AFDC 27 (2573) DTZS  
                                                                                                                                           
@TBS 2024 - All rights reserve d                                                                                                           First Edition 2024 
 
                              DRAFT TANZANIA  STANDARD  
 
 
Code of p ractice for weed control to prevent and reduce pyrrolizidine alkaloid 
contamination in food and feed   
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
TANZANIA BUREAU OF STANDARDS  
 
 
                                                                                                                   AFDC 27 (2573) DTZS  
                                                                                                                                           
@TBS 2024 - All rights reserve d                                                                                                           First Edition 2024 
  
 
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from it may be reproduced, stored or transmitted in any form for any other purpose without prior written 
permission from TBS.  
Requests for permi ssion to reproduce this document for the purpose of use should be addressed as shown 
below:  
Tanzania Bureau of Standards  
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16103 Dar es Salaam  
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Tel +255 (22) 2450206/2450949/2450298  
Fax +255 22 2450298  
Hotline: 080 0110827  
E-mail: info@tbs.go.tz  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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@TBS 2024 - All rights reserve d                                                                                                           First Edition 2024 
 Code of practice for weed control to prevent and reduce pyrrolizidine alkaloid 
contamination in food and feed   
 
Foreword  
This Tanzania Standard was approved under the authority of the Board of Directo r of Tanzania Bureau of Standards.  
Tanzania Bureau of Standards (TBS) is the statutory national standards body for Tanzania established by the 
Standards Act Cap. 130 . 
Tanzania Standards are developed through Technical Committees that are representative of key stakeholders 
including government, academia, consumer groups, private sector and other interested parties. The Technical 
Committees work under the supervision of Divisional (sectoral) Committees. The Standards are developed in 
accordance with the Guide  and Procedure for Development of Tanzania Standards and TZS 0, Guide for 
presentation of Tanzania Standards .  
Tanzania Standards are subject to review, to keep pace with  science and  technological advances. Users of the 
Tanzania Standards are therefore exp ected to ensure that they always have the latest versions of the standards they 
are implementing.  
This Tanzania Standard was developed under the supervision of the Agriculture and Food  Divisional Standards 
Committee [ AFDC ]. The Technical Committee responsi ble for the standard is AFDC 27 - Contaminants.   
The reporting of the result of a test or analysis made in accordance with this Tanzania Standard, if the final value, 
observed or calculated is to be rounded off, shall be done in accordance with TZS 4.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 Introduction  
Pyrrolizidine alkaloids (PAs) are natural toxins occurring in a wide variety of plants. Over 6 000 plant species 
throughout the world contain PAs. Examples of  weeds containing PAs in Tanzania include  Parthenium 
hysterophorous  (Gugu karoti) and Crotoraria  species . PAs are probably the most widely distributed natural toxins 
that can affect animals  and humans.   
 
PAs have a common toxicity profile with the liver being the main target organ of toxicity. Major signs of  toxicity in 
animal species include various degrees of progressive liver damage (centrilobular hepatocellular necrosis), and 
veno -occlusive disease. Furthermore, the International Agency for Research on Cancer (IARC) has classified 
three PAs (lasiocarpine,  monocrotaline and riddelliine ) as ‘possibly carcinogenic to humans’ (Group 2B). PAs may 
differ in potency, the relative potencies are currently not known due to lack of oral toxicity data on individual PAs, 
which hampers risk assessment for PAs.   
 
Risks to humans may arise from the intake of PA contaminated food of vegetable or animal origin s and outbreaks 
of toxicity in farm animals cause economic losses to farmers and rural communities. PAs can cause food 
poisoning that can result into death.  PAs are also found in products of animal origin, e.g. milk and eggs, indicating 
the carry over of PAs from feed.   
 
Although there are gaps in the information available on the toxicity and relative potency of individual PAs and the 
contribution of different foods to overall exposure, dietary exposure to PAs should be as low as possible .  To 
achieve this, management practices aimed at the prevention and reduction of contamination of food and feed with 
PAs must be undertaken.   
 
Management practices to prevent or reduce  PA contamination of food and feed can comprise weed management 
(removal/reduction) practices to reduce exposure of food -producing animals, including livestock and bees, to PA -
containing plants, and practices to reduce presence of PAs in raw and processed commodities. This Code of 
Practice focuses on weed control. Deliberate use of PA -containing plants for foods and feed cannot be justified 
for any reason without appropriate assessment.  
 
