Effect of stocking rate and supplementation on performance of grazing lambs in a rice-pasture rotation system

P Rovira

Meat and Wool National Program, Instituto Nacional de Investigación Agropecuaria (INIA Uruguay)
Ruta 8 km 281 - INIA, CP 33.000. Treinta y Tres, Uruguay
provira@tyt.inia.org.uy

Abstract

Lamb fattening has been integrated to rice-pasture systems of production in lowlands to keep the livestock sector competitive.  The objective of the present study was to evaluate the effect of stocking rate (SR) and supplementation (S) on lamb performance grazing improved pastures (legumes + ryegrass) after rice harvesting. Sixty castrated Romney Marsh lambs 9 months old with an initial liveweight (LW) and body condition score (BCS, scale 1 to 5) mean ± SD of 35.1±2.8 kg and 2.8±0.27 points were divided randomly into balanced groups of six animals and assigned to five treatments with two replicates: 1) 10 lambs/ha, 2) 16 lambs/ha, 3) 16 lambs + S, 4) 24 lambs/ha, 5) 24 lambs/ha + S. The supplement used was high moisture sorghum grain (HMS) daily delivered at 1.1% of liveweight (as-fed basis). 

Botanical composition at the beginning of the study was 53% rice stubble, 38% legumes and 9% ryegrass and other grasses; with an overall chemical composition of 13.4% crude protein and 52.2% acidic detergent fiber. In unsupplemented treatments, final liveweight (LW) decreased from 50.8 to 37.5 kg as SR increased from 10 to 24 lambs/ha as a result of a significant SR effect on average daily gain (ADG)  (206, 151 and 40 g/a/day for 10, 16 and 24 lambs/ha; respectively). Maximum ADG was achieved at an estimated daily allowance of 1.2 kg DM/head/day. There was a quadratic effect of SR on LW gain per hectare which decreased from 158 to 62 kg/ha (16 and 24 lambs/ha, respectively). For supplemented treatments, HMS improved ADG when compared at 24 lambs/ha (40 and 122 g/a/day without and with supplement, respectively) achieving a feed efficiency of 4.6 kg of supplement per kg of added gain above the unsupplemented animals. In addition, supplementation improved average BCS from 3.4 to 4.1 at 24 lambs/ha but 8% of those animals still failed to meet minimum target BCS for slaughter (3.5) at the end of the experiment. Although poor-drained rice soils can be a predisposing factor for the occurrence of footrot, lambs did not show clinical symptoms of the disease probably due to the low mean minimum temperature (6.0ºC). Stocking rates between 10 and 16 lambs/ha are recommended for fattening lambs grazing improved pastures in poor-drained lowland rice soils. At higher stocking rates supplementation and/or extended grazing periods would be required to improve animal performance and to meet industry requirements for slaughter.

Keywords: crop-livestock, feed, sheep, system of production

Introduction

Due to the lack of competitiveness of extensive sheep production systems, traditionally oriented to wool production, high quality meat derived from lambs has gained in importance among sheep producers.  This product is characterized by a young animal at slaughter (milk teeth female or male lamb), 34-45 kg of liveweight and 3.5 value in the corporal condition scale from 1 to 5 (Azzarini 2000). The success of those more intensive production systems with elevated costs associated with feeding depends on the adequacy of the nutrient supply to meet animal requirements (Galvani et al 2008). Lamb meat production competes for highly productive land with other farming systems, such as agriculture. 

In the last decade, the number of mixed crop-livestock farming enterprises has increased in different regions of the world in order to diversify and increase their production base (Garcia Prechac et al 2003; Russelle et al 2007; Tarawali et al 2011; Bell and Moore 2012). As an example, lamb meat production could be integrated to rice (Oryza sativa)-pasture operations in lowlands. In general, rice fields have a previous history of a three-year pasture as the weeds and plant diseases do not allow rice monoculture (Irisarri et al 2001). This pasture is preferably grazed by sheep as heavier livestock such as cattle can compact soil structure and reduce subsequent crop growth and yield. However, there are concerns if the production and quality of pastures sown on rice stubble can support the activity of fattening lambs or whether an appropriate combination of forage and concentrate is required to obtain optimum lamb growth. In addition, lowland rice soils can remain saturated and/or surface-flooded during prolonged periods of time increasing the risk of developing footrot, a world-wide contagious disease which clinical symptoms are foot lesions and lameness (König et al 2011). 

