Influence of multinutrient blocks on liveweight gain of young bulls grazing sorghum stubble during the dry season
R Ricca and J Combellas
Facultad de Agronomía, Universidad Central de Venezuela Apartado 4579, Maracay, Venezuela
An experiment was carried out to evaluate the influence of multinutrient blocks (MB) on live weight (LW) gain of young bulls of 235 kg initial LW grazing sorghum stubble during the dry season. Thirty two animals were divided in two groups of similar weight and continuously grazed two paddocks of 14 ha for 21 weeks. One group was offered MB in four places and the other received a mineral mixture (MM). Both groups were rotated between paddocks every two weeks to minimize the effect of plot. The MB had 32.3% crude protein, and the main ingredients were 40% chicken manure, 32.5% molasses, 10% lime, 5% salt, 5% minerals, 5% urea and 2.5% ground hay. LW gains of animals offered MM and MB were 1.18 and 1.33 kg/day during the first eight weeks (P<0.05) and 0.30 and 0.31 kg/day during the remaining 13 weeks (P>0.05). The higher response to MB at the beginning of the trial was associated with a high availability of stubble of low nitrogen content. The reduction of stubble availability towards the end of the trial limited the LW gain response to MB.
KEY WORDS: Multinutrient blocks, cattle, sorghum stubble, live weight gain.
Grazed sorghum and maize stubbles are the basal diet of growing cattle during the dry season in cereal areas of Venezuela. The grain is harvested at the end of the rainy season and the animals continuously graze the stubbles for about five months until the end of the dry period. The in vitro organic matter digestibility (IVOMD) of sorghum leaves and stems is over 65% (J D Reed, cited by Preston and Leng 1987), high energy values compared to other cereal stubbles. But their crude protein contents are low and decrease with time after grazing (Orellana y Rauseo 1982, Escobar y Parra 1981). The pattern of live weight (LW) change during the dry season presents a rapid increase at the begining of grazing, followed by the maintenance of LW or its decrease towards the end of season (Arias et al 1980).
The main objective of this work was to evaluate the influence of multinutrient blocks (MB), as sources of degradable nitrogen (N) and other nutrients during the dry season. The variation in chemical composition and weight of standing stubble fractions through the season were also evaluated.
Materials and methods
The experiment was carried out in the farm La Unión at 4 km from Tucupido, Guárico State, Venezuela, at an altitude of 143 m above sea level. Average annual rainfall in the area is 1053 mm, with a dry period from December to May. Mean annual temperature is 26.3 °C with a maximum and minimum monthly averages of 32.9 °C and 21.8 °C. Mean relative humidity is 74%, with a monthly average range from 65 to 82%. The soils of the farm are clay or sandy-clay.
Experimental design and treatments
Thirty two young bulls of 235 kg average initial LW were balanced according to weight and allocated to two treatments consisting of supplementation with MB or with a mineral mixture. The animals continuously grazed two paddocks of sorghum stubble of 14 ha each, and were rotated between them every 14 days to minimize the effect of plots. The experiment started on the first December 1991 and last 21 weeks until the 25 April 1992, with the start of the first rains.
Animals and management
The animals had undetermined proportions of Zebu, Brown Swiss and Holstein breeds, and were deparasited with 12.3 cc/animal of Albendazole at the beginning of the trial. LW was measured every 14 days between 0800 and 0900h with a portable electronic scale. LW gain was estimated as the linear regression of LW on time in two stages, from weeks 1 to 8 and 9 to 21, because a change in slope was observed between these periods. LW gains were compared following a Student "t" test (Steel and Torrie 1980). MB and mineral mixture were always available according to treatment in four feeding troughs in each paddock. The consumption of supplements was determined by difference every two weeks.
Crop management and feeds
Sorghum hybrid Chaguarama III was sown at a density of 200,000 - 250,000 plants/ha on 1 September 1991 and machine harvested on 19 November 1991. It was fertilized at sowing with 300 kg/ha of a complete formula (12-24-12) and 100 kg/ha urea were applied 25 days after sowing. The grain yield was 2600 kg/ha. The standing stubble per ha was estimated at the start of the experiment and approximately every month afterwards, throwing ten quadrants of 1.00 m x 0.50 m per paddock following a fixed transect and cutting the samples at ground level. The samples were individually weighed, mixed to obtain two samples and dried at 65 °C for 48h. One sample was previously fractioned in leaves, stems, panicles, loose material and weeds. Loose material was defined as the parts of the stubble separated from the plant and lying above the soil; initially, a large proportion is composed of fibrous waste discarded by the grain harvester and afterwards by other parts of the plant falling down as the grazing period advances.
