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  • Writer's pictureMark Hunt

May 23rd - Do PGR's apply a selection pressure to the grass species on your surfaces ?



Hi All,


In last week's blog I was comparing the output from the GFS and ECMWF for this week and wondering which of the two contradictory forecasts would turn out to be accurate and I'd say a bit of both on balance.


There's some strange features in the weather at the moment with a really lackadaisical jet stream. This is allowing weather systems to move from east to west when normally we see the opposite, with weather systems pushing in from The Atlantic. I think this slow jet stream speed is adding to the complexity of forecasting past the 5-day sort of point and making life tricky for meteorologists. So whilst we will get some warmer, settled weather for a time at the end of this week, it won't last for long as a cooler, low pressure system will push in from the east and affect continental Europe and the U.K & Ireland.


On a nature theme, I mentioned awhile back about the late arrival of Swallows, House Martins and Swifts in this locality and it appears that I was correct because this year the RSPB have confirmed that they were 10 days later than normal due to poor weather in southern Europe (wet, cold) and a predominance of easterly and northerly winds which work against their migration. Numbers are definitely down as well, particularly Swifts and House Martins. I wonder if the jet stream is playing a part here as well ?

On a more positive nature note, I saw my first two Painted Lady butterflies at the weekend whilst out walking. Like the Swallows and Martins, these migrate along part of the same route from northern Africa and southern Europe to the U.K. Difficult to comprehend how something as fragile as a butterfly can cross thousands of miles isn't it ?


You can read about them on the Butterfly Conservation website here



I also mentioned last week that we were likely to have some thunderstorms last Wednesday / Thursday and so it came to pass with some big lightning and torrential rain in places. The ATD lightning data from netweather.tv shows the areas affected and the timing of the strikes. We picked up 30 mm plus here overnight from 18th - 19th of May. I expect there was some significant nitrogen in that rainfall with the conversion of NO2 - NO3 in the atmosphere. It sure greened things up big time in the garden !


OK, time to put some detail on the weather ;


General Weather Information


So as predicted last week, the start of this week is cool and unsettled with rain currently affecting the south east of England, north west of Scotland and the west of Ireland. Monday will see these showers consolidate through the day across Ireland, the west coastline of the U.K and across the south east and east of England, with later rain moving inland across all areas. A gentle westerly wind and cloud cover will keep temperatures around the mid-teens.


Tuesday starts off drier but we will see a re-occurrence of showers forming in the 2nd part of the day, this time along the easterly coastline of the U.K and Ireland, with western and central areas drier. The wind picks up a little on Tuesday and swings round to the north west and again that'll keep temperatures down to the mid teens, 15-17°C sort of area.


Wednesday sees rain push across Ireland overnight and into the west of Scotland, some of this will be on the heavy side. By mid-morning, it'll clear Ireland and push into Wales, The South West, The North West extending all the way up to Scotland. This consolidated band of rain will then move eastwards across Scotland and northern England but lessen in intensity as it does so. Remaining cool despite the wind coming from the west / south west. 15-17°C again the norm.


Thursday sees high pressure nudge into the south of England and that means any rain will be more north and north west. On this note, heavy rain will push into the northern half of Ireland overnight and maybe extend from Mayo to Dublin before pushing north eastwards later in the morning towards the west of Scotland. Tricky to put an exact line on it but it looks mainly to affect the north and north west. Across The Irish Sea and we have a more settled situation with a drier, windier picture and temperatures pushing up into the high teens.


Closing out the week on Friday and a drier picture for the UK & Ireland overall with just some showers over the north west of Scotland to blot the copybook. These showers will cross Scotland during the course of the day. Normally high pressure at this time of year means warm temperatures but due to the position of the weather system, it'll drag down north westerly winds so remaining on the cool side as we see out the last days of May with 15-17°C the norm for this week.


The weekend looks dry and sunny for all areas, plenty of cloud about as well though and still that cool, north westerly wind keeping temperatures on the cool side. This wind will swing round to the north east for Saturday so still remaining cool.

Weather Outlook


At the end of this week we have high pressure pushing up, but on the GIF above you can see why this isn't likely to remain into the start of next week. That mass of cooler air is moving westwards against the normal run of play weather-wise and it'll continue to pull down those cooler northerly winds as it does so. So cool and unsettled at the start of next week with showers down easterly coasts. These look to persist through the first part of next week and then we have the distinct possibility of a BOB coming into play. In my books whenever we have a slow jet stream, we increase the probability of low pressure systems pushing up from The Bay of Biscay (hence the acronym BOB) and affecting the southern half of the U.K and Ireland. I think this is an increasing feature of our weather and it could mean a cool and pretty wet end to next week.


Time will tell.


Agronomic Notes



Daily Growth Potential in May


Last week I mentioned that we were experiencing a real growth flush which incidentally isn't unusual in May so to start off this week, I thought I'd put some more detail on it.


Above are the Growth Potential charts for 3 locations across the U.K and you can clearly see the flush that occurred across England (and Wales) during the mid-part of the month with G.P nearly at optimum. For Scotland and likely Ireland, there wasn't a pronounced flush as we have see further south (east) but growth levels were / are still good with an average G.P of 0.6 - 0.7.


With a cooler wind direction from mid-week, we can expect things to settle down a bit as the daily G.P drops to around 0.6. This will make growth a good bit more manageable, especially on higher height of cut areas.


Since we are well into the Poa annua seedhead flush, many of you would have seen this translating to a lot of seedheads as Poa annua kicked into life after a very dry end to April / start of May. Fortunately this flush will actually shorten the period of seeding because Poa annua currently is growing well with the combination of both temperature and moisture. I normally work on the seedhead flush lasting around 200 GDD in total, so on that basis we are starting to come to the end of it at some locations. That statement though is entirely conditional on the type of Poa annua (biotype) that you have present.


PGR performance and GDD in May


Below is a track of GDD from the start of April to show how the growth has panned out and where we are currently ;

You can see how May has been a really productive growth month from the near straight line on the cumulative GDD graph. It is and has definitely been a month for applying PGR's !


Now to make matters slightly confusing we have 2 different A.I's on the market, the old stager, Trinexapac-ethyl, which has been around since the days it was marketed as Shortcut in soluble (or sometimes not so soluble) sachets and Prohexadione calcium, the new kid on the block.


Work from the U.S claims different longevities for these two PGR's with applications based on 200 and 300 GDD respectively. These GDD figures utilise a base temperature of 0°C in their calculation, whereas I've always worked on 6°C. The reason for the difference is that I think a growth model should measure just that, growth, and at 0°C, we don't get any. it's not until we reach 6°C that we start to see cool season grass growth.


Now we don't have to get hung up on the maths too much as 200GDD at 0°C translates roughly to 130GDD at 6°C. Likewise 300GDD at 0°C translates roughly to 170GDD at 6°C. You choose which system works for you but bear in mind if you're using GDD to show potential growth, a model using 0°C doesn't do that.


From a PGR perspective (in this case TE) and working on re-application every 200GDD (0°C base) / 130GDD (6°C base), that means a TE application made on the 1st of May will be running towards the end of its effectiveness around the 16th of May. Since most superintendents are applying nutrient + PGR on a fortnightly basis, this should be working well in holding back growth without a rebound effect.


Interestingly, if we look at Prohexