Chemical composition, energy content, intake and in situ crude protein degradability of the forage consumed by goats in a thorn scrubland in the semiarid region of North Mexico

A S Juárez-Reyes, G Nevarez-Carrasco, M A Cerrillo-Soto

Universidad Juárez del Estado de Durango. Facultad de Medicina Veterinaria y Zootecnia.
Carretera Durango-Mezquital, Km. 11.5. CP 34620. Durango, Dgo. México
ajuarez52@yahoo.com.mx

Abstract

Current protein systems for ruminants describe the availability of the protein consumed by ruminants as metabolizable protein. Those systems consider that the protein consumed by those animals is incorporated into the rumen microorganisms as microbial protein or that it flows to the small intestine without being attacked by the rumen microflora. Some of those systems also consider the precise determination of the fermentative characteristics of the protein, as an important part of the system structure, in order to predict animal performance or to appropriately implement supplementation practices. The objective of this study was to determine the nutritive value and to estimate the protein ruminal degradation parameters of the diet consumed by goats in a thorn scrubland in the semiarid region of North Mexico. Three goats fitted with ruminal and esophageal cannulae (38 ±1.7 kg BW) belonging to a flock of 250 animals were used to obtain diet samples during the 1999 dry season (February to June) and to incubate samples of the forage consumed by the animals. Meanwhile, three intact bucks (38.7 ± 7.6) were used to determine fecal production and from it to estimate OM intake (OMI = OM fecal production/undigestible fraction of the diet). Extrusa samples were analyzed for CP, OM, in vitro organic matter digestibility (IVOMD) using pepsin-cellulase, digestible organic matter (DOM) and metabolizable energy (ME). The extrusa samples collected the previous month were placed in nylon bags (5 g DM) and incubated in the rumen of two of the same goats for 0, 3, 7, 24, 48, 72 and 96 h. The protein degradation characteristics of the forage consumed were described using the equation P = a + b (1 - e ^-ct).

The CP content (% DM), OM (g/kg DM), IVOMD (%), ME (Mcal/kg DM), OMI (g/d) and OMI (g/kg BW ^0.75/d) were 12, 822, 55, 1.7, 927 and 61.3, respectively. The mean values for the protein degradation parameters were: 29.6% for the soluble fraction (a), 35.5% for the insoluble but fermentable fraction (b), 4.0 %/h for the constant rate of degradation (c), 64.9% for the potential degradation (PD) and 48.8% for the effective degradation (ED). The soluble fraction represented more than 60% of the protein effectively degraded in the rumen, which may indicate that the rumen microbial capacity to uptake and utilize the chemical compounds could have been overwhelmed in the initial phase of the protein degradation. The constant rate of degradation and the effective degradability of the protein in the forage consumed by goats would indicate a fairly small amount of degradable protein to sustain the activity of the rumen microbial population during the dry season.

Results from this study suggest that the range goats meet their energy requirements for maintenance and medium activity, although in situ data indicate the need for supplementing a source of slowly degradable protein during the harsh season.

Key words: energy, grazing goats, intake, protein degradability

Introduction

Protein nutrition in ruminants is a dynamic and complex process. The fact of digestion and metabolism of the consumed feeds by ruminants occur in two steps, first by the rumen microorganisms and then by the host animal (Broderick et al 1991), implies that two separate but dependent ecosystems should be considered. Research performed during the last 30 years has developed protein systems that allow an accurate knowledge of the flow of the protein through the gastrointestinal tract and particularly its close relationship with the consumed energy (Burroughs et al 1975; Roffler and Satter 1975; Roy et al 1975; Van Soest et al 1982; NRC 1985; Vérite et al 1987; AFRC 1993). In any case, those systems consider new concepts directed to describe protein availability in terms of metabolizable protein, taking into account that the protein consumed by ruminants takes two routes: a) it is incorporated to the rumen microorganisms, b) it flows to the small intestine as undegraded protein. The knowledge on nitrogen digestion and metabolism have progressed accordingly to the methods that measure those phenomena, and thus, feeding systems have become accurate tools to improve animal performance (Vérite et al 1987). Nevertheless, the new systems require to accurately determine the dynamic aspects of the protein degradation in the rumen (Huntington and Givens 1995). The structure of the new feeding systems is based on the in situ technique proposed by Ørskov and Mc Donald (1979) to perform such determinations and to emphasize the relevance of the degradable and undegraded protein fractions to adequately apply the metabolizable protein systems in ruminants (Klopfenstein et al 2001).

