126 Lead Paper 13
Trade-offs Between Stocking Rate, Range Productivity and Livestock
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consequences, it influences the growth dynamics, the botanical composition and nutritional quality of the pasture (Henkin et al., 2011; Sternberg et al., 2000). The constantly changing availability and quality of the pasture vegetation that determine both the nutritional intake of the grazing animals and the role of supplementary feeding, create trade-offs between the multiple responses of the vegetation, the livestock and the cost/benefit ratio of the grazing enterprise (Baron et al., 2002). Quantifying the trade-offs can contribute to more effective enterprise management (Savory and Butterfield 1998).
In eastern Mediterranean grasslands that are dominated by annual species, the strongly seasonal pasture cycle is characterized by a mild, rainy winter-spring growing season and a hot and dry summer-autumn season. At the beginning of the rainy season (Nov.-Dec.), the biomass is too sparse to enable beef cattle to forage effectively on the predominantly annual herbaceous pasture vegetation. As the biomass increases, livestock ingest increasing amounts of forage. During the spring (Feb.-Apr.), there is a surge of growth that provides the abundant quality forage on which livestock production on the range is critically dependent. Peak biomass production at the end of spring is followed by a sharp reduction in amount and quality of the herbaceous vegetation caused by seed dispersal, followed by rapid desiccation and a gradual decline in the amount of dry herbaceous biomass.
Herbaceous plant production determines the carrying capacity of rangeland for livestock husbandry. It is difficult to determine over the large area required to maintain viable herds because it is influenced not only by the prevailing climatic factors but also by the vegetative composition of the range and the complex habitat characteristics (Seligman and Van Keulen 1989). Nevertheless, it is an essential factor in the analysis of herd management decisions, especially where supplementary feeding interacts with the other elements of the production system. Appropriate stocking rates under such conditions are dependent not only on productivity of the pasture vegetation and the supplementary feeding protocol but also on the response of vegetation and livestock to grazing pressure and timing (Gutman et al., 1990).
As a rule, the amount of standing biomass in the pasture decreases with increasing stocking rate (Baron et al., 2002). Change in the standing biomass of a growing sward depends on the balance between the rate of pasture growth and the rate of forage consumption by the grazing herd. Heavy grazing at the beginning of the growth season can prevent the increase of standing biomass and maintain the sward at a low equilibrium level at which the growth rate of the sward equals the forage consumption rate of the herd, which is then far below that required for adequate animal nutrition (Noy-Meir 1975). Deferment of heavy grazing at the beginning of the growing season has been shown to prevent a fall to a low stable equilibrium (Gutman et al., 1999). The aim of the current study was to quantify the trade-offs in the multiple responses of pasture growth, performance of the grazing livestock and amount of consumed supplementary feed to relatively high stocking rates in a predominantly annual Mediterranean rangeland.
Materials and Methods
Study site and Experimental design
The experiment was conducted during the years 1994-2010 at the Karei Deshe experimental farm, located in the eastern Galilee in the north-east of Israel (long. 35035'E; lat. 32055'N;
altitude 60 - 250 m a.s.l.). The topography is hilly and the landscape is covered with basaltic rocks. The soil is a fertile brown basaltic protogrumosol of variable depth, but seldom deeper than 60 cm. The area has a Mediterranean climate, characterized by wet, mild winters and hot dry summers. The average seasonal rainfall is 560 mm, falling mostly in winter and spring.
The rainy season begins in October-November and ends in April. As a result, the vegetation
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is green between November and April, but during May most plants mature, disperse seed and dry out.
The study included different grazing scenarios based on two stocking rates and two management protocols: continuous (C) grazing of a single paddock throughout the grazing season, and a split-grazing (S) protocol with animals being moved between multiple paddocks. The two stocking rates were moderate (M, 0.55 cows ha-1) and high (H, 1.1 cows ha-1). In the split-grazing protocol the paddock was divided into two equal sub-paddocks.
Grazing began in one sub-paddock early (E) in the growing season after deferment of grazing since germination. The herd was moved to the second un-grazed sub-paddock later (L) in the grazing season when the amount of standing biomass was reduced to a level that subjected the herd to nutritional stress. The two stocking rates (M, H) and two grazing management protocols (C, S) constituted four main grazing treatments (MC, HC, MS, HS). The trial was maintained for 17 years to cover the wide annual variation in climatic conditions and pasture productivity. All treatments were replicated twice and were maintained without change. The area of the 8 main paddocks ranged between 21 and 35 ha.
Livestock and grazing season
The paddocks were stocked with mature, medium-frame Simford (Simmental × Hereford) crossbred cows, with about 20% blood from local eastern Mediterranean breeds with an average body weight of 432 ± 15 kg. The pasture year is basically divided into two seasons: a green and a dry season. The average date of the first effective rains (>20 mm) in Karei Deshe experimental farm during the years was November 4th. The herbaceous vegetation was usually well established within 14 days after the first effective rains, by which time the standing biomass (reported on a dry matter basis throughout) was ca. 10 g m-2 (Svoray et al., 2008). Grazing was deferred in all paddocks during the initial growth period to allow the herbage to reach a threshold beyond which the livestock could forage a substantial part of their daily requirement. The herds were introduced to the experimental paddocks at the end of the deferment period, usually during the latter half of January, when the standing biomass averaged 76 ± 5 g m-2. In the summer, the herds remained in the moderately grazed paddocks till October – November, depending on the timing of the first rains. In the heavily grazed paddocks, the herds had to be removed towards the end of August. At weaning, the cows were weighed and in September the pregnancy of each cow was determined to calculate the conception rate of the herd. All calves were weighed at weaning and then removed from the paddocks.
