration over the duration of the experiment. Although forage CP concentrations can vary with precipitation and temperature28, the responses of CP to N fertilization observed could not be attributed to the climatic variables. The highest and lowest precipitation did not coincided with the experimental years that the highest and lowest CP concentrations occurred (Fig. 1 and Table 2).
The average CP concentration for Marandu palisadegrass is usually less than 10%7,13,33. The minimum value observed in our study was 12% (Tables 1 and 2). Our results are similar to those found previously in our experimental site (11%–14%; Barbero et al25; Koscheck et al.27) and that observed by McRoberts et al.8, which was 13%–15%. Nitrogen concentration, together with the cell wall concentration, is the most important factor in the supply of the required quantity of nutrients26. Our results suggest that five years of grazing at a fixed pasture height that corresponds to the 95% light interception during the growing season can result in high CP concentrations for Marandu palisadegrass, even without N fertilization. Therefore, a high animal performance could be obtained.
Soluble protein: Fraction A and B1
The fraction of soluble protein increased linearly with increasing N levels (P < 0.0001; Table 1) and with experimental year (P < 0.01; Table 2). Previous research8,21,22 has indicated that increased N fertilization levels increase nitrate accumulation in the plant, which is a portion of Fraction A.
The soluble protein fraction varied from 325 to 586 g kg−1 CP throughout the experiment (Table 2). Several researchers have shown that the proportion of leaves and stems, leaf expansion rate, and tillering adapt to new grazing management targets24,34. In our study, the grazing management strategy allowed the growth of a greater proportion of green leaves, which probably explains the increase in soluble protein. High concentrations of Fraction A and B1 are desirable, as this fraction is rapidly degraded in the rumen and can result in greater animal performance35. Animal performance depends on microbial protein production that can be optimized with a greater amount of soluble N since the amount of energy does not limit microbial growth considering de NFC and NDFpd levels (Table 1).
Fractions B2 and B3
We did not find any effect of increasing N fertilization on the protein fractions with moderate and low degradation rates. Conversely, these fractions varied quadratically with experimental year (P < 0.01; Table 2). Fraction B2 was approximately 40% lower during the fourth and fifth experimental years. The highest average value of Fraction B3 was observed in the year with the lowest precipitation (2016; Fig. 1). Fractions B2 and B3 are associated with membranes and extensins that are bound to hemicellulose and are dependent on the temperature. In our study, the temperature remained similar throughout the year, varying from 19 to 31 ºC (Fig. 1).
In contrast to our results, Fraction B2 has previously been found to vary with N fertilization. Rogers et al.20 found that Fraction B2 increased by approximately 35% in bermudagrass, and Johnson et al.21 observed an increase of up to 57% for warm-season grasses in Florida with N fertilization. Theses variations between theses studies and our findings is likely due to the pastures management differences. Berça et al.14 showed that pasture management play essential role in the variation of fiber concentration in marandu palisade grasss However, our results are in line with those of Santos et al.22, who only observed an effect of seasonality, with the highest values Fraction B2 being associated with low rainfall and high NDF.
Protein fraction C: non-degradable nitrogen
Fraction C decreased linearly with N fertilization (P < 0.01; Table 1). This result differs from previous studies. Johnson et al.22 observed that Fraction C depends of the N fertilization dose for bahiagrass and stargrass being lower or higher to the lowest dose. Rogers et al.20 and Santos et al.22 did not find any effect of N fertilization on Fraction C. However, similar to our results, Zhang et al.36 found a decrease in Fraction C by 55.3% in annual ryegrass forage with increasing N fertilization.
Fraction C corresponds to N linked with lignin, tannin-protein complexes, and Maillard products, which are highly resistant to enzymes produced by the microbes in the rumen, being considered unavailable to the animal10,11,17. Increasing N fertilization improved protein availability in Marandu palisadegrass. Growing Marandu palisadegrass under a grazing management strategy with a pasture height corresponding to a light interception of 95% resulted in a low concentration of structural carbohydrates associated with lignin and a low concentration of Fraction C25,27.
The crude protein:organic matter digestibility ratio increased linearly in response to N fertilization (Table 1). According to Poppi and McLennan37, a CP:DOM of 160 g resulted in high efficiency of microbial growth; however, CP:DOM values above 210 g caused high N losses. Maximum N utilization efficiency values in post-weaning beef cattle reared in tropical grass pasture were observed below 200 g of CP/kg DOM, and losses occurred above 200 g of CP/kg DOM38. In the present study values above 160 g of CP/ kg DOM were observed at doses of 180 and 270 kg of N/ha.
Conclusions
Estimations of carbohydrate and protein fractions can increase the nutrient utilization efficiency and determine the type of supplementation needed under each pasture management strategy. We observed an increase in CP and soluble protein with increasing N fertilization, leading to less protein being required in supplements. Therefore, it is necessary to include soluble carbohydrates, starch, and pectin in the diet to maintain protein and carbohydrate degradation synchrony in the rumen and optimize microbial growth. We observed higher CP, soluble protein, and TDN concentrations when 90 kg N ha−1 was applied, suggesting that this dose is the most suitable for Marandu palisadegrass under a continyos stocking and a pasture management strategy with canopy target of 25 cm during the growing season. Future studies should be directed toward understanding undegradable protein supplementation, inclusion of proteins from legumes, and high degradable carbohydrate fractions in tropical diets.
Source: Ecology - nature.com