There is scarce information on the protein fermentation characteristics of the protein consumed by grazing goats, such characteristics will permit to know more accurately the quality of the protein consumed by the animals. Therefore, the objective of this study was to determine the nutritive value and to apply the in situ degradation technique to estimate the rumen fermentation parameters of the protein consumed by goats grazing on a thorn scrubland during the dry season in North Mexico.

Material and methods

The study was conducted in Santa Clara, Durango, Mexico, in a vegetative community characterized as a thorn scrubland (COTECOCA 1979). The site has a dry climate with total annual rainfall of 350 mm. Three esophageal (Stevens et al 1985) and ruminal (Hecker 1969) cannulated goats (38±1.7 kg BW) belonging to a herd of 250 animals were used to obtain diet samples. Sampling was performed during two consecutive days each month. The animals were allowed to graze at 9:00 and the extrusa samples were collected at 11:30 and 15:30 for 45 minutes (Holecheck et al 1982). The samples were stored on ice, dried in an air forced oven for 48 h and milled through 1 and 2 mm screen. The samples of three animals were further composited to obtain a representative extrusa sample of each month. Organic matter intake was calculated by: OMI = OM fecal production/[1- diet digestibility] (Handl and Rittenhouse 1975). To estimate diet digestibility, the pepsin-cellulase enzymatic procedure was utilized (Aufrère 1982). Diet CP and OM was determined according to AOAC (1985) and ME according to Kirchgessner and Kellner (1978). About 5 g of DM esophageal extrusa collected the previous month were weighed into nylon bags (Ankom Technology, Inc) with a pore size of 50 mµ. The bags were incubated in the rumen of two goats for 0, 3, 7, 12, 24, 48, 72 and 96 h. These animals were kept grazing as the rest of the flock throughout the study. After incubation the bags were washed in polyethylene bags several times until the rinsing water was clear, approximately 5 litres of water for two bags, and dried at 60°C for 48 h. Protein degradation parameters of the forage consumed were estimated by fitting the in situ data to the exponential equation P = a + b (1 - e ^-ct) according to Ørskov and McDonald, (1979), where a represents the immediately soluble fraction of the protein, b the insoluble but rumen degradable fraction, a + b the potential degradation,and c the rate constant. The estimations were performed as outlined by the Statistical Analysis System Procedure (SAS 1997). The effective degradability (ED)of the protein was calculated considering a rate of passage of 3% h^-1 using: a + b * c/c + 0.03 (AFRC 1993). The data of chemical composition and degradation parameters were analyzed using ANOVA according to a completely randomized block design using PROC MEANS (SAS 1997). Means comparisons were performed using Turkey's test.

Results and discussion

Chemical composition of the diet

Table 1. Chemical composition, energy content and forage intake by grazing goats during the dry season.
Months	CP, % in DM	OM, g/kg DM	OMD^1,%	ME^2,Mcal/kg DM	OMI³,g/d	OMI³, g/kg BW^0.75/d
February	13.2	851^ab	56.6^b	1.80	774^b	49.5^b
March	11.8	813^bc	60.8^a	2.01	773^b	49.5^b
April	12.7	792^c	56.6^b	1.83	917^b	61.5^ab
May	9.1	777^c	56.5^b	1.71	1250^a	82.0^a
June	13.1	873^a	45.0^c	1.24	923^b	64.0^ab
Mean	12.0	822	55.2	1.7	927	61.3
SEM⁴	1.76	11.8	0.99	0.21	77.6	5.2
¹ OMD = Organic matter digestibility ²Metabolizable Energy calculated by: ME = 0.152 (OMD-cellulase) - 1.16/4.184. ³OMI = Organic matter intake ⁴SEM = Standard error of the mean

The CP content (Table 1) ranged from 9 to 13%. The OM value varied between months (P<0.05). The higher value was registered in June (873 g/kg) whereas the lower value was obtained in May (777 g/kg).

