Improvement of biosustainability of a goat feeding
system with key supplementation
A.R. Morales
b,c, M.A. Galina
a,b,*, S. Jimenez
b, G.F.W. Haenlein
d aFacultad de Estudios Superiores Cuautitlan, UNAM, Cuautitlan 54000, MexicobPosgrado Interinstitucional en Ciencias Pecuarias, Universidad de Colima, Colima 28000, Mexico cPuma Ranch, Municipio del Marques, Queretaro, Mexico
dDepartment of Animal and Food Sciences, University of Delaware, Delaware, Newark 19717-1303, USA
Accepted 25 May 1999
Abstract
A one-year study was conducted to measure the effects of balanced supplementation to improve biosustainability of a 110 head goat herd in Mexico in terms of economic feasibility on limited pasturing range land complemented with an alfalfa hay forage bank and grazing corn stubble. Average weight of adult females was 52.4105 kg, yearlings 40.03 kg at ®rst kidding. Average total lactation milk production was 45521 kg in 210 days. Feeding strategy included balanced concentrate (BC) from December to May, gradually changing the supplementation when grazing started by offering a multinutritional block (MB) and complex catalytic granulate feed (CCF), which was used at the beginning of range pasture and continued, when goats were fed on corn stubble starting October. The initial BC was composed of corn, barley, wheat bran, soybean oil meal, but the latter was withdrawn from the mixture in May (3 Mcal ME and 12% CP; 10% from May on), offered twice a day. Animals on pasture from May until November were supplied ad libitum with MB prepared from molasses, urea, salt, cottonseed meal, limestone, cement kiln dust, corn stubble, and a mineral mixture, composed of triple superphosphate and a commercial mineral mix for ruminants and salt. Beginning in August, 200 g of CCF was added from a mixture of molasses, urea, salt, limestone, cottonseed meal, rice polishing, corn, poultry litter, commercial mineral salts, ammonium sulfate, cement kiln dust, and animal lard. Late pasture on corn stubble was from October to December. CCF was kept with the diet. Average voluntary feed intake (VFI) was 1.880 kg DM/d with an annual total of 828 kg; of which 248 kg DM was provided by alfalfa hay from a forage bank corresponding to 30% of the total feed intake; 182 kg or 22% of the diet was concentrate (BC, MB, CCF), and 398 kg or 48% was from range land (grasses, shrubs and tree leaves) or corn stubble pasturing. Balancing concentrate diminished the protein intake from 2.10 times requirements to 1.19. Changes in supplementation according to forage availability permitted nutritional optimization of the system. It was possible to improve biosustainability (forages produced on the farm) from 33% before to 48%, while increasing milk production from 400 to 455 kg/year and diminishing production cost from 20 to 17 US cents per liter of milk.#2000 Published by Elsevier Science B.V. All rights reserved.
Keywords:Goats; Range land; Biosustainability; Supplement feeding; Grazing Small Ruminant Research 35 (2000) 97±105
*Corresponding author. Address: AP 22 Colima, Colima 28000, Mexico. Tel.: +52-331-411-33; fax: +52-331-275-81.
E-mail address:[email protected] (M.A. Galina).
0921-4488/00/$ ± see front matter#2000 Published by Elsevier Science B.V. All rights reserved.
