Ruminal degradation and crude protein content of native pastures with or without Arachis pintoi, 
in the humid tropics of México

C A Sosa, E Castillo, J Jarillo, L ‘t Mannetje*, A Aluja
 and R A Monsalve**

Centro de Enseñanza, Investigación y Extensión en Ganadería Tropical, FMVZ, UNAM. Apartado Postal 136, Martínez de la Torre, Ver. CP 93600. Tel y fax: (2) 3243941.
clarin@karlinka.net.mx.
*Wageningen Agricultural University, The Netherlands.
**Universidad de Ciencias Ambientales, Santa Fé de Bogotá, Colombia.

 
Abstract

An experiment was conducted to determine the nutritive value of forage collected via oesophageal fistula with two cows that grazed two pastures: native grass (NP) and NP associated with Arachis pintoi CIAT 17434 (NP+Ap), both in the morning (between 7 and 8 AM) and in the afternoon (between 5 and 6 PM). Rumen degradation was expressed as y = a + b(1 - e^-ct), where ‘y’, is the dry matter degraded at time ‘t’, ‘a’ is the highly soluble dry matter when t = 0 (%), ‘b’ is the insoluble but slowly degradable dry matter (%), ‘a+b’ is the extent of degradation (%), ‘c’ is the degradation rate of b (%/h) and ‘t’ is the rumen incubation time (h). 

Neither cow nor time of grazing had a significant effect upon the immediately soluble fraction (‘a’) or the insoluble but rumen degradable fraction (‘b’). However, the effect of pasture was significant upon ‘c’, the degradation rate of ‘b’, being the values 4.07 %/h and 7.66 %/h for NP and NP+Ap, respectively. Crude protein content (CP) was not affected  by cow or time of grazing, but the effect of pasture was highly significant. The cows in NP harvested material with 9.8% CP while those in the NP+Ap selected forage with 13.5% CP in dry matter. The relationship between CP and percent A. pintoi in extrusa was not significant due to a limited number of observations (n=7). The NP and NP+Ap pastures showed 9.12 g of CP/MJ of ME and 11.84 g of CP/MJ of ME, enough to support maintenance, pregnancy and 4 and >6 kg of milk/cow/day, respectively. 

It was concluded that the NP+Ap pasture presented a higher nutritive quality than the NP pasture and, as a consequence, a higher milk production potential.

Keywords: Humid tropics, native pastures, Arachis pintoi, ruminal degradation, crude protein content

Introduction 

The mixture of pasture legumes and grasses has been proposed as an alternative to improve the nutritive quality of the diet of the grazing ruminant in the tropics (Norton and Poppi 1995). If the grass basal diet is limiting in nutrients for the microbial population of the rumen, the inclusion of a legume increases the digestibility and dry matter intake and thus leads to increases in animal production. Also, if the diet is adequate in nutrients and besides there is protein with a low rumen degradability supplied by the legume, dry matter intake may not increase, but animal production can increase due to the presence of by-pass protein. Since cattle consume mostly leaves from the legume when it is mixed with a grass, intake increases because rumen retention time decreases (Poppi and Norton 1995). For these reasons, it is necessary to know the nutritional characteristics of the mixed pasture ingested by the grazing ruminant with the aim of evaluating its animal production potential. 

In general, it is recommended to use oesophageally fistulated animals to obtain representative samples of the ingested forage (McManus 1981), because the grazing ruminant can select material of a very different nutritive quality than that collected by hand, either when clipped (Salman et al 2000) or when grazing is imitated by the "hand plucking" sampling technique (Ibrahim 1994). However, sampling with oesophageally fistulated animals also has its limitations. Short time grazing periods of fistulated non-resident animals could lead to a higher proportion of legume in the diet, in comparison to non-fistulated resident animals (Jones and Lascano 1992). 

The in situ or in sacco technique allows the evaluation of characteristics such as the rate and extent of digestion, which are highly related to the nutritive quality of a forage (Marten 1981; Minson 1981). 

The objective of the present study was to compare the rate and extent of digestion, as well as the crude protein content, of the oesophageal extrusa from native grass pastures and native grass mixed with Arachis pintoi CIAT 17434, in order to observe if the introduction of this legume into native grass pastures was of benefit to the diet of F1 (Holstein x Zebu) cows. 

