Battling Clover Root Weevil
Clover root weevil has infested productive pastures in many Nth Is areas. It reduces Nitrogen fixation by clover rhizobia and hence both clover and pasture growth. In an effort to compensate and avoid going broke, many farmers now topdress with fertiliser N, a little and often. This is expensive and is seen as environmentally unfriendly, so they have banded together to get some research funded on how much N is actually needed on various soil types to keep both clover and grasses going, and how much is taken up by plants, how much lost through leaching and volatilisation.
Earlier this year we ran a story on the clover root weevil, and how this little critter is devastating clover plants in many North Island pastures. Scientists had identified a parasitoid a small wasp-like insect that showed promise as a biological control for the weevil, and the Environmental Risk Management Authority has just granted an application for paddock trials.
However, its still a long way from a satisfactory solution, and a group of concerned farmers has formed to gather information and encourage research that will help them and other farmers manage the problem.
In particular, they want information on how much nitrogen to apply and how often to apply it to pastures to boost clover growth throughout the year (as distinct from boosting pasture growth early in spring).
Nitrogen and clover root weevil
About 10 years ago some farmers in the Waikato discovered that they were facing a new pasture pest -- a clover root weevil that attacked the rhizobia on clover roots and reduced their nitrogen fixing ability to almost zero. Clover will typically fix up to 400 units of nitrogen per hectare, but once the evil weevil did its damage that figure dropped away.
Clovers fix nitrogen for their own use and share it with other pasture plants. As a result of clover root weevil infestation, pastures become much less productive and clover plants are stunted and had small leaves. Clovers are normally a rich source of nutrition for stock, but the stunted plants are hard to graze.
Affected farmers were sometimes advised to put on more phosphate, potash and/or lime, but this had no effect. They coped by buying in additional feed, growing crops of turnips and/or maize, and putting on extra nitrogen fertiliser. It was a serious problem, and some farmers were slowly going broke according to Lorraine Bilby, an environmentally conscious dairy farmer from Tauhei in the Waikato.
"It was having a massive impact in our area and we needed some answers, so we held farmer meetings, formed an action group and started the quest for funding," says Lorraine.
"Several farmers had identified that if you use strategic applications of fertiliser nitrogen to replace the nitrogen that clover was no longer fixing you could get both a clover and a grass response. We believed that if you topdressed immediately after grazing with a small amount of nitrogen that it was totally used by plants."
"Up until then I had vowed that we would never used nitrogen on our property, but although we had tried organic fertilisers we had not got the required response. With weevil infestation we couldnt grow enough grass to stay in business. So we started putting nitrogen on often and in very small quantities, and that increased production by a third, which is what we had lost."
Essentially the problem is one of nitrogen depletion. Most soils have a reserve of nitrogen, but in weevil infested pastures this had been used up and had to be replaced by fertiliser nitrogen. The idea is topdress with N as soon as the cows have finished grazing an area. The amount varies at different times of the year and no topdressing is done if it is wet, but typically about 10-15 units of N is put on about every three weeks. If pasture and clover are growing well, the amount can be cut right back.
"In the past year we had been able to miss the odd round because we have built up a bit of a nitrogen bank again, which we lost during the period when there was no clover to fix any nitrogen -- you could dig up the clover plants and inspect the routes and there was no nodulation at all," says Lorraine.
"In years gone by farmers never had to take special steps to look after clovers, but farming practices have changed. They used to make hay and then feed it out and that would reseed pastures, but that is not done much any more. Grazing management has also changed and most clover never gets the chance to flower these days so you don't get natural seeding. All those things coupled with nematodes, clover flea and clover root weevil have taken their toll, but the clover root weevil is the last straw."
Fertiliser nitrogen is expensive and hugely time-consuming to apply in the small doses necessary, so farmers don't really want to do it from that point of view let alone an environmental one. However, with the recent focus on artificial nitrogen as a potential contaminant in groundwater and waterways, farmers using bagged nitrogen on infested pastures have become the "naughty boys" of the environment, according to Keith Holmes, another Tauhei dairy farmer.