1. Introduction
Costly nutritionally unbalanced management of goats on range land supplied with an alfalfa hay forage bank has been discussed (Galina et al., 1998b), which is a common practice for goat milk production in Mexico, but wastes protein by two times the amount of nitrogen fed above that needed. Other studies showed the feasibility of producing goat's milk eco-nomically by feeding agricultural by-products and range land vegetation, which otherwise would be wasted, thus proposing seasonal utilization of range land, when vegetation growth permits, leaving it idle in the dormant season (Galina et al., 1995). Pastoral goat management on Mexican range and shrub land has contributed 33% of the annual DM intake, 28% of energy and 39% of the protein, making the system partially biosustainable (Galina et al., 1998b). Goats select carefully their forage during grazing, balancing nutritional requirements and avoiding toxic materials (Provenza, 1995; Peraza, 1996). Goats prefer shrub land and brush over grasses when compared with other small ruminants. Sheep like grazing better than brows-ing (Wilson et al., 1985; Genin and Pijoan, 1993). However, other studies have demonstrated that when disposability ¯uctuates in shrub land, goats are 50% grazers and 50% browsers (Rios, 1983). Behavioral research has shown that grasses were 92% of the diet in summer and shrub land and forbes were the sources of key nutrients in winter (Coblentz, 1977). In con-trast, Trujillo and GarcõÂa (1995) showed insigni®cant utilization of grasses and cactus by grazing goats. However, recent goat behavior research demonstrated that goats are opportunistic browsers or grazers, depending on the availability and quality of forage (Galina et al., 1998b). Thus milk production was sustained by supplementary feeding, when forage growth was reduced, avoiding over-grazing of the range land, and consistent production of goat cheese by the farmer was maintained. It was concluded that the alfalfa hay forage bank was the key to success. Use of corn stubble can add to the biosustainability of the system and balancing nutritional needs will also diminish the amount of yearly concentrate feeding (Puga, 1998).
Fiber rich, low protein crop residues are the most abundant and appropriate feeds for ruminants in developing countries (Preston, 1995). Recently in
Mexico, studies have been conducted to achieve eco-nomical performance with local forages, using com-plex catalytic supplementation (Galina et al., 1997). Previous work discussed strategies to improve the utilization of those feeds suggesting supplements to correct nutrient imbalances for rumen bacteria (érk-sov, 1994; Fondevila and Dehority, 1995; Krouse and Russel, 1996; Morrison, 1996; Wells and Russell, 1996; Russell and Wilson, 1996; Weimer, 1996), and to increase availability of energy to rumen microbes by offering simple carbohydrates as molasses, which is metabolized rapidly in the rumen when mixed usually with urea (Alvarez et al., 1976; Elliot et al., 1978; ValdeÂs and Delgado, 1990; ValdeÂs and Castillo, 1993; FernaÂndez, 1996). Key work has concluded that the most limiting nutrients for rumen microbes are ammonia, sulfur and phosphorus (Pre-ston, 1995). Strategies to feed rumen bacteria included continuous supply of non-protein nitrogen to keep a high level of ammonia nitrogen in the rumen, and introducing to the diet key amino acids, degradable carbohydrates, and mineral salts (Galina et al., 1998a). Animal growth performance has to be complemented by adding by-pass protein, low degradable carbohy-drates (starch) and long chain fatty acids (Gusnet and Demarne, 1987; INRA, 1988). Multinutritional blocks and complex catalytic supplementation, as de®ned by Preston (1995), have been used to correct nutritional imbalance for rumen bacteria (Galina et al., 1998a).
The objective of this study was to improve biosus-tainability by introducing high energy, low protein, cheap forages such as corn stubble, substituting alfalfa in the dry period with changes in the supplementation according to available forages. Use of key feeds with complex catalytic mixtures in multinutritional blocks or granulate feeds should result in productive inter-actions to allow a balance of nutrients between those needed by the rumen and suf®cient by-pass ingredi-ents for body maintenance and milk production under economically feasible conditions of using shrub land and corn stubble. Traditional balanced diets were employed as controls (INRA, 1988; AFRC, 1998).
2. Materials and methods
The study was done at the Puma Farm in the Cerro Prieto region, Queretaro, Mexico, at 208350
latitude North and 1008180
longitude West. The altitude was 1950 m above sea level with a climate Bs 1 kw (w) (e) described as dry semiarid with isolated rains in winter with a total 460 mm average precipitation per year (GarcõÂa, 1973). The grazing herd consisted of 110 goats of different ages and levels of cross-breeding with French Alpine, Saanen and Toggenburg. Average weight of the adult females was 53.4105 kg, and 40.03 kg of the yearlings after ®rst pregnancy. Pro-duction average was 45521 kg in 210 days lactation. Management of the goats included seasonal grazing on range land from May to November, use of alfalfa hay from December to April, and pasturing corn stubble from October to December.