Materials and Methods

The experiment was conducted at the Centro de Enseñanza, Investigación y Extensión en Ganadería Tropical (Center for Education, Research and Extension in Tropical Livestock Production) of the Faculty of Veterinary Medicine and Zootechnics of the National Autonomous University of Mexico. The Center is located in the coastal plains of the Gulf of Mexico at 23° 04’ N latitude, 97° 03’ W longitude and 105 meters above sea level. There are three climatic seasons: a) ‘dry’ from March to June (30.6 ± 1.7 °C and 19.9 ± 1.7 °C of maximum and minimum temperatures, respectively, and 477 ± 343 mm of rainfall), b) ‘rainy’ from July to October (31.2 ± 1.3 °C and 20.5 ± 1.8 °C of maximum and minimum temperatures, respectively, and 1032 ± 612 mm of rainfall) and, c) ‘winter’ from November to February (25.0 ± 2.1 °C and 15.1 ± 1.3 °C of maximum and minimum temperatures, respectively, and 482 ± 268 mm of rainfall). The soils are classified as Ultisols, acid (pH 4.1-5.2) of low fertility (1-2 ppm of available P), with no Al toxicity problems. Soils are shallow (1-20 cm) and the horizon ‘A’ overlays a hardpan that makes drainage difficult during the rainy season. Native pastures are the main source of feed for grazing ruminants. These pastures consist of a mixture of grasses from the genera Paspalum, Axonopus, Cynodon and Setaria and legumes of the genera Desmodium and Centrosema. The content of native legumes is very low, from 2.5% to 15.4% and, on the average, the legumes comprise only 5% of the native pasture dry matter yield (Bosman et al 1990). In general terms, the year-round carrying capacity of these pastures is about 1 cow/ha (Aluja and Mc Dowell 1984).

The experiment took place between September and November 1999. There were two pastures (P): native pasture (NP) and NP associated with Arachis pintoi CIAT 17434 (NP+Ap), each one with an area of 2.5 ha pasture. Each pasture had 21 sections for the rotational grazing with one day of grazing and 20 days of recovery. The stocking rate was 2 cows/ha.

 Two oesophageally fistulated cows were used for 0.5 h periods from 7 to 8 in the morning (AM) and from 5 to 6 in the afternoon (PM), in order to obtain oesophageal extrusa samples. The NP+Ap mixture was sampled six times on 26 and 29 September and 2, 8, 13 and 23 October, whereas the NP pasture was sampled seven times on 4, 10, 11, 16, 19, 22 and 24 November. Sampling periods were confounded with pasture treatment. However, it was considered that if the legume was indeed ingested, it would lead to higher contents of crude protein and digestible dry matter. Once the extrusa were collected, the liquid portion was eliminated by applying gentle pressure against a cheesecloth and then the sample was frozen (-20 °C) and stored until processed.

The rate and extent of the digestion were estimated by rumen incubation in a fistulated bull with a permanent cannula, using incubation times of 3, 6, 9, 12, 24, 48 and 72 h; 20 g of previously unfrozen extrusa were put into a 10 cm x 20 cm dacron bag; triplicates of each incubation time were used. The “zero time” was obtained by washing the bag with the sample, in water at 39 °C for 30 min. Dry matter degradation was described with the equation proposed by Ørskov and McDonald (1979):

y = a + b(1 - e^-ct),

where ‘y’, is the dry matter degraded at time ‘t’, ‘a’ is the highly soluble dry matter when t = 0 (%), ‘b’ is the insoluble but slowly degradable dry matter (%), ‘a+b’ is the extension of degradation (%), ‘c’ is the degradation rate of b (%/h) y and ‘t’ is the rumen incubation time (h). The parameters of the equations corresponding to each available combination among pasture (P), date (D), sampling time (S) and cow (C) were estimated with a least squares iterative procedure devised by Chen (2000), which is incorporated in the NEWAY software, provided on line by The International Feed Resources Unit (IFRU), Aberdeen, Scotland.

The literature mentions that the use of use of at least three animals do not compromise accuracy when in situ degradability is estimated (Mehrez and  Ørskov 1977). However, studies conducted by our research group indicated a non significant effect of the animal on in situ degradability estimations of dried and ground native legumes (Coutiño et al 2000) or proteinacious meals from vegetal and animal origin (Ramírez et al 1999), when six rumen fistulated bulls grazing African Stargrass (Cynodon nlemfuensis) were used. This suggests that for our conditions one bull is sufficient to estimate the rate and extent of degradation.

A portion of the extrusa was not frozen but was dried at 60°C/72 h, ground in a Wiley mill to pass a 2 mm screen and its crude protein determined in duplicate according to the Kjeldahl procedure (AOAC 1980).