There are no scientific published reports of fattening lambs in such challenging conditions in integrated rice-livestock systems.  The present study therefore, was planned to validate the technology of heavy lamb production integrated in a lowland rice-pasture system, and to evaluate the effect of three stocking rates and two grain supplementation levels on lamb performance.

Materials and methods

The experiment was carried out from June 6^th to August 23^rd 2012 (78 days) at the Research Unit of the National Institute of Agricultural Research (INIA) located in Treinta y Tres (latitude 33º 14’ S, 54º 15’ W) in Uruguay. The study site consisted of silt loam soil, which is deep, poor-drained lowland soil with low to moderate fertility. The experiment site was integrated in a standard rice-pasture rotation. Rice was planted in October 2010 and harvested in mid-March 2011. At the beginning of April 2011 the stubble was sown by plane. The seeding rate and species were 3 kg/ha of white clover (Trifolium repens), 6 kg/ha of birdsfoot trefoil (Lotus corniculatus) and 12 kg/ha of annual ryegrass (Lolium multiflorum). During the first year the pasture was grazed by yearling steers. It was fertilized by plane in April 2012 with 100 kg/ha (0-47-46-0) and kept without grazing until the beginning of the present experiment.  

All animal procedures in the experiment were approved by the INIA ethical and animal care committee. Sixty castrated Romney Marsh lambs 9 months old with an initial liveweight and body condition score (scale 1 to 5) mean ± SD of 35.1±2.8 kg and 2.8±0.27, respectively, were used in a complete randomize experiment to determine the effects of stocking rate and supplementation on animal performance. Lambs were divided randomly into balanced groups of six animals and assigned to five treatments with two replicates: 1) 10 lambs/ha, 2) 16 lambs/ha, 3) 16 lambs + supplement, 4) 24 lambs/ha, 5) 24 lambs/ha + supplement. The grazing area was adjusted to each treatment according to the stocking rate corresponding 6000 m² (10 lambs/ha), 3750 m² (16 lambs/ha) and 2500 m² (24 lambs/ha) per replicate. Within each treatment the grazing system was alternated in two plots with changes every 14 days. At the beginning of July, on day 30 of the experiment, lambs were shorn using the Tally-Hi method. The supplement used was high moisture sorghum grain (11.1% crude protein, 12.1% acidic detergent fiber, 15.8% neutral detergent fiber, 1.9% ashes and 3.10 Mcal metabolizable energy/kg dry matter) harvested with 23% dry matter, ground and conserved under conditions of anaerobiosis in a silo bag. In this study, processing of sorghum grain was practiced with the purpose of improving performance of cattle supplemented with the same silage. It has been reported that cereal grain processing is of no value in fattening lambs (Economides et al 1990). Before starting the experiment, a 14 day adaptation period was implemented in which the animals became accustomed to the routine of supplementation and to the characteristics of sorghum grain. During the experiment the supplement was delivered early in the morning from Monday to Friday at a level of 1.1% of liveweight (as-fed basis). Herbage mass and sward height were estimated every 14 days by cutting 0.1 m² quadrats to ground level in one of the two plots before and after grazing. Three sward surface height readings were recorded in each quadrat before cutting using a common ruler. For each treatment replicate, samples from individual quadrats were weighed and combined. Then one subsample was weighed and dried in a forced-air oven at 110ºC for 24 h to determine dry matter. Botanical composition was performed at the beginning of the experiment considering the fractions white clover, lotus, ryegrass, straw (stubble), and others (natural grasses and weeds).  Unfasted lamb liveweight and body condition score was recorded every 14 and 28 days, respectively. Liveweight gain per hectare was calculated based on total liveweight per lamb and the respective stocking rate. Supplemental efficiency was calculated as kg of high moisture sorghum grain (dry matter basis) required per kg added gain per animal compared with the unsupplemented treatment at each stocking rate. 