Feeding strategy included use of the balanced con-centrate (BC) from December to July (300±700 g/d), changes in pasture with employment of multinutri-tional blocks (MBs) and complex catalytic granulate feed (CCF) supplementation (200 g/d), while pastur-ing on shrub land or corn stubble. The concentrate employed had 60% corn, 16% barley, 20% wheat bran, 4% soybean oil meal, which was withdrawn from the mixture in May (3 Mcal ME and 12% CP, 10% from May on), offered twice a day. Animals on pasture were supplied ad libitum MB prepared from 45% molasses, 10% urea, 3% salt, 6% cottonseed meal, 10% lime-stone, 5% cement kiln dust, 20% corn stubble, and 1% mineral mixture, composed of triple superphosphate, commercial mineral mix for ruminants and salt.
Ani-mals using corn stubble in late pasturing of shrub land were provided 200 g/d of a complex catalytic granu-late feed (CCF) composed of a mixture of molasses (14±18%), urea (3±4%), salt (3±4%), limestone (3± 4%), cottonseed meal (20±25%), rice polishing (10± 13%), corn (11±12%), poultry litter (9±10%), com-mercial mineral salts (1.3%), ammonium sulfate (0.3± 0.5%), cement kiln dust (1.5%), and animal lard (10± 15%). After milking and pasturing from 10.00 a.m. to 5.00 p.m. or alfalfa bunk feeding, goats were kept in overnight con®nement. All milk was used for Chevre type cheese production. The BC ration was offered at milking.
The ranch has 40 ha of range vegetation with grasses: Bouteloua curtipendula, Chloris virgata, Bothriochloa saccharoides, Leptochloa saccharoides, Leptochloa dubia, Rhyncheltythurum roseum, Pani-cum obtusum, Bouteloua repens, Aristida adscensio-nis, Setaria parvi¯ora, Urochloa fasciculata; leguminous trees:Prosopis laevigata, Acacia farnesi-ana, Acacia schaffneri, Mimosa biuncifera; shrubs: Celtis pallida, Jatropha dioica, Zalazania augusta, Verbasina serrata, Opuntia spp. During this study the body weight and milk production of the goats was measured monthly, and the supplement and forage consumption daily. Grazing intake was calculated with the method described by Galina et al. (1995). The program was evaluated by comparing the energy and protein supply from feed composition tables or chemical analyses of the forages and concentrate used on the farm with nutrient requirement tables (Mench-aca et al., 1988; Camacho, 1991).
The statistical analysis was multiple regression comparing variables (SAS, 1985). Monthly VFI and nutrient requirements as suggested by INRA (1988) were dependent variables. By regression equation the nutrient density of the feeding treatments and the total requirements of the grazing goats were the determi-nants of probable consumption.
3. Results
Table 1
Monthly relationship of nutrient supply and requirements of Mexican dairy goats fed concentrates, alfalfa hay bank and grazing
Month Dry matter supply (g/day/goat) Total supply per day Total requirements per day
Concentrates
(g/d)g
Alfalfa
hay (g/d)g
Grazing shrub or
corn stubbleg
Energy (Mcal)
Protein (g)
Energy (Mcal)
Protein (g)
January concentratec 650 1100 400 5.25 150 5.45 124
February concentrate 600 1300 0 5.29 140 4.71 127
March concentrate 650 1300 0 5.35 150 4.38 136
April concentrated 700 1300 0 5.65 145 4.17 134
May concentratedblocke 600 1000 900 5.12 140 4.32 128
June concentratedblock 400 900 1100 4.17 160 3.85 124
July concentratedblockgranulatef 400 600 1550 4.29 160 4.00 126
August granulateblock 400 0 1800 4.32 160 4.04 121
September granulate 200 0 2100 4.12 150 3.88 108
October granulate 200 0 2300 4.21 140 3.62 111
November granulate 200 200 2000 4.22 110 4.32 91
December concentrate 1000 700 700 5.02 120 3.79 111
Average 4.750.57 14316 4.130.32 12012
Percentage from total 115a 119b 100 100
Total (kg/year) 18217 24814 39811
Percentage 22 30 48
Total requirements calculated based on INRA (1988).
aPercentage above total requirements of energy.
bPercentage above total requirements of protein.
cConcentrate (BC): corn 60%; barley 16%; wheat bran 20%; soybean oil meal 4%.
dWithout soybean oil meal.
eBlock (MC): molasses 45%; urea 10%; salt 3%; cottonseed meal 6%; limestone 10%; cement kiln dust 5%; corn stubble 20%; minerals 1%.
fGranulate (CCF): molasses 14±18%; urea 3±4%; salt 3±4%; limestone 3±4%; cottonseed meal 20±25%; rice polishing 10±13%; corn 11±12%; poultry litter 9±10%; minerals
1.3%; ammonium sulphate 0.3±0.5%; cement kiln dust 1.5%; animal lard 10±15%.
gMultiplied by the total days of the monthly accumulated totals in kg/year.