The equation parameters and the crude protein content were analyzed by analysis of variance with a model that included the main effects of P, S, and C, using the variation among days (D) to generate the “experimental error”. Interactions among P, S and C were not included because the treatment design was not balanced; for the same reason, type 3 sums of squares were used. These analyses were performed with the PROC GLM procedure of SAS (SAS 1982).

The crude protein content of the extrusa was related to the contribution of A. pintoi to the botanical composition of the same extrusa, which was estimated according to a technique adapted for microscopic observation (Harker et al 1964), the model was:

CP = b₀ + b₁(CPTAP),

where CP = crude protein in dry matter and CPTAP = contribution of A. pintoi to the botanical composition, b₀ is the intercept or the value of CP when CPTAP=0 and b₁ is the slope or units of increase in CP per unit of increase in CPTAP. The model was fitted with the SAS PROC GLM procedure (SAS 1982).

The metabolizable energy (ME, MJ/kg of DM) of extrusa samples was calculated with the formula:

ME = GE*(D₄₈/100)*0.81,

where ME is the metabolizable energy in MJ/kg DM, GE is the gross energy of dry matter (18.5 MJ/kg DM), D₄₈ is the dry matter digestibility after 48 h of rumen incubation, predicted from the equation of Ørskov and Mc Donald (1979), and 0.81 is a conversion factor from digestible energy (DE) to ME (AFRC 1993). The protein to energy ratio (P/E, g CP/MJ of ME) was calculated from the CP and ME data, and the resultant P/E values were compared to those given by Martin (1998) for different levels of production of cattle grazing tropical grasses.

Results and Discussion  

Only  sampling time had a significant effect upon ‘a’, being 12 percent points higher for the AM sampling time. The effect of cow was close to being significant (P=0.06). Cow “A” had a mean 11.3 percent points higher value than that of cow “B” (Table 1).

The effects of P, S and C were not significant upon ‘b’ and ‘a+b’. The degradation rate ‘c’, was significantly affected  by P; the forage harvested by the cows in pasture NP+Ap was degraded twice as fast as that harvested in pasture NP (Table 1; Figure 1). Neither S nor C had a significant effect on ‘c’.  

The ‘c’ values range from 3 to 6%/h for tropical grasses and are higher in young regrowth. Alayón (1996) found that Stargrass (Cynodon nlemfuensis) hay used in integral rations with foliage of Gliricidia sepium plus molasses and minerals had a degradation rate of 3.6%/h. The ‘c’ value for Guineagrass (Panicum maximum) harvested 90 days after planting was 2.9%/h, while the value for a 33 day regrowth was 4.8%/h (Singh and Gupta 1996). Elephant grass (Pennisetum purpureum) harvested when it reached a height of 1 m, showed ‘c’ values from 3.0%/h to 4.0%/h, depending on whether the supplement used was Gliricidia sepium or Leucaena leucocephala (Abdulrazak et al  1996). There is no published information about degradation rates of tropical native grass pastures of México. The 4.07%/h for NP of this experiment (Table 1), lies within the typical values for young grass regrowth and this was a product of the short recovery period given to the pastures. 

Some legumes, particularly the leaves, show a high ruminal degradation, which is independent of age. Berseem clover (Trifolium alexandrinum) showed a ‘c’ value of 10.8%/h for material harvested 79 days after planting, and for 26 day regrowth it was 10.5%/h, (Singh and Gupta 1996). Respective degradation rates for stems were 10.1 and 8.9%/h. Abdulrazak et al  (1996) found that ‘c’ values of foliage from G. sepium and L. leucocephala fluctuated from 5.2 to 7.6%/h and from 4.0 to 5.2%/h, respectively, when these legumes were given as supplements to steers with a basal diet of Elephant grass. However, if the basal diet was corn stover, the values went from 9.2 to 12.0%/h for G. sepium and from 7.2 to 9.4%/h for L. leucocephala (Abdulrazak et al 1997). Alayón (1996) estimated a degradation rate of 10.7%/h for G. sepium foliage. 

There is no known published information on degradation rates of forage harvested by oesophageally fistulated cows from tropical grass/legume mixtures. Hess et al (1999) found that dried leaves of A. pintoi had a ‘c’ value of 6.02%/h. In the present case, a value of ‘c’ of 7.66%/h for NP+Ap was inferior to that of most legumes mentioned above, but it was almost two times that estimated for NP. This indicated that the addition of A. pintoi to the diet of the cow, increased the rate of degradation of dry matter (Figure 1). 