Treatment effects on pasture and animal parameters were evaluated by analysis of variance using the GLM procedure of the statistical package of SAS, in a completely randomized design. Each group of six lambs was the experimental unit and animals within treatment serves as the error term for treatment. For the variables final liveweight, average daily gain and liveweight gain per hectare there were a total of 10 observations (5 treatments x 2 replicates). In the case of pasture variables there were 30 observations as a result of 10 observations x 3 sampling dates considering sub-samples (each quadrat) within experimental units the error term. Mean of the treatments were compared by LS means procedure when a significant (P<0.05) F-test was observed.   

Results and discussion

None of the pre-grazing pasture attributes were different among treatments averaging over the 78-day period (P>0.05) which might be explained on basis of the rather short resting period between grazing periods (14 days) (Table 1). Botanical composition (dry matter basis) at the beginning of the experiment was 53.0% dry forage (mostly rice stubble), 22.4% white clover, 15.4% birdsfoot trefoil, 4.9% ryegrass, 4.3% other grasses and broad-leaf weeds.  Overall chemical composition (%) was 13.4±2.6% crude protein, 52.2±4.2% acidic detergent fiber and 61.8±5.6%. It significantly contrasts with the low nutritive value of the rice stubble characterized by low  protein content and high level of lignification (Sarnklong et al 2010). Clover/grass pastures sown immediately after rice crops are generally dominated by the clover fraction due to an improvement in soil Phosphorus availability, the main limiting factor for legume growth in Uruguayan soils, as it increases after rice flooding (Wright et al 2001). Legume incorporation in rice-cropping systems not only benefits the livestock phase but also may increase yield of the following rice crop due to Nitrogen supply through biological fixation improving soil fertility (George et al 1992; Dunn and Beecher 1998). According to Benoit and Laignel (2010), the main way to improve overall energy efficiency in a mixed crop-sheep farming system is through eliminating nitrogen fertilizer purchases and introducing legumes into the rotation. This results in a “win-win” relationship between the rice grower and the livestock producer, generally different persons but engaged in a land use rental agreement, encouraging the adoption of the rice-pasture rotation. Post-grazing pasture attributes were different among treatments (P<0.05). The lower stocking rate (10 lambs/ha) determined a higher post-grazing herbage mass and sward height due to a lower utilization of the forage produced.  The fact that there were no differences among the rest of the treatments in residual herbage mass can also explain the absence of differences in pre-grazing pastures attributes as similar post-grazing forage mass might yield a similar pasture regrowth pattern (Martinez-Hernandez et al 1997). 

Overall animal performance is shown in Table 2. There was a significant effect of treatment on final liveweight which decreased 13.3 kg as stocking rate increased from 10 to 24 lambs/ha for un-supplemented treatments as a result of a significant stocking rate effect on average daily gain. It agrees with the general concept that average daily gain per animal decreased with increasing stocking rate (Jones and Sandland 1974; Schlegel et al 2000).  Data presented by Nolan (1972) suggest that this trend also extends to mean hot carcass weight, indicating that differences in liveweight will ultimately be reflected in the carcass weight of lambs. A decrease in liveweight gain from 206 to 40 g/d was observed with increasing the number of animals from 10 to 24 lambs/ha, which was attributed to limited individual diet selection and intake as a result of increased inter-animal competition at high stocking rate. On the other hand, increasing stocking rate from 10 to 16 lambs/ha did not affect liveweight gain per hectare for un-supplemented treatments as  the poorer performance of each animal was compensated for by the larger number of lambs/ha. An increase in animal productivity in terms of either individual performance or per hectare can “dilute” the main energy costs (concentrate feed and production of forage) generating an improvement in overall efficiency of the system (Benoit and Laignel 2010).

Daily allowance of forage had an effect on the average liveweight gain of lambs in unsupplemented treatments. Lamb growth rate increased as daily allowance increased from 40 g/day at a daily allowance of 0.4 kg DM/head/day up to 206 g/day at a daily allowance of 1.2 kg DM/head/day. Overall liveweight gain was not high compared with some other reported short-term, post-weaning lamb liveweight gains (Fraser et al 2004; Speijers et al 2004; Nicol et al 2010). Animal growth rate achieved on a ryegrass/clover sward will depend on the stocking rate, severity of grazing and the clover content (Nolan and Grennan 1998).  In pastures implanted after rice cropping the uneven conditions of the terrain caused by animal trampling and traffic of heavy machinery during the rice phase, among other factors, can affect the spatial and temporal distribution of forage production in such poor-drained soils. Depending on the topography of the area, around 15 to 20% of rice area corresponds to soil walls with 20-30 cm of vertical height built to hold the irrigation water during critical phases of the crop. Before planting the pasture species by plane, these walls can be mechanical broken or not, but in any case the herbage mass production in this disturbed area is different than in the rest of the field.