A.R.
Moral
es
et
al.
/
Small
Ruminant
Resear
ch
35
(2000)
months of the year (May to January) with grazing grass, brush and tree leaves, followed by pasturing in early autumn on corn stubble (Table 1 and Fig. 1). Key to maintaining milk production level all year was the change in supplementation, when the quality of the range land in the rainy season during the summer increased, and the use of supplements was decreased in the diet. MB and CCF supplementation were offered to stimulate rumen bacterial protein produc-tion.
Average daily VFI by the goats was 1.880 kg, but lower in summer and at the end of lactation, while increasing at the beginning of lactation. Use of CCF increased VFI during range land pasturing. Annual accumulated consumption was 828 kg DM, of which 248 kg DM was from alfalfa hay corresponding to
30% of total feed DM intake; 182 kg or 22% of the diet was supplement (BC, MB, CCF); and 398 kg DM or 48% was from range land or corn stubble during May to January (Table 1). To maintain level of production, 1669 Mcal ME was required, of which 73% were for maintenance of the goats and 27% for production of milk and weight gain. In terms of protein, 42 kg DP was needed annually, of which 45% was for main-tenance and 55% for production. Energy was the principal limitation in the feeding system. The range land contributed 48.0% of the DM, 66.2% of the energy, and 47.4% of the protein (Table 2). The monthly feed intake from all sources is shown in Fig. 1 and Table 1.
Tables 2 and 3 summarize the total data of this biodiverse and biosustainable feeding system. The Fig. 1. Monthly feed supply to goats.
Table 2
Summary of feeding system per year
Concentrate (DM) per goat 182.0 kg
Concentrate per kg of produced milk 0.400 kg
Grazing area 40 ha
Grazing area per goat 0.52 ha
Grazing days 210
Nutrient supply from shrubs and corn stubble per year
Dry matter 48.0%a
Energy 66.2%b
Protein 47.4%c
Total vegetation, shrubs and corn stubble apparently eaten by a goat per year (DM) 398.0 kg
Milk produced per year in average per goat 45521 kg
Concentrate:forage ratio 24:66
Percentage of alfalfa hay bank providing nutrient requirements (DM) 30%
aDry matter provided by shrubs and corn stubble from the total consumed per year.
bEnergy provided by shrubs and corn stubble from the total required per year.
cProtein provided by shrubs and corn stubble from the total required per year.
Table 3
Voluntary feed intake (g/d), balanced concentrate (BC), multinutritional block (MC), and complex catalytic granulate (CCF) feed supply (g/d) (urea offered as protein (g/d) from urea (g/d))
January February March April May June July August September October November December
VFI (g/d) 215056b 180071 195056 200095 250075 240045 255036 220089 230076 2500115 240058 240035
BC (g/d) 65042 60039 65055 70014a 50035a 20021a 90037
MB (g/d) 10012 20016 20011
CCF (g/d) 20017 20016 20021 20032 20017 1009
Urea (g/d) 10 20 26 6 6 6 6 6
PFU (g/d) 25 50 65 15 15 15 15 15
VFIvoluntary feed intake, BCbalanced concentrate, MCmultinutritional block, CCFcomplex catalytic granulate feed, PFUprotein from urea.
aBalanced concentrate without soybean oil meal.
aStandard errors.
A.R.
Moral
es
et
al.
/
Small
Ruminant
Resear
ch
35
(2000)
grazing strategy permitted the conservation of the range land so that approximately 95% of the forage cover was present at the beginning of the next rainy season, i.e. over-grazing was avoided by relying on the alfalfa hay forage bank during the dry months of January to May and additional nutrients were supplied from supplements. The stocking rate was 5 goats/ha, which varied from 15 in the summer to none in February to April.