"The clover root weevil is biocontrol slip-up, and when that happens you either lose that part of the economy or you do something to compensate. Clover used to supply up to 400 units of nitrogen per hectare, but now that is down to 70 or less. That makes a deficit of about 300 units, which we make up using bagged nitrogen and bought-in feed," says Keith.
"The total amount of nitrogen per hectare is no different, just the source. The lawmakers have yet to understand this, and with nutrient budgeting being proposed we need to have rules that allow farmers like us to continue farming."
Keith puts the cost of applying the extra nitrogen at around $500 per hectare per annum -- a cost that farmers would avoid if they could. Research is beginning to come up with promising partial solutions, such as the parasitoid that has been recently approved for paddock trials by the Environmental Risk Management Authority. In the longer term a new generation of resistant clovers may be bred. In the meantime, says Keith, there needs to be more work done on soil nitrogen.
"We need to understand what happens to nitrogen in different soil types, and how much nitrogen is required to grow grass to make the land economic," he says.
"We need to be able to put an economic value on the clover root weevil and on nematodes, and then we can put in place some farming practices that will give us optimum production with optimum environmental allowances. If the rules come in and don't allow us to put on enough nitrogen then we will end up with no grass."
Bruce Willoughby, an entomologist at AgResearch, is leader of a research programme to determine how much nitrogen is necessary to sustain infested pastures. They have chosen three sites in the Waikato with different soil types, and will monitor clover growth and nitrogen use.
"We will make detailed measurements of nodulation on the clovers, their response to nitrogen applied and how long the response lasts, and then we will take the major findings from that out to demonstration paddocks in Taranaki, Northland and Bay of Plenty and Hawkes Bay to see if there are regional differences. We expect to hold field days at the sites in the autumn," says Bruce.
"The farmers who have asked for this research are keen to see that all nitrogen goes into clover and grass growth and is not volatilised, leached away or otherwise lost. Both environmental and economic considerations are important, and farmers are to be congratulated for asking the hard questions and being responsible about the amount of nitrogen they are applying."
Bruce also wants feedback from farmers who have problems with the clover root weevil.
"We need to get some observations about how this pest is behaving in different environments, for example in Hawkes Bay where they have dry summer conditions how quickly there is re-infestation after a dry period, what sort of crop rotations will effectively act as a barrier to renovation," he says.
"Photographs of those sorts of observations would also be hugely helpful, and I can be contacted by e-mail bruce.willoughby@agresearch.co.nz or phone 07-8385032."
Earlier this year we ran a story on the clover root weevil, and how this little critter is devastating clover plants in many North Island pastures. Scientists had identified a parasitoid a small wasp-like insect that showed promise as a biological control for the weevil, and the Environmental Risk Management Authority has just granted an application for paddock trials.
However, its still a long way from a satisfactory solution, and a group of concerned farmers has formed to gather information and encourage research that will help them and other farmers manage the problem.
In particular, they want information on how much nitrogen to apply and how often to apply it to pastures to boost clover growth throughout the year (as distinct from boosting pasture growth early in spring).
Nitrogen and clover root weevil
About 10 years ago some farmers in the Waikato discovered that they were facing a new pasture pest -- a clover root weevil that attacked the rhizobia on clover roots and reduced their nitrogen fixing ability to almost zero. Clover will typically fix up to 400 units of nitrogen per hectare, but once the evil weevil did its damage that figure dropped away.
Clovers fix nitrogen for their own use and share it with other pasture plants. As a result of clover root weevil infestation, pastures become much less productive and clover plants are stunted and had small leaves. Clovers are normally a rich source of nutrition for stock, but the stunted plants are hard to graze.
Affected farmers were sometimes advised to put on more phosphate, potash and/or lime, but this had no effect. They coped by buying in additional feed, growing crops of turnips and/or maize, and putting on extra nitrogen fertiliser. It was a serious problem, and some farmers were slowly going broke according to Lorraine Bilby, an environmentally conscious dairy farmer from Tauhei in the Waikato.
"It was having a massive impact in our area and we needed some answers, so we held farmer meetings, formed an action group and started the quest for funding," says Lorraine.
"Several farmers had identified that if you use strategic applications of fertiliser nitrogen to replace the nitrogen that clover was no longer fixing you could get both a clover and a grass response. We believed that if you topdressed immediately after grazing with a small amount of nitrogen that it was totally used by plants."