Vegetation sampling, primary productivity and animal nutrition
The standing biomass in the paddocks was estimated by placing 25x25 cm quadrats at random along permanent transects that crossed all the experimental paddocks. All above- ground herbaceous standing biomass in the quadrat was harvested. At each sampling date, 20 samples were harvested in each of the paddocks. The harvested plant material was oven-dried at 65°C and weighed. The biomass was sampled every year at the beginning of grazing (January-February), at the peak of vegetation growth (April), at the end of the abundant dry pasture in early summer and the beginning of supplementary feeding (June), and at the conclusion of the grazing season towards the end of summer (August-September). We estimated primary productivity under grazing as the sum of the standing biomass at the end of the growing season and the assumed quantity of herbage that was ingested during the growing season by the cattle. During the green season and the beginning of the dry season, the pasture was the only feed source of the experimental herds. During the later dry pasture
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period in summer the herds were given ad lib access to supplementary feed, mainly poultry litter, that provided a nitrogen supplement to the protein-poor pasture.
Statistical analysis
The distribution of explainable sum of squares of the biomass data among season, year, grazing treatment, replicates and interaction between season and treatment was calculated with a GLM procedure (SAS Institute 2002). For comparisons among the secondary treatments, the years were treated as independent nominal values and the standard deviations of the average biomass values over the years are given as an indication of the inter-annual variability.
Results
Duration of the grazing season
At the moderate stocking rate, the herds were on the pasture for almost 280 days of the year and at the heavy stocking rate, just over 200 days (Table 1). Grazing started at the end of the grazing deferment period and ended at the beginning of autumn when the dry pasture biomass was grazed down to between 55 and 116 g m-2 (Figure 1). The variation in the number of grazing days per hectare in the different treatments is a function of the stocking rate and the duration of the grazing season in the different treatments (Table 1).
Table 1. Pasture utilization, supplementary feed consumed, live-weight and conception rate of herds is the experimental treatments (Average ± S.D.).
Treatment MC MS HC HS
Days in treatment paddocks 279 ± 19 277 ± 18 198 ± 26 205 ± 27 Grazing days per hectare 156 ± 10 168 ± 29 223 ± 28 233 ± 60 Supplementary feed (kg day-1) 4.4 ± 2 4.5 ± 2 7.9 ± 2 6.7 ± 3 Cow live-weight on entry (kg) 429 ± 30 448 ± 25 421 ± 19 430 ± 28
Conception rate (%) 73 ± 13 76 ± 13 70 ± 6 78 ± 5
Weaned calf production (kg ha-1) 46 ± 12 41 ± 7 89 ± 17 87 ± 17 Standing biomass and primary productivity
The average long-term green standing biomass when the herds entered the paddocks was between 71 and 84 g m-2. Subsequently, the amount of standing biomass reflected the grazing pressure imposed on the different paddocks (Figure 1). The standing biomass in the continuously grazed paddocks was inversely proportional to the stocking rates in the MC and HC treatments. An estimate of primary productivity of the herbaceous vegetation in a grazed paddock requires an estimate of herbage consumption by the herd during the growing season.
We were obliged to use the normative values of biomass consumed by cows in a beef herd.
Adding the biomass consumption by the herd to peak biomass gave 17-year average primary production between 219 and 397 g m-2.
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Karei Deshe: Average standing biomass dry matter (1994 - 2010)
0 100 200 300 400 500 600
MC HC MSe HSe MSl HSl
Grazing treatments
Biomass, g m-2
Onset of grazing Peak season Mid-summer End of grazing
Figure 1. Annual cycle of standing biomass in the moderate and heavy continuous stocking rate treatments and in the sub-paddocks of the split paddock grazing protocol (Averages and standard deviations, 1994-2010). (Main treatments: MC – moderate continuous, MS – moderate split, HC – heavy continuous, HS – heavy split; Sub-paddocks: MSE – heavy early, HSE – very heavy early, MSL – heavy late, HSL – very heavy late).
Performance of the herd
The amount of poultry litter consumed was clearly related to the stocking rate and to the availability of dry pasture biomass during the late summer. Even though the animals at the heavy stocking rate were on pasture for a shorter period, the consumption of supplementary feed per head was 64% higher (Table 1). The live-weight of the cows and their conception rates were higher in the split grazing treatments but the differences were relatively small so stocking rate had an overwhelming, almost proportional effect on weaned live-weight per unit area of pasture: the weaned calf production was 87-89 and 41-46 kg ha-1 at the high and moderate stocking rates, respectively (Table 1).