In the feeding system of this study, alfalfa hay use was greatest (Table 1) during the time of zero grazing which was from February to April to avoid overgrazing and maintain biosustainability of the range land. From July to November grazing was the main source of nutrient supply because of the previous rain, thereby saving alfalfa hay and supplement feeding, which diminished the cost of production. BC supplied energy while feeding alfalfa hay, being gradually substituted by MC and CCF, also providing time for changes in the ruminal ecology.
4. Discussion
Goats in this feeding system lived in two major seasons, from October to May with low temperatures and short days when the feed intake was reduced from alfalfa hay and grazing (1.3±0.6 kg in the rainy sea-son). At the end of this season there was an increase in temperature that produced heat stress, lower feed intake and less milk production (Peraza, 1996). In this phase the feeding of the BC had to be increased from 0.3 to 0.9 kg, bringing it up to 40% of the total energy intake.
The second seasonal stage began at the end of spring with the warm weather (218C). The beginning of the rainy season favors the growth of forages on the range land with excellent nutritional value and an average of 1.8 Mcal of ME/kg of DM. Changes in supplementation with MB and later CCF changed ruminal ecology favoring bacterial protein formation as discussed by Preston (1995). However, forage with supplementation provided 2.5 Mcal/kg DM and 0.18± 0.20 kg DP/kg DM with much soluble carbohydrates (15% of the forage DM) and protein from bacteria. This was re¯ected in a second peak in lactation with increased cheese yield because of higher protein
content in the milk, and accompanied by an increase in live weight of the goats.
It can be suggested from the present results that goats adequately supplemented with small amounts of essential nutrients performed equally well on a lower digestibility feed as animals on higher ones. Pasture management was a key to obtain gains from grass as discussed before (ValdeÂs et al., 1996). Heat increment of added acetogenic substrates (forages) will depend on the pattern of fermentation in the rumen, VFA production, and the balance of protein available from the intestines. Goats need to oxidize acetate, the major thermogenic substrate. Ruminants con®ned to calori-meters are usually maintained at what is supposed to be thermoneutrality, however, in our study animals were exposed to heat and cold (rain) stress, and responded metabolically according to the insulational properties of the skin. Variation in monthly gains re¯ected environmental effects on goats in the present study. Acetogenic substrates are the major source of ATP for muscle metabolism (Pethick, 1984), therefore acetate formation in this study could explain the production by the supplemented animals as also sup-ported by calculations of available nutrients from stoichiometric considerations (Leng, 1990).
5. Conclusions
Utilization of ®brous diets by ruminants can be manipulated in various ways. Digestibility and intake was improved, apparently due to elevation of rumen pH and augmentation of degradable bacterial synth-esis from supplementation with essential amino acids, non-protein nitrogen, sulfur and phosphorus, which improves cellulose utilization. Use of local resources allowed the economical feasibility of the diet and consistent production levels.
farmer was maintained. It is concluded that the com-bined use of balanced concentrates with non-conven-tional supplementation from multinutrinon-conven-tional blocks and catalytic granulate feeds allowed an optimal use of forages. Goats did adapt fast to new ruminal condi-tions to produce bacterial protein and maintain milk production.
Acknowledgements
Research supported by projects SIMORELOS CONACYT 9501075 and 970301029 and PAPIIT 217798 UNAM.
References
AFRC, 1998. The nutrition of goats. AFRC Technical Committee on Responses to Nutrients, Report 10. CAB International. UK and New York, 118 pp.
Alvarez, F.J., Wilson, A., Sutherland, T.M., Preston, T.R., 1976. Studies in urea utilization in sugarcane diets: effect of different methods of incorporating urea in the ration. Trop. Anim. Prod. 1, 186±192.
Camacho, R.G., 1991. ProduccioÂn de leche y consumo estimado en un hato caprino alimentado a base de esquilmos agrõÂcolas (Estimated milk production and feed intake in a goat herd fed agricultural by-products). Thesis, FES-CuautitlaÂn, UNAM, Mexico, 75 pp.
Coblentz, B.E., 1977. Some range land relationships of feral goats on Santa Catarina Island, California. J. Range Manage. 16, 37± 47.