The effect of P upon CP content was highly significant (P<0.01). The CP of the NP pasture was 3.7 percent points lower than that of the NP+Ap pasture (Figure 2). The effects of S and C were not significant (P>0.05; Figure 2).

Our data indicate that the CP of the oesophageal extrusa from the NP+Ap pasture increased due to the inclusion of A. pintoi in the diet. Nevertheless, both pastures showed CP contents above the critical value of 7% (Milford and Minson 1966). It has been shown that when CP falls below this value, the dry matter intake of stalled animals eating forages only is reduced in response to a N deficit in the rumen (Siebert and Kennedy 1972; Humphreys 1991). 

Even though the difference was not significant, the CP content of extrusa collected in the morning was higher than that from the afternoon sampling, which was probably due to the fact that the cows entered into a new pasture every morning, and for this reason they had a higher dry matter availability which allowed them to choose forage of higher quality. By the afternoon sampling, the cows had already ingested the most nutritious portion of the pasture and therefore, the CP was lower. The reduction in pasture quality with progressed grazing has been reported by other researchers (Chacón and Stobbs 1976; Ibrahim 1994). Therefore, our finding justifies sampling two times during the day (AM and PM) if we expect to collect representative data on forage ingested by grazing cows (Chacón and Stobbs 1977). 

Sampling periods were confounded with pasture treatments and this could have lead to significant differences among treatments. However, standing dry matter (SDM, kg/ha) before grazing was high: 6200 and 7202 kg/ha for NP and NP+Ap, respectively. Thus, grazing pressure was very lenient: 29.3 and 35.1 kg of SDM/100 kg of live weight/day. Respective pasture utilization rates were 11.4% and 13.6% of SDM before grazing (Monsalve 2000). Therefore, it was unlikely that sampling period was a factor responsible for differences in the nutritive quality of ingested forage. 

In this experiment, the relationship between the CP content of the extrusa and the contribution of A. pintoi to the botanical composition of the extrusa was not significant (P>0.05).  The equation: CP = 11.8 +0.077(CPTAP), R² = 0.43, n = 7, predicted that without a legume, the CP content would be 11.8% and that for a unit of increase of the legume in the botanical composition (range from 2% to 56%), the CP would increase by 0.077 percent points (Figure 3). This result was not very different to that of Ibrahim (1994) in spite of the fact that he did find a highly significant relationship  between extrusa CP and percent A. pintoi in the botanical composition (range from 5% to 76%). His equation was: CP = 12.0 + 0.11(CPTAP), R² = 0.89, which indicated that for each unit of increase in A. pintoi, the CP increased by 0.11%. In our case, the lack of significance for the regression was due to the scarce number of observations, but also to a tendency to have high CP values with intermediate A. pintoi contents (Figure 3).

The P/E ratio was 9.12 and 11.8 g of CP/MJ of ME for NP and NP+Ap pastures, respectively. Martin (1998) found that the mean P/E ratios for a group of grasses were: 12.30 and 9.74 for young and mature grasses, respectively. For the rainy and dry seasons the values were 12.0 and 11.5, respectively. According to this author, even mature grasses had enough nutritive quality to support maintenance, pregnancy and a milk production of 4 to 6 kg/cow/day. 

Our data indicate that the potential production for NP pastures is enough to give a milk yield of 4 kg/cow/day, besides maintenance and pregnancy. On the other hand, the potential production for the NP+Ap pasture appears to be well beyond that of 6 kg/cow/day plus maintenance and pregnancy, cited by Martin (1998). Therefore the grass/legume association had a higher estimated potential for milk production than that of the NP pasture. However, it should be considered that the nutrient requirements are covered not only with nutritive quality but with dry matter intake also. Thus, two forages can have the same P/E ratio, but their intakes can be widely different (Martin 1998). For this reason, in this type of study the measurement of dry matter intake is also advisable. 

Conclusions

It was concluded that the native grass/introduced legume association showed a higher nutritive quality than that of the native grass/native legume pasture, and that as a consequence, the former has a higher estimated production potential.

Acknowledgements

Financial support received from the Consejo Nacional de Ciencia y Tecnología (National Council for Science and Technology - CONACYT) through the Sistema de Investigación del Golfo de México (Gulf of Mexico Research System - SIGOLFO) through the research grant 97-01-014-V “Improving a native pasture with the legume Arachis pintoi CIAT 17434” is gratefully acknowledged. The authors also thank the provision of facilities by the Faculty of Veterinary Medicine and Animal Production of the National Autonomous University of México.

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Received 28 June 2001

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