It should be emphasised that total eradication of PA -containing plant s is not feasible or ecologically desirable. 
Under normal circumstances,  grazing animals avoid eating most growing plant species containing PAs. 
Generally, livestock graze on PA -containing plants when feed gets scarce in conditions of drought or on over -
grazed pastures. Livestock may also consume PA -containing plants when they are present in dried form in feed. 
Therefore, good feeding practice s are important in addition to  management through weed control.   
 
This Code of Practice aims to provide good manage ment practices for weed control of PA -containing plants to 
prevent and reduce the contamination of food and feed with PAs. In this regard, this code will cover control 
measures for the management of the PA -containing plant as well as measures for control o f plant release and 
spread.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 1. Scope  
This Code of Practice provide s guidance on managing PA containing weeds to prevent contamination of food and 
feed with P As. This Code of Practice should be read in conjunction with other relevant Codes  of Practice for the 
prevention and reduction of other contaminants in food and feed.  
 
2. Normative references  
There are no normative reference s in this document . 
 
3. Terms and definitions  
There are no terms and definition s in this document.  
  
4. Evaluation of com pliance with relevant legislation   
All management practices presented in this Code of Practice shall be followed in compliance with relevant 
national   legislation s and standards, including general requirement s for consumer and worker protection.   
  
 
5. General principles for weed control of PA-containing plants  
To ensure adequate prevention of the spread of PA -containing plants, and to lower the costs of control measures, 
early   identification of these plants is essential followed by action s to prevent co ntamination of food and feed.  To 
achieve early detection, raising awareness by providing good information  (e.g., the most important PA -containing 
plants, and their  ecology ) to the farmers and local population (including contractors and roadside maintenanc e 
staff) is critical .  
 
Once PA -containing plants are identified and the suitable data are available, the risks for human and animal 
health must be established in order to establish the need for an integrated weed management plan.  However , it 
must be reco gnised that different  PA-containing plants may react in different way s to a particular management 
measure. Therefore, it is   important to consider the ecology of the specific plant and the influence of weather .  To 
effectively control the spread of the PA -containing plants, all stakeholders  must take collective responsibiliti es.   
 
6. Evaluation of the need to proceed to action   
Before considering any action  to control PA -containing plants , risk assessment should be done to  identify the 
risks posed by the pr esence of PA -containing plants. This could be done by setting up a tiered risk 
characterisation approach based on:   
a) toxicity of the particular PAs, if known, present in the plant;  
b) the relevant contributions of the various PA -containing plants to the spec ific or total PA intake of the 
livestock or presence in food/feed, if known;  
c) proximity of the PA -containing plants to arable fields and meadows/pastures/grasslands;  
d) level of infestation;  
e) local circumstances;   
f) climate;   
g) soil type; and   
h) vegetation cove r of receiving land.  
  
The likelihood of PA -containing plants spreading to land used for agricultur e or grazing  should be the determining 
factor in the  assessment of the risk.   
 
As an example, principles for assessing and managing the risk s posed to live stock by ragwort ( Jacobaea 
vulgaris ), a common PA -containing plant, have been identified. These have been based on practical 
considerations of the proximity of the ragwort to pastures for livestock ( c above):  
• high risk: ragwort is present and flowering/se eding within 50 m of land used for grazing by food  producing 
animals or land used for feed/forage production;  
• medium risk: ragwort is present within 50 m to 100 m of land used for grazing by food -producing animals 
or land used for feed/forage production;  
• low risk: the land on which ragwort is present is more than 100 m from land used for grazing by food -
producing animals or land used for feed/forage production.  
In the example of ragwort control, when a “high risk” situation is identified, the guidance is  that immediate action 
should be taken to control the spread of PA -containing plants using appropriate control techniques taking account                                                                                                                   AFDC 27 (2573) DTZS  
                                                                                                                                           
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 of the status of the land. In case of a medium risk, a control policy may be established to ensure that when the 
situat ion changes from a medium to a high risk of spread, it is identified and dealt with in a timely manner using 
appropriate control techniques taking into account of the status of the land. In case of a low risk, no immediate 
action is required.   
 
Similar ri sk assessments and resulting actions could be carried out for other PA -containing plants, but noting that 
defining risk zones and appropriate actions in other situations, will require the different ecology of the relevant PA -
containing plants to be taken i nto account alongside the bullets in above .  
 