Elliot, R., Ferreriro, H.M., Priego, A., Preston, T.R., 1978. Rice polishing as a supplement in sugarcane diets: the quantities of starch (glucose polymers) entering the proximal duodenum. Trop. Anim. Prod. 3, 30±35.
FernaÂndez, J., 1996. ComplementacioÂn alimenticia en el rancho ``Puente Quemado'' Colima, Col. MeÂxico (Nutritional com-plementation on ``Puente Quemado'' Ranch in Colima, Mexico). Primer Foro Internacional en pastoreo intensivo en zonas tropicales. FIRA, Banco de MeÂxico, Veracruz, Ver., Mexico, pp. 2±12.
Fondevila, M., Dehority, B.A., 1995. Interaction between
Fibro-bacter succinogenes, Prevotella ruminicolaandRuminococcus flavefaciensin the digestion of cellulose from forages. J. Anim. Sci. 74, 678±684.
Galina, M.A., 1994. Sistemas Pecuarios biosostenibles y biodi-versos. Una alternativa de produccioÂn animal ante el deterioro ambiental y la contaminacioÂn (Biodiverse and biosustainable livestock production systems. An alternative to animal produc-tion and deterioraproduc-tion of the environment and polluproduc-tion). Avances en InvestigacioÂn (Agropecuaria) 3, 48±75.
Galina, M.A., Palma, J.M., Morales, R., Hummel, J., 1991. Consumo voluntario aparente en caprinos con el meÂtodo de
unidades empanzonantes (Apparent voluntary feed intake by goats by the method of fill intake units). Avances en Investigacion (Agropecuaria) 14, 92±108.
Galina, M.A., Palma, J.M., Morales, R., Aguilar, A., Hummel, J., 1995. Voluntary dry matter intake and nutritional management by dairy goats grazing on range land or agricultural by-products in Mexico. Small Rumin. Res. 15, 127±137.
Galina, M.A., Pineda, J., Rosado, J., Aguilar, A., Puga, C., Rubio, C., Murillo, J.C., 1997. Fattening of steers ZebuF1 cross fed high fermentable carbohydrate diet of a continuous non-protein nitrogen and by-pass protein supplement. Adv. Agric. Res. 6(3), 22±32.
Galina, M.A., Pineda, L.J., Rosado, J., Puga, D.C., PeÂrez-Gil, F.,
Murillo, J.C., 1998a. ZebuF1 cross steers fattening on tropical
grass with a complex catalytic supplementation in Colima, Mexico. Cuban J. Agric. Sci., in press.
Galina, M.A., Puga, D.C., HernaÂndez, A., Haenlein, G., 1998b. Biodiverse and biosustainable production system with goats in Mexico: importance of a forage bank. Small Rumin. Res. 27, 19±23.
GarcõÂa, E., 1973. Modificaciones al sistema de clasificacioÂn climaÂtica de KoÈppen (Modifications of KoÈppen climatic classification system). Instituto de GeografõÂa, Universidad Nacional AutoÂnoma de MeÂxico. MeÂxico, DF, 33 pp. Genin, D., Pijoan, P., 1993. Seasonality of goat diet and plant
acceptabilities in the costal shrub of Baja California, Mexico. Small Rumin. Res. 10, 1±11.
Gusnet, P., Demarne, Y., 1987. La regulation de la lipolyse et la lipogenese chez les mammiferes (The regulation of lipolysis and lipogenesis in mammals). INRA, Paris, France.
INRA, 1988. Alimentation of bovines, ovins and caprins (Nutrition of cattle, sheep and goats). INRA, Paris, France, 475 pp. Krouse, D.O., Russel, J.B., 1996. How many ruminal bacteria are
there? Symposium: Ruminal Microbiology. J. Dairy Sci. 79, 1467±1475.
Leng, R.A., 1990. Factors affecting the utilization of ``poor quality'' forages by ruminant animals particularly under tropical conditions. Nutr. Res. Rev. 3, 277±303.
Menchaca, M.A., Ruiz, R., GarcõÂa-Trujillo, R., Geerken, C.M., Larduet, R., JordaÂn, H., 1988. Un paquete de programas para el anaÂlisis de la situacioÂn alimentaria del ganado vacuno (A package to analyse the alimentary situation of cattle). En Taller de computacioÂn. ANALIT. XI ReunioÂn de la AsociacioÂn Latinoamericana de ProduccioÂn Animal. ALPA, La Habana, Cuba, 75 pp.
