Effect of agronomic management practices on lettuce quality including the risk of enteric pathogen
By Sabine Rattler, Ulrich Köpke, Daniel Neuhoff, Institute of Organic Agriculture, University of Bonn, and Angelika Ploeger, Department of Organic Food Quality and Food Culture, University of Kassel
||In Organic Agriculture, fast growing crops like lettuce with comparatively low nitrogen requirements have approximately the same yield potential as conventional fertilized crops. Also, the present experiments indicated that the risk of lettuce contamination induced by organic manure application is rather low.
Product quality of lettuce
Product quality of lettuce depends on the agronomic strategies used. Thus, soil fertility management and irrigation regime influence grading and utility value as well as nutritional quality.
Nutritional quality describes the inherent biological or health value of produce including the ratio of beneficial to harmful compounds, taste, fragrance, freshness, shelf-life as well as the risk of toxic pathogens as important quality characteristics that govern consumer behaviour.
Fertility input in organic farming
Beside positive pre-crop effects of leguminous or leafy crops, the main homemade fertility input used in organic vegetable production systems is compost, e.g., farmyard manure (FYM). Composted farmyard manure has several advantages compared to fresh manure. Mature compost quickly comes to an equilibrium with the soil, acting as a source for nitrogen which is slowly released and often resulting in higher product quality.
Prevention of pathogen transfer
One strategy for the prevention of pathogen transfer from manure is the controlled aerobic composting of FYM. Composting temperatures above 60° C are known to effectively kill pathogenic germs and weed seeds. However, the impact of composting on the population density of Enterobacteriacea and enteric pathogens found on lettuce grown in soil fertilized with manure/compost has rarely been quantified.
Sprinkler irrigation is commonly used in many lettuce growing regions throughout Europe, because it is cheaper than the use of irrigation tape and other drip irrigation systems. However, it is also supposed to increase the risk of enteric pathogen transfer from manured soil to lettuce leaves. Food contamination with manure-born pathogens is of concern in some European countries.
Assessment of the role of different fertility inputs
The objective of this project was to assess the effect of different fertility inputs on marketable yield, desirable and undesirable ingredients and the sensory quality of lettuce. Another aim was to quantify the enteric bacteria transfer risk associated with the use of fresh farmyard manure, composted farmyard manure and liquid manure, derived from fermented plant exudates of Urtica dioica shoots and to compare the enteric bacteria transfer risk of overhead with soil-based irrigation methods.
The field experiments were conducted in 2004 and 2005 on the organic research farm “Wiesengut”. The farm is located at the southern border of North Rhine Westphalia in the River Sieg valley around 25 km in the east of Bonn, Germany.
With an annual average temperature of 10.2°C (average of 1972-2001) the climate in this area is comparatively mild. The annual average precipitation is about 850 mm (average of 1972-2001). Normally the distribution of rainfall is relatively balanced, rarely resulting in severe drought conditions.
The climatic conditions during the growing periods of all four trials were characterised by heavy rainfalls, making additional irrigation unnecessary. In 2004, the first trial was planted at the end of April and the second was planted in August. The first trial of 2005 was established in May, the second in July.
The lettuces were harvested at optimal maturity, when marketable yield was highest. After harvest the following parameters were assessed:
- Yield: total yield - weight of 18 heads per plot. Marketable yield - cleaned lettuce heads > 200 g
- Histological parameter: Cell size, Cell density,Tissue strenght
- Morphological parameter: Number of leaves until heart, Mass of lettuce hearts, Leaf colour
- Ingredients: Nitrate content, Phosphor, potassium, magnesium, iron, C (total)/N (total)
- Microbiological parameters: Coliform bacteria, E. Coli, Salmonella enteriditis and Enterococcus.
- Sensory evaluations: Appearance, texture, aroma, mouthfeel, flavour, aftertaste/afterfeeling (evaluated by a trained panel).
Our experiments showed showed that comparable lettuce yields can be obtained with both mineral and organic fertilizer application. The experimental factors fertilizer type and N-availability level had no clear effect on total yield. Furthermore no clear effect of fertilizer type and level was observed on marketable yield.
Contents of nitrate and minerals
Compared to organic manures calcium ammonium nitrate fertiliser resulted in significantly higher nitrate contents, but absolute contents were generally low. Nitrate concentration increased with the amount of fertilisers applied. The mineral composition of lettuce was only slightly affected by the type of fertiliser applied and was mainly a function of the mineral content of the corresponding fertiliser used.
Mineral nitrogen fertilization tends to increase the number of leaves apart from the heart, the leaf area and the ratio of leaf mass to heart mass. Fertilizer type and amount resulted in a different histology of lettuce leaves. Increased availability of nitrogen resulted in lower cell density and higher cell size, an effect supposed to result in a more spongy tissue and lower crunchiness. This hypothesis was indirectly confirmed by several correlations between histological and chemical parameters but not confirmed by the direct measurement of tissue strength, presumably for methodical reasons given with the Instron penetrometer used.
Low risk of contamination
Overall, the contamination levels of enteric bacteria, which are relevant for human health, were comparatively low in our experiments. The hygienic quality of lettuce was only slightly affected by the experimental factors. Thus, according to our results, the risk of lettuce contamination with enteric bacteria induced by organic manure application is considered as overestimated.
Good agricultural practice, i.e. incorporation of FYM, composting of FYM and no manuring prior to ready-to-eat crops are considered as sufficient tools to keep the enterobacteria contamination of organic lettuce low.