7. Recommended practices  
7.1 Management  of the presence of PA -containing plants   
7.1.1  For managing the presence of PA -containing plants, preferably a combination of non -chemical and 
chemical methods, i.e. integrated weed  management, should be applied to obtain the most effective results.    
 
7.1.2  The use of an integrated weed management plan could reduce the use of and reliance on herbicides, 
thereby lowering the chance of herbicide resistance, and allows weed management in mo st environments. 
However, it should be noted that in those cases where appropriate herbicides are available, their application s 
alone could be sufficiently effective to manage weed presence.   
 
7.1.3  Furthermore, an integrated weed management plan should be acco mpanied with practices to reduce the 
spread of PA -containing plants thereby preventing infestations from spread ing.  
 
7.1.4  It should be kept in mind for the management practices described in this section that their application s 
should not result in harmful cons equences for agriculture, the livestock or the pasture. Some methods may be 
destructive for other plant species (such as the crop) as well as to the target species. Applying these methods 
must be directed to ward  the eradication of individual plants and don e after good planning taking into account 
possible risks to the environment.  
 
7.2  Mechanical methods  
7.2.1  PA-containing plants can be controlled by mechanical methods such as pulling, ploughing, milling and 
slashing. The timing of applying mechanical methods is important. These practices are best applied before 
flowering of the PA -containing plants to prevent seed production and spread. When handling the PA -containing 
plants, suitable precautions should be taken to protect operators’ skin (contact with some plant s might cause 
allergic reaction s) and prevent inhalation of pollen.   
 
7.2.2  Effective manual control requires removal of the root crown and all larger roots. Therefore, manual control 
may only be effective for seedlings and young rosettes in contrast to bigger plants, which normally develop deep 
roots. In addition, effective hand pulling is useful for small infestations but is not cost -effective for large ones, nor 
is it suitable for large areas of land. In case of hand pulling, the plants should be handled and transported  in a 
manner that prevents their spread, e.g. in hermetically sealed bags, and destroyed (burned) afterwards. It should 
be noted that disturbance of the soil may lead to more germination since buried seeds will be exposed to (sun) 
light.   
 
7.3 Chem ical methods  
7.3.1  When applied carefully at the recommended dose s of the herbicide s, che mical spraying with appropriate 
herbicides may be an effective way of controlling PA -containing plants. Herbicides used should be registered for 
application s in that specif ic situation. Also, herbicides should preferably be used in combination with other control 
methods to increase their effectiveness. The choice of herbicide s depends on the specific PA -containing plant 
species and availability of appropriate herbicides.  
   
7.3.2  For most PA -containing plants, in general the most effective time to spray herbicides is when the plants 
are actively growing and commencing flowering, i.e. in the spring before bloom and in the autumn when they are 
applied to the new rosettes. Some herbic ides require other timing due to their mode s of action. PA -containing 
plants should not be sprayed when the plants are stressed through lack of either  water, too much water, disease, 
insect s or mechanical damage, as spray effectiveness will diminish.                                                                                                                     AFDC 27 (2573) DTZS  
                                                                                                                                           
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 7.3.3  The use of non -selective herbicides may damage the crop species and surrounding crops, pastures and 
environment. Hence, it is better to use selective herbicides or limit the use of non -selective herbicides for spray 
topping the PA -containing plant s. Further, some PA -containing plants may develop resistance against a particular 
herbicide over time. It should be ensured that active substances are registered for the specific purpose s in each 
country. In addition, as these substances are herbicides they may still have inhibiting effect s on crops, so care 
should be taken in case of possible bordering arable land.   
 
7.3.4  In case of established PA -containing perennial plants, it is better to use systemic herbicides. Systemic 
herbicides are absorbed by either roots or foli ar parts of a plant and are then translocated within the plant system 
to tissues that may be remote from the point of application.   
 
7.3.5  In addition, care should be taken that herbicides are applied in suitable weather conditions, since the 
effective concentr ation s of herbicides could be reduced when applied in unfavourable weather conditions, such as 
rainfall  within 5 hours of application.   
 
7.4 Biological methods  
7.4.1  Natural enemies of plants  may be used to control PA -containing plants. It may be an economical and  
effective method. However, efficacy must be established and the natural enemy must not present an 
environmental problem.  
 
