Are we turning chickens into cows: How much grass do free range broiler eat?

Free-range broilers and layers are less efficient converters of feed into saleable meat and eggs and generally have higher mortality than conventionally-reared poultry. In broilers, the performance gap has been quantified as a 10-12 point increase in FCR and a 2-3% increase in mortality in free-range compared with conventionally-reared birds (Durali et al., 2012). This increase in FCR, however, may still be under represented considering free range birds also have access to supplementary feed sources on range. Although it has been established by observation studies that chickens eat grass while on range (Glatz et al., 2005, Miao et al., 2005), there have been minimal attempts to quantify the amount of grass consumed and its effect on performance and digestibility in birds. Some of the methods that have been used so far are by measuring reduction in sward height (Elbe et al., 2004), or comparing herbage mass in areas grazed by hens to an area from which they have been excluded (Jondreville et al. 2011). Other methods involve invasive procedures such as analysis of crop, gizzard and faecal contents which cannot be repeated for individual birds (Antell and Ciszuk 2006; Jondreville et al. 2011; Milby; 1961; Takahashi et al. 2006). One of the most suitable methods is to use the n-alkanes (Hatt et al. 2001; Ordakowski et al. 2001; Premaratne et al. 2005). So far there have been very few attempts to use this methodology in birds. The first reported study on use of alkanes in chickens was conducted by Hameleers et al (1996), who were able to determine their faecal recovery. In another study, alkane analysis was used to study intake and nutrient digestibility in pigeons (Hatt et al. 2001). Recently, soil and herbage intake was measured for free range layers using this methodology to evaluate the impact of nutrient restriction on ingestion (Jondreville et al. 2011). If grass consumption can be quantified accurately, then an important outcome would be to provide birds with feed that would compensate for the nutritional imbalances caused. This study attempts at establishing the use of alkanes as internal markers for estimating the intake of grass.

 

A total of 1440 Cobb 500 as hatched broilers were allocated to one of four treatments each with twelve replicates in a 2x2 full factorial design, the factors being conventional or freerange production system and conventional (with in-feed antibiotic growth promoter (AGP+)) or free range (AGP-) diet. Day old chicks received a numbered wing tag at placement and were randomly allocated to 48 pens (30/pen, density of 15 kg/m²) with ad libitum access to feed and water in a tunnel ventilated shed. Broilers were kept at a temperature of 31°C for days 1-4 and thereafter this was reduced by 0.5°C/day to 24°C. Although chicks were assigned to treatment diets on day 1, free range access was available to birds only from day 21 onwards. Ten percent of birds in the free range treatment were chosen randomly and manually assigned to the range from day 21-28 for 2 hours. The number was increased to 20% birds from day 28-35 and to 30% from day 35-42 for three hours (all chosen randomly) on range to represent the free range preference by broilers on a commercial production farm (pers. comm., Durali 2012). The range had a homogenous growth of Kikuyu grass (Pennisetum clandestinum) as the main herbage and was mowed down to 2.5 inches before assigning the birds. Pen wise body weights and feed intake were recorded at weekly intervals during the 42-day trial, corrected for mortality and FCR calculated.

Litter samples which consisted of woodchip along with excreta were collected on day 42 from each pen using the "coning and quartering" technique to represent an even distribution of the representative excreta. Grass, diet pellets and clean wood chip samples were also collected on day 42, dried to a constant weight in a freeze drier and ground through a 0.5 mm screen Cyclone mill. Alkane concentration in grass, diet pellets, woodchip and litter samples was determined using a modification of Mayes et al. (1986) methodology and gas chromatography. The identity of odd chain alkanes (C25 to C33) was determined from their retention times relative to the known standard. The area under the peak for each alkane was determined using an integrator (Model 3393A, Hewlett Packard), and peak areas were converted to amounts of alkane by reference to the internal standard C32.

Recovery of each alkane was used to calculate the proportion of the ingested ingredient, which was recovered in excreta. Diet proportions were estimated using a nonnegative least squares procedure in the software "EatWhat" (Dove and Moore, 1995). In this study, only five odd chained alkanes (C25, C27, C29, C31, and C33) that were found in traceable concentrations were used for diet proportion estimates. The concentration of individual alkanes in excreta was corrected allowing for incomplete recovery based on published values by Hameleers et al. (1986) and number of birds per pen. By subtracting the contribution of woodchip, the intake of pellets and grass was established. Feed intake and FCR were corrected by adding the calculated grass consumption for day 21-42. Performance and alkane data were analyzed using JMP 9.0, 2010. ANOVA was conducted to evaluate the system and diet effects on performance as well as effect of individual alkanes on excreta recovery rates. In all cases, significance was set at P < 0.05.

 

Before accounting for grass consumption in the calculations, performance of birds on free range system appeared to be better than those raised on conventional system with a significant decrease seen in feed intake of birds ( 121gm vs. 130 gm/b/day, P < 0.001) and a significantly lower FCR for birds with range access ( 2.15 vs. 2.33, P < 0.001). However, birds on range were seen to be eating grass. Alkane analysis was used to quantify grass consumption. The alkane profiles of the diet components showed that the predominant alkanes recovered were the odd chain alkanes C25, C27, C29, C31 and C33. While C31 (27.77 mg/kg DM) and C33 (21.12 mg/kg DM) showed up in higher concentrations than any other alkanes in grass, all alkanes showed up in more abundance in grass as compared to the other diet components. Woodchip showed the lowest amount of alkanes ranging from 0.028 mg/kg DM of C25 to 0.256 mg/kg DM of C29 recovered from it, whereas the two diet pellets showed higher concentrations of C25 (1.013 and 1.069 mg/kg DM), C27 (1.283 and 1.360 mg/kg DM), and C29 (1.260 and 1.407 mg/kg DM).

The n-alkane concentrations in litter for the different production systems and diets are presented in Figure 1. Alkanes recovered from litter samples showed significant rise in C31 and C33 alkanes in birds on a free range system and with access to grass ( 2.17 + 0.082 and 1.13 + 0.041 mg/kg DM) as compared to the conventional system (0.54 + 0.082 and 0.27 + 0.04 mg/kg DM). No significant difference was seen in alkane recoveries between the two diets.

Figure 1 - Alkane recoveries (mg/kg DM) for conventional and free range systems and diets.

No grass was detected in excreta of birds reared under the conventional production system. Figure 2 shows the proportion of ingested ingredients as recovered from excreta of birds that had access to grass and the two diet treatments.

Figure 2 - Amount of grass and pellet diets ingested by birds in a free range production system.

Diets intended for consumption by free-range birds are not routinely formulated to accommodate the modifying effects of grass consumption on digestible nutrient intake. However, the consumption of grass at the levels outlined above would have a substantial diluting effect for dietary energy and would result in a significant increase in dietary potassium concentrations. The implications of these changes in dietary nutrient supply are currently being explored in a subsequent trial where both standard and high energy density broiler diets were systematically diluted with grass. The outcomes of this work will shed light on possible nutritional contributions to the relatively poor performance of free-range broilers in Australia. Inadvertent changes in either dietary energy density or in dietary electrolyte balance (DEB) may be of importance, especially during summer months where DEB balance becomes critical to control metabolic alkalosis.

 

ACKNOWLEDGEMENTS: The Authors are grateful to the Poultry CRC for the financial support of this study. We thank Ajantha Horadagoda and Ravneet Jhajj of DRF for their help with the alkane analysis.

 

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