Values for OM digestibility (OMD) differed between months (P<0.05). March registered the higher value (60.8%), whereas June registered the lower value (45%). February, April and May registered intermediate values. The OMD value recorded in June may reflect the scarcity of forage in the area of study at this time of the year. The mean content of ME of the diet was 1.7 Mcal/kg DM and was similar among months (P>0.05). The OM intake was different (P<0.05). The higher intake was registered in May (1250 g/d) when twigs from shrubs are available in the pasture, whereas the lower intake was in February (774 g/d) and March (773 g/d) with a mean of 927 g/d. Expressed as g/kg BW ^0.75/d, OMI was also different between months; the higher value was registered in May (82 g/kg BW ^0.75/d) whereas lower values were observed in February and March (49.5 g/kg BW ^0.75/d) with a mean of 61.3 g/kg BW ^0.75/d.

Although differences in chemical composition were obtained, the diet consumed by the animals during the months of the study allows to evaluate the utilization of the natural resources available in the range. The variations registered in chemical composition are affected either by the availability of major vegetative species or by the selectivity of the animals which may consume a variety of forages in the grazing vegetative communities (Bartolome et al 1998). The CP content obtained during the dry season (12%) may be considered adequate, whereas the range (9-13%) is indicative of a wide (45 to 61%) range of OM digestibility (Topps 1992). Papachristou (1997) also reported similar values of CP and OM digestibility in a grazing pasture composed by 56% fodder trees.

The mean OM content of the diet in this study was 822 g/kg MS; similar values (857 g/kg MS) were observed by Lopez-Trujillo and Garcia-Elizondo (1995) in shrublands of North Mexico. The OMD ranged from 45% in June, when forage availability was scarce, to 61% in March when regrowth of shrub foliage was available, this result may have influenced the important variations in ME in the forage consumed by the animals in this study (1.2 to 2.0 Mcal/kg DM). Other researchers (Ramirez et al 1991) have reported a range of OMD from 25 to 43% with a mean value of 34% during the dry season, which is lower than the value registered in this study (55%), whereas similar variations in the digestibility of forage selected by range goats (44 to 60%) were reported by Lopez-Trujillo and Garcia-Elizondo (1995). Differences in OMD between rangelands may be highly influenced not only by climate and the physiological stage of the plant (Masson et al 1991), but also by differences in chemical composition of the forage consumed by goats, particularly compounds such as lignin and tannins which are negatively correlated with the digestibility (Ramirez et al 1991).

The mean ME content was 1.7 Mcal/kg DM while the DM intake during the dry season was 1.5 kg DM, therefore the ME consumed by the animals (2.5 Mcal/d) is higher than the recommended requirement for maintenance plus medium activity for a 38 kg goat (2.3 Mcal d^-1) in semiarid regions with slight hills (NRC 1981). Lower values of ME intake (1.9 Mcal d^-1) have been reported for range Spanish goats in North Mexico (Fierro et al 1989). Differences in ME intake may be explained by forage availability in different ecosystems (Masson et al 1991).

Although organic matter consumption is probably the main limiting factor of range ruminant production, scarce information on intake in grazing goats from North Mexico is available. Ramirez (1999) reported that organic matter consumption (822 g/d) by grazing goats during dry season was sufficient to supply nutrient requirements for 40 kg goats. Thus, the higher value in this study (927 g/d) may indicate an adequate level of consumption. Kearl (1982) indicated OMI maintenance requirements of 66 g kg ^-0.75 for grazing goats, which are similar to the values obtained in this study (61.3 g kg ^-0.75). Higher values in OMI registered in this study during the late dry season may be explained by higher intake of shrub foliage with high content of soluble compounds (Schacht 1992).

Protein degradation parameters

Protein degradation parameters in the forage consumed by grazing goats estimated by the equation: P = a + b (1 - e ^-ct) are presented in Table 2.