Tansy ragwort ( Jacobaea vulgaris ) densities may for example be reduced by the natural enemies Longitarsus 
jacobaeae  (ragwort flea be etle) and a combination of Longitarsus jacobaeae  and Tyria jacobaeae  (cinnabar 
moth). Also Cochylis atricapitana , a ragwort stem and crown boring moth from Europe, was found to reduce the 
plant height of flowering plants , the size and survival of rosettes.  Another biocontrol agent used is Platyptillia 
isodactyla  (ragwort plume moth) which has a common host marsh ragwort ( Senecio aquaticus ). Deuterocampta 
quadrijuga  (blue heliotrope leaf -beetle) can completely defoliate blue heliotrope ( Heliotropium amplexic aule), with 
both the larvae and adults feeding on the leaves.   
 
7.4.2  However, good bio control is only feasible for a limited number of species as costs associated with 
finding, screening and testing potential agents can be very high. As such, successful biolo gical control requires 
extensive development and establishment phases and costs. For most of the PA -containing plants , no effective 
biological control agent is available. Research has shown that these methods are generally only very effective in 
the case o f non -native plants.  
 
7.5 Other methods  
7.5.1  Soil solarisation, flaming (burning) and use of boiling water are other controlling methods that may be 
used for small infestations.  
 
7.5.2  As there is some evidence that changing soil moisture and nutrient availability may  influence the PA 
content of the roots, leaves and flowers of PA -containing plants, cultivation methods may change the PA content 
of remaining plants. For example, increasing soil moisture will lead to higher PA -concentrations in the roots. PA 
concentratio ns are expected to be higher when nutrient availability is low, i.e. higher concentrations were found in 
plants grown in sand without nutrients than with nutrients. It is, however, not clear whether the same effect may 
be expected in flowering plants.  
 
7.5.3  Do not transport PA -containing plants unnecessarily and only when stored in hermetically sealed bags or 
containers.  
 
Not all management practices are suitable to be used on every type of land. Therefore, specific management 
practices to control PA -containin g plants are discussed separately hereafter specified by type of land: arable 
fields, pastures, and areas bordering the crop or pasture.  
 
7.6 Arable fields  
In the case of crops, the best timing of applying mechanical methods is at the start of crop growth. O nce the crops 
are dense, weeds have little chance to grow. In crops such as wheat and millet , etc., fields should be weeded 
prior to planting and periodically during the first six weeks of the growth cycle. A final weeding, about two weeks 
before harvest, if feasible, could reduce the possibility of contamination of the harvest with toxic plant parts 
significantly. In fact, in legume crops, mechanical or manual weeding may be the only option if infestation is large.                                                                                                                   AFDC 27 (2573) DTZS  
                                                                                                                                           
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 Attention should be paid to areas borderi ng the crop, as these may constitute a continuous reservoir for the weed 
infestation s.   
 
7.7 Pastures and areas bordering the crop or pasture  
7.7.1  Landowners are generally not legally responsible for the areas bordering the crop or pasture, such as 
road verges, s ides of a ditch and ruderal places. Therefore, for this type of land it is extremely important that all 
landowners, occupiers and managers take a collective responsibility to ensure that effective control of possible 
spread of PA containing plants is achie ved.  
 
7.7.2  For large -scale restorations in pastures, mowing and cutting can be more easily applied. Cutting or 
slashing tansy ragwort ( Jacobaea vulgaris ) at the start or end of anthesis will reduce the number of flower heads. 
Therefore, it is recommended to do  the first mowing when half of the plants start anthesis, and the second 
mowing when half of the re -established plants start anthesis again. On the other hand, fireweed ( Senecio 
madagascariensis ) should not be slashed in late spring or when more than 25% o f the plants are flowering, as the 
mature plant, that otherwise might have died, may begin re -shooting. However, these mechanical methods are 
not always effective in killing the plants and may even encourage them to re -shoot as is observed with tansy 
ragwo rt (Jacobaea vulgaris ) and Paterson’s curse ( Echium plantagineum).  As a consequence, slashing or 
mowing may need to be executed on a very regular basis and be applied in combination with other control 
measures as part of an integrated weed management plan.  For example, high mowing frequencies can be 
combined with the use of additional nitrogen that will lead to the promotion of fast growing grass species which 
will impair the germination and establishment of PA -containing plants.  Attention should be paid t o areas 
bordering the pasture, as these may constitute a continuous reservoir for the weed infestation.  
 