"Up until then I had vowed that we would never used nitrogen on our property, but although we had tried organic fertilisers we had not got the required response. With weevil infestation we couldnt grow enough grass to stay in business. So we started putting nitrogen on often and in very small quantities, and that increased production by a third, which is what we had lost."
Essentially the problem is one of nitrogen depletion. Most soils have a reserve of nitrogen, but in weevil infested pastures this had been used up and had to be replaced by fertiliser nitrogen. The idea is topdress with N as soon as the cows have finished grazing an area. The amount varies at different times of the year and no topdressing is done if it is wet, but typically about 10-15 units of N is put on about every three weeks. If pasture and clover are growing well, the amount can be cut right back.
"In the past year we had been able to miss the odd round because we have built up a bit of a nitrogen bank again, which we lost during the period when there was no clover to fix any nitrogen -- you could dig up the clover plants and inspect the routes and there was no nodulation at all," says Lorraine.
"In years gone by farmers never had to take special steps to look after clovers, but farming practices have changed. They used to make hay and then feed it out and that would reseed pastures, but that is not done much any more. Grazing management has also changed and most clover never gets the chance to flower these days so you don't get natural seeding. All those things coupled with nematodes, clover flea and clover root weevil have taken their toll, but the clover root weevil is the last straw."
Fertiliser nitrogen is expensive and hugely time-consuming to apply in the small doses necessary, so farmers don't really want to do it from that point of view let alone an environmental one. However, with the recent focus on artificial nitrogen as a potential contaminant in groundwater and waterways, farmers using bagged nitrogen on infested pastures have become the "naughty boys" of the environment, according to Keith Holmes, another Tauhei dairy farmer.
"The clover root weevil is biocontrol slip-up, and when that happens you either lose that part of the economy or you do something to compensate. Clover used to supply up to 400 units of nitrogen per hectare, but now that is down to 70 or less. That makes a deficit of about 300 units, which we make up using bagged nitrogen and bought-in feed," says Keith.
"The total amount of nitrogen per hectare is no different, just the source. The lawmakers have yet to understand this, and with nutrient budgeting being proposed we need to have rules that allow farmers like us to continue farming."
Keith puts the cost of applying the extra nitrogen at around $500 per hectare per annum -- a cost that farmers would avoid if they could. Research is beginning to come up with promising partial solutions, such as the parasitoid that has been recently approved for paddock trials by the Environmental Risk Management Authority. In the longer term a new generation of resistant clovers may be bred. In the meantime, says Keith, there needs to be more work done on soil nitrogen.
"We need to understand what happens to nitrogen in different soil types, and how much nitrogen is required to grow grass to make the land economic," he says.
"We need to be able to put an economic value on the clover root weevil and on nematodes, and then we can put in place some farming practices that will give us optimum production with optimum environmental allowances. If the rules come in and don't allow us to put on enough nitrogen then we will end up with no grass."
Bruce Willoughby, an entomologist at AgResearch, is leader of a research programme to determine how much nitrogen is necessary to sustain infested pastures. They have chosen three sites in the Waikato with different soil types, and will monitor clover growth and nitrogen use.
"We will make detailed measurements of nodulation on the clovers, their response to nitrogen applied and how long the response lasts, and then we will take the major findings from that out to demonstration paddocks in Taranaki, Northland and Bay of Plenty and Hawkes Bay to see if there are regional differences. We expect to hold field days at the sites in the autumn," says Bruce.
"The farmers who have asked for this research are keen to see that all nitrogen goes into clover and grass growth and is not volatilised, leached away or otherwise lost. Both environmental and economic considerations are important, and farmers are to be congratulated for asking the hard questions and being responsible about the amount of nitrogen they are applying."
Bruce also wants feedback from farmers who have problems with the clover root weevil.
"We need to get some observations about how this pest is behaving in different environments, for example in Hawkes Bay where they have dry summer conditions how quickly there is re-infestation after a dry period, what sort of crop rotations will effectively act as a barrier to renovation," he says.
"Photographs of those sorts of observations would also be hugely helpful, and I can be contacted by e-mail bruce.willoughby@agresearch.co.nz or phone 07-8385032."