Concentrate supplements may be offered to lambs on pasture to sustain lamb growth where forage is scarce or to finish lambs earlier in the season (Grennan 1999).  Many reports confirm the positive effects on average daily gain of grain supplementation of grazing lambs (Freer et al 1988; Daura and Reid 1991; Karnezos et al 1994; Jabbar and Anjum 2008; Felix et al 2012). In recent years it has become more popular among livestock producers the use of high moisture sorghum grain silage as supplement due to the increased problems of availability and price variability of dry feed grains (Rovira 2012). In this experiment, intake of high moisture sorghum grain averaged 387 and 378 g/a/d for 16 and 24 lambs/ha. In similar conditions it has been reported that such levels of grain sorghum supplementation do not affect ruminal pH and are close to optimal values for pasture dry matter digestibility (Aguerre et al 2009). No difference was detected between supplemented and unsupplemented animals at 16 lambs/ha for average daily gain (P>0.05) which was attributed to the substitution rate of pasture by grain due to the increase in pasture allowance (Bargo et al 2003). Average rate of gain was significantly increased (P<0.05) comparing supplemented and unsupplemented treatments at 24 lambs/ha (122 and 40 g/a/d, respectively) as grain supplementation increases soluble carbohydrates available for rumen fermentation and supply of energy to animals (Poppi and McLennan 1995; Alvarez et al 2001; Abdelhadi et al 2005). Because no differences for average daily gain were observed at 16 lambs/ha, estimate for supplement conversion was made only for the 24 lambs/ha group. In this case, kilograms of supplement per kilogram of added gain above the unsupplemented animals were 4.6.  This in general agreement with Fahmi et al (1989) who reported that lambs on a high-energy diet (concentrates) needed 4.3 kg of dry matter to produce 1 kg of body gain, while those on a low-energy diet (roughages) needed 5.7 kg dry matter.   

In general, body condition score followed the same trend than liveweight. Comparing unsupplemented treatments, lambs at the lowest stocking rate showed the highest final score compared with animals at the maximum stocking rate (4.7 and 3.4 for 10 and 24 lambs/ha, respectively) (Table 2). In addition, 25% of the animals for the 24 lambs/ha treatment had a body condition lower than 3.5 at the end of the experiment, minimum score for slaughter according to industry requirements.   Although supplementation improved body condition from 3.4 to 4.1 at 24 lambs/ha, 8% of the animals still failed to meet minimum target for slaughter. Lambs did not show clinical symptoms of foot rot or similar lesions.  Although poor-drained lowland soils can be a predisposing factor for the occurrence of the disease, transmission of foot rot occurs only when the mean ambient temperature is consistently above 10ºC (Stewart and Claxton 1993). During the 78 days of the study, minimum air temperature (mean ± SD) was 6.0±6.1 ºC with 55 out of the 78 days registering air temperatures below 10ºC.   Other barrier against the foot rot in this rice pasture based systems can be the anticipated tillage in the summer previous to the rice crop followed by glyphosate application and minimum tillage (or no tillage) previous to rice sowing in the spring. It can impact the transmission of the disease exposing the bacteria, if present in the soil; to less favorable environmental conditions interrupting the cycle of the disease. Finally, British breeds as the Romney Marsh used in the current experiment and extensively adopted in lowlands regions are more resistant than other breeds (i.e. Merinos) to the development of foot rot (Emery et al 1984).

Conclusions

• Finishing lambs in lowland rice-pasture systems of production is a viable alternative of production for sheep farmers to adapt to increasing competition for land. Optimizing animal production from these pastures requires careful management of stocking rate and supplementation. This study indicates that a stocking rate between 10 and 16 lambs/ha represents a reasonable compromise between animal productivity per head and per hectare in unsupplemented systems of production. When higher stocking rates are desired, grain supplementation must be provided and the grazing period must be extended to achieve the required fat and liveweight target for slaughter. 

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Received 12 March 2013; Accepted 2 June 2013; Published 1 July 2013

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