Fourteen large tree legumes were also present within the grazing area: 3 Pithecolobium saman, 5 Enterolobium cyclocarpum and 6 Prosopis juliflora. These plants were used as shade for the animals and their pods were feed sources at the end of the dry season.
The MB had the following composition (%): chicken manure 40.0, sugar cane molasses 32.5, urea 5.0, lime 10.0, common salt 5.0, mineral mixture (Venefostracal) 5.0 and ground hay 2.5. Two samples of blocks were taken at 5 and 70 days of the trial for chemical analysis. The samples of MB, sorghum stubble and its fractions were ground through a 1 mm screen and analysed for ash and crude protein (AOAC 1984), neutral detergent fiber (NDF) (Goering and Van Soest 1970), calcium (Fick et al 1973) and phosphorous (Harris and Popat 1954). The mineral mixture used had 50% common salt and 50% Venefostracal (FONAIAP, Maracay). Venefostracal is formulated mainly with calcium diphosphate and smaller amounts of other elements.
Weight and composition of standing stubble
The weight of standing stubble was 8.23 ton DM/ha just before grazing, slightly increased during the first two months and was appreciably reduced towards the end of the experiment (Table 1). Some regrowth on the stubble and seed germination in the field was observed during the first two months, associated with light precipitation at the start of the trial. The rainfall (mm) during the experiment was: December 13.3, January 0.0, February 9.0, March 0.0 and April 29.8. Large variation in the proportions of plant parts were observed during the grazing period. The availability of leaves and panicles almost disappeared after 57 days of grazing and significant proportions of only stems and loose material remained afterwards.
The crude protein (CP) of sorghum stubble was 5.8% before grazing and decreased to 3.2% at 81 days (Table 2). The weeds were the fraction with highest CP content, but their proportion in the stubble was very small, except at the end of the trial when the less palatable species with lower CP content remained. They were followed by leaves, loose material and the lowest values, less than 3.6% CP, were observed in stems. CP also declined with days of grazing in all the fractions analysed. The NDF followed an inverse pattern, increasing as the experiment advanced. A large proportion of fiber was observed in all fractions except in panicles during the first two samplings, when them included a significant proportion of grains. The phosphorous content followed a similar trend to CP, decreasing with days of grazing. Calcium had erratic variations and no trend was observed. The chemical composition of MB in DM basis was: crude protein 32.6%, ash 35.8%, calcium 7.59% and phosphorous 1.36%.
|Table 1. Weight and composition of standing stubble|
|Days of grazing|
|Stubble weight (ton DM/ha)||8.23||9.02||8.65||6.33||2.71|
|Percentage of (DM basis):|
|Table 2 Chemical composition of the standing stubble (DM %)|
|Days of grazing|
|Neutral detergent fiber|
Live weight gain of animals and intake of supplements
LW gain was over one kg/day in both treatments during the first eight weeks of stubble grazing (Table 3), and the use of blocks resulted in a significant increase of 0.15 kg/day (P=0.07) compared with the control group receiving mineral mixture during this period. This variable was appreciably reduced during the last 13 weeks of the experiment to about 0.31 kg/day (Table 3) and no differences between treatments were observed. The LW gain response to MB on average during the trial was 0.70 kg/day. The stocking rate used was 1.14 animals/ha, with 284 kg average LW during the experiment, equivalent to 0.81 animal units (AU)/ha.
|Table 3. Live weight gain of animals and consumption of multinutrient blocks (MB) and mineral mixture (MM)|
|Live weight gain (kg/day)|
|Weeks 1 to 8||1.18||1.33||0.070*|
|Weeks 9 to 21||0.30||0.31||0.51|
|Weeks 1 to 21||0.63||0.70||0.031*|
|Consumption of MB (kg DM/day)|
|Weeks 1 to 8||0.26|
|Weeks 9 to 21||0.43|
|Weeks 1 to 21||0.37|
|Consumption of MM (g/day)|
|Weeks 1 to 8||57|
|Weeks 9 to 21||45|
|Weeks 1 to 21||50|
Monthly MB consumptions from December to April were 0.22, 0.30, 0.31, 0.39 and 0.60 kg DM/animal/day, associated with decreases in stubble availability and quality. The intake of mineral mixture in the control group was 50 g/day, with small variations during the experiment (Table 3).