Morales, A.R., 1986. Necesidades de energõÂa y proteõÂna de la cabra lechera en estabulacioÂn total (Requirements of energy and protein of milking goats in total confinement). Thesis, FES-Cuautitlan, UNAM, Mexico.
Morrison, M., 1996. Do ruminal bacteria exchange genetic material? J. Dairy Sci. 79, 1476±1486.
érksov, E.R., 1994. Recent advances in understanding of microbial transformation in ruminants. Livest. Prod. Sci. 39, 53±60. Peraza, C., 1996. Systems of goat feeding in arid regions of
Pethick, D.W., 1984. Energy metabolism in skeletal muscles. In: Baker, Gawthorne, Purser (Eds.), Ruminant Physiology: Concepts and Consequences, Perth, Australia, pp. 277±287. Preston, T.R., 1995. Tropical Animal Feeding. A manual for
research workers. FAO Animal Production and Health Paper 126. Rome, Italy, 305 pp.
Provenza, D.F., 1995. Postingestive feedback as an elementary determinant of food preference and intake in ruminants. J. Range Manage. 48, 2±17.
Puga, D.C., 1998. Mejoramiento de un sistema de alimentacioÂn parcialmente biosostenible en cabras bajo pastoreo racional tecnificado moÂvil (Improvement of a partial biosustainable goat feeding system with rational technical mobile grazing). Master's Thesis. PICP, Universidad de Colima, MeÂxico, 174 pp. Rios, A.G., 1983. Estudios preliminares sobre la produccioÂn caprina con dietas a base de ramoneo en monte bajo en la zona henequenera de Yucatan (Preliminary studies on goat produc-tion with animals fed on shrubland on Yucatan). Master's Thesis. Universidad de YucataÂn, Mexico, 66 pp.
Russell, J., Wilson, D. 1996. Why are ruminal cellulolytic bacteria unable to digest cellulose at low pH? J. Dairy Sci. 79, 1503±1509. SAS, 1985. Statistical Analysis System. User's Guide: Statistics, Version 5th ed. SAS Institute, Inc., Cary, NC, USA, 956 pp. Trujillo, L.R., GarcõÂa, E.R., 1995. Botanical composition and diet
quality of goats grazing natural and grass reseeded shrublands. Small Rumin. Res. 16, 37±47.
ValdeÂs, G., Castillo, F., 1993. Sistema de manejo de machos bovinos en pastoreo. I. SuplementacioÂn con miel y un alto nivel de urea (Management system for male steers on grass. I. Supplementation with high levels of molasses urea). Rev. Cubana Cienc. AgrõÂc. 27, 163±169.
ValdeÂs, G., Delgado, A., 1990. SuplementacioÂn proteõÂco energeÂtica para la ceba de ganado en pastos y forrajes (Protein and energy supplementation on cattle fattening with grass and forages). En. ProduccioÂn de carne en el troÂpico. EDICA, Instituto de Ciencia Animal, La Habana, Cuba.
ValdeÂs, G., Molina, A., Castillo, F., 1996. Estudio del manejo de diferentes categorõÂas de ceba en la produccioÂn de carne bovina y la estabilidad del pastizal. I. Indicadores del comportamiento animal y econoÂmico (Study on management of different levels of fattening for bovine meat and pasture stability. I. Data on animal behaviour and economical feasibility). Rev. Cubana Cienc. AgrõÂc. 30, 277±328.
Weimer, P., 1996. Why don't ruminal bacteria digest cellulose faster? J. Dairy Sci. 79, 1496±1502.
Wells, J., Russell, J., 1996. Why do many ruminal bacteria die and lyse so quickly?. J. Dairy Sci. 79, 1487±1495.
Wilson, A.D., Leigh, J.H., Hindley, N.L., Mulham, W.E., 1985.
Comparison of the diets of goat and sheep on a Causarian
Cristata±Heterodendrum oleifolium woodland community in New South Wales. Aus. J. Exp. Agric. Anim. Husb. 15, 45± 52.