7.7.3  In pastures, PA -resistant livestock can be quite effectively used in grazing management to reduce PA  
containing plants since it may weaken the plants and prevent prolific seeding. Anti -methanogenic therapy with 
bacteria may be used to increase ruminant resistance to PA toxicity. Animals with no previous exposure to PAs 
are very susceptible to poisoning while animals with prior exposure to PA -containing plants show enhanced 
rumen detoxifying activity. The bacterium Peptostreptococcus heliotrinreducans  most likely plays an important 
role in this process.  
 
7.7.4  In addition, preferably non -food producing animals should be used  to graze on PA - containing plants  as 
PAs may transfer from feed into milk and edible tissues. The best livestock to use are sheep, especially non -
pregnant, non -food producing Merino sheep or goats. If food -producing animals are used, the edible products 
could potentially contain high levels  of PAs, and as a precautionary approach, these edible products must be 
segregated and not sold for human consumption until it is confirmed that they do not contain PAs. When 
removing animals from affected areas it is necessary to avoid transfer of seeds o n their hooves, coats and 
digestive tracts, which can infest a new area. That is, livestock can spread seeds by consuming and passing 
viable seeds through their digestive tract. The seeds that survive the digestive tract are eliminated in the manure, 
which  is rich in nutrients that can increase weed emergence. Thus, for some weed species it may be appropriate 
to prevent animal grazing when the plants are setting seeds, or the spreading of seeds by livestock can be 
prevented by placing them in quarantine. Gr azing management can be applied to low-level, widespread 
infestations. However, significant numbers of grazing animals must be available; water and fencing or herding to 
control movement must be set up and the timing, intensity and duration of grazing must  be closely monitored and 
managed to prevent overgrazing. It must be recognised that overgrazing may lead to loss of the competitive 
nature of the pasture or of native plants, allowing PA -containing plants to return and spread over the bare soil, 
which cou ld result in livestock poisoning. Hence, it is recommended to stop grazing during flowering of (a number 
of) PA -containing plants as their PA -production is then very high.  
   
7.8 Control of plant release and spread  
7.8.1  Identify  alternative plant sources to reduce  undesirable growth  
For crops, sound crop rotations can also minimise weed problems, since they will help to build up soil fertility and 
structure to produce increasing yields. Increased fertility in its turn will reduce the impact of weeds, and rotating 
crops can reduce the seeding and germination of weeds. In pastures and areas bordering the crop or pasture, 
use alternative plant sources to reduce undesirable growth, i.e. by planting vigorous perennials that will suppress 
the introduction and growth of P A-containing plants.   This should be done in accordance with Good Agricultural 
Practice, such as appropriate sowing time and depth, adequate fertility and moisture at sowing, which are 
important to ensure good pasture management. Furthermore, it is recomm ended to use agricultural methods such 
as water and nutrient management or mulching. The plant material used for mulching must be free of PA -plants 
and their seeds.  
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 7.8.2  Control movement of plants/seeds over agricultural zones and pastures  
Assure planting of  high quality, weed -free crops and weed -free grass seeds. Whe re possible use certified seeds 
by the national competent authority.  
 
7.8.3  Control plant seed movement on vehicles and agricultural machinery  
Clean vehicles, machinery and equipment that are used in infested areas to prevent introduction of the PA  
containing plant to pastures or other agricultural land by spread of seeds. Weed -free buffer zones between 
infested and un -infested lands will help to contain any infestation.  
 
7.8.4   Control plant seed movement on animals  
In the event  that livestock has grazed in infested areas, place them in quarantine for several days as seed can be 
carried on the hooves and coats, and in the digestive tracts of livestock. Inspect these quarantine areas regularly 
to assure that no PA -containing plants will start infesting those areas.   
  
7.8.5  Control of plant and seed movement from within and across borders  
Provide educational material to farmers  and other stakeholders  to correctly identify PA  containing plants to 
prevent propagati on of unwanted plant species. This information may be supported by national regulations on the 
propagation, sale and distribution of PA -containing plants. Advise the general public on how to prevent the spread 
of PA -containing plants within the country.  
 
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 Bibliography  
CAC/RCP 74 -2014 , Code of P ractice for weed control to prevent and reduce pyrrolizidine alkaloid contamination 
in food and feed