Performance of growing animals on sorghum stubble
The high increases in LW gains of animals in both treatments at the beginning of the experiment (Table 3) were related to the large allowance of sorghum stubble and to the selection of the better quality stubble parts by the young bulls.
The weight of standing stubble was similar to the values obtained with the same hybrid by Arias et al (1980) and to those obtained by Parra et al (1978). Lower yields have been observed with this by- product with other varieties (Arias et al 1980, Orellana y Rauseo 1982). The high variability observed in this measurement is influenced by plant genotype, agronomic practices, soil conditions and climatic factors. The weight of standing stubbles in 23 hybrids evaluated by Arias et al (1980) varied from 2582 to 8680 kg DM/ha.
Selection during the first two months of grazing was in favor of leaves and panicles (Table 1), the stubble parts with higher contents of CP and phosphorous. Arias et al (1980) and Escobar y Parra (1981) have also observed higher CP contents in these parts compared to stems. The energy content was not evaluated, but J D Reed (cited by Preston and Leng 1987) reported average IVOMD of 70 and 65% for leaves and stems of 24 varieties of sorghum evaluated, showing the high energy value of this fibrous resource. The quality of the diet consumed at the beginning of the trial was also influenced by the regrowth of the stubble and the growth of new plants mentioned before; the presence of sorghum grains in the panicles remaining in the field, could have also increased the energy content of the diet and contributed to the high LW gains observed at this stage. The high LW gains observed could have also been influenced by compensatory growth due to a poor feeding before the trial. These animals were grazing native pastures, and at the end of the rainy season, florescence occurs and the quality of pastures decreases.
After 57 days of grazing the leaves and panicles had almost disappeared and the base diet was constitued by stems and loose material. The stubble allowance was reduced (Table 1) and the nutritive value of available parts was also decreased. The LW gains were consequently reduced to 0.31 kg/day and these small increments remained until the end of the dry season. The presence of tree legumes within the paddocks could have contributed to the maintenance of small gains at the end of the trial through the supply of pods. The fall of legume pods started on week 15 of the trial, at the beginning of March, and lasted until the end of the experiment. Average LW gains obtained with sorghum stubble in this trial, over 0.6 kg/day with 0.81 AU/ha, were similar to those obtained by Orellana y Rauseo (1982) of 0.62 kg/day during 101 days of grazing with a lower stocking rate of 0.35 AU/ha and higher than those of Arias et al (1980) of 0.18 kg/day during 131 days with 0.67 AU/ha. The latter authors observed losses of weight at the end of the dry season, affecting the overall LW gain of the experiment. The efficiency of utilization of stubble under grazing conditions is low. In this trial, even assuming a high intake of 3 kg DM/100 kg LW, only 17.4% of the stubble available at the start of the trial was consumed. Alternative systems to reduce these losses, such as hay making, have not been adopted in the area because of the high costs of the machinery required.
The influence of mb on lw gains
The positive response of MB at the beginning of the experiment was associated with the presence of a large amount of stubble of high energy quality and low N content. The IVOMD of sorghum stubble is high compared with tropical forages (Minson 1981) and with most agricultural fibrous by-products. But its content of CP is low even in the leaves, which have the highest concentration of this fraction. The supply of degradable N in the MB, derived from the urea and chicken manure, could have contributed to covering N deficiencies of rumen microorganisms, increasing the supply of microbial protein to the animal and the utilization of the organic matter consumed. The concentration of calcium and phosphorous was also low in the stubble and supplied by the MB. But the control treatment was supplemented directly with a mineral mixture to prevent the confounded effects of mineral and N deficiencies.
During the last 13 weeks of grazing, LW gains were reduced in all animals, and the response to MB disappeared (Table 3). The CP content of the standing stubble was lower at this stage (Table 2), and its availability was also much reduced and could have limited the intake of grazing animals. Results obtained at this Institute (Mata and Combellas 1992) with stall fed animals receiving low quality hays have shown that the main effect of blocks in these conditions is through an increase in roughage intake and not by a raise in the basal diet digestibility.
The use of MB is justified at the beginning of the stubble grazing in most price situations, where the high feed availability and low N content enables a significant LW gain response. The MB included 40% chicken manure, which is a very cheap and abundant ingredient. The MB consumption during the first eight weeks was 14.6 kg DM/animal and the LW gain response to MB during the 147 days of trial was 10.3 kg/animal. It is assumed that LW gains in the MB treatment are maintained at 0.31 kg/day without offering it during the last 13 weeks of the trial.
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(Received 18 July 1993)