Table 2. In situ protein degradability parameters of the forage consumed by grazing goats
	Parameters
Months	a	b	c	a+b	ED
February	27.0^ab	32.8^b	0.049	59.8^ba	45.9^ab
March	27.6^ab	49.2^a	0.026	76.8^a	50.5^a
April	33.6^a	33.9^b	0.037	67.5^ab	52.3^a
May	34.3^a	31.5^b	0.046	65.7^b	53.3^a
June	24.6^c	30.2^b	0.041	54.7^c	41.9^b
Mean	29.4	35.5	0.040	64.9	48.8
SEM	1.82	3.44	0.014	2.64	1.8
a=Immediately soluble fraction b= Insoluble but rumen degradable fraction c= Rate of degradation of b a+b= Potential degradability (a + b) ED= Effective degradability of the protein expressed by: a + b * c/(c + 0.03). SEM= Standard error of the mean

The mean value for the soluble fraction a of the protein was 29.4%, with a range from 24.6% in June to 34.3% in May. The higher value for the insoluble but fermentable fraction b was in March (49.2%) whereas a value of 32% was registered the rest of the dry season. The mean protein degradation rate was 4.0 % h^-1, the lower values were registered in March (2.6% h^-1) and April (3.7 h^-1), and the rest of the season was higher than 4.0% h^-1. The potential degradability (a + b) of the protein ranged from 54.7 to 76.8% with a mean of 64.9%. The higher value was obtained in March (76.8%) and the lower in June (54.7%). The mean value for effective degradation (ED) was 48.8% with a range from 41.9 to 53.3%.

The soluble fraction of the protein, a , includes small water soluble molecules (NPN, free AA and small peptides) which are released after the feeds reach the rumen and are rapidly converted to ammonia-N (NRC 1985); the contribution of these compounds to the non degradable protein in the rumen is negligible (Klopfenstein et al 2001). A criteria to know if the proportion of this fraction is physiologically acceptable, is that it should not be greater than 40% of the protein effectively degraded (AFRC 1993). In this study, this fraction was greater than 60% of the protein affectively degraded, which may indicate that the capacity to uptake these compounds by the rumen microorganisms has been overwhelmed during the first phases of the protein degradation.

The amount of insoluble but rumen degradable feed protein, b, depends on the time it is exposed to rumen microorganisms. This fraction represents the consumed protein that potentially may escape rumen degradation but that is absorbed in the small intestine (NRC 1985) and contributes mostly to the potential degradation (a + b), which corresponded to 54% in this study. The fraction b obtained in this study (35.5%) is lower than the value indicated for green forages (65%) but similar to that of cereal straw treated or non-treated with NaOH (AFRC 1993).

The ED of the protein is an estimate of the total amount of nitrogen captured and utilized by the rumen microbiota for growth and synthesis of microbial protein (AFRC 1993). Protein in most forages is susceptible to rapid degradation in the rumen (Klopfenstein et al 2001) especially green forages (Preston and Leng 1989) in which the protein is degraded up to 73% (Vérite et al 1987). However, in this study, the ED of the protein consumed by goats, which consisted of a variety of forages, was only 48.8%. It has been shown that a major factor that affects nitrogen availability in the diet consumed by grazing goats is the amount of nitrogen bound to ADF, which may constitute up to 50% (Ramirez et al 1991; Gutierrez y Garcia 1998). Moreover, the microbial access to protein seems to play a key role in protein degradation in the rumen (NRC 1985). The latter is particularly true for the protein in the primary wall of the cell plant which is closely related to cellulose and hemicelluloses in the cell wall (Grenet 1997). It is possible that this might be reflected by the constant rate of degradation (c) in this study (4% h^-1), which is low, compared to that of cultivated green forages (12% h^-1) and agricultural by products (6% h^-1) (AFRC 1993).

Conclusions

The results from the present study indicate that the variations in the CP content may reflect the variations in OM digestibility and ME. Nevertheless, the ME consumed by the animals was adequate for maintenance and medium activity for goats grazing semiarid regions. Moreover, the level of OM consumption indicates an adequate supply of nutrients.

Regarding the in situ protein degradability, it can be concluded that the highly degradable protein made up more than 60% of the protein effectively degraded, which may indicate that the nitrogen uptake capacity by the microbes could have been overwhelmed during the early phases of protein digestion, whereas the effective degradability was 48.8%. This suggests the need to supplement a source of slowly degradable protein to the grazing goats during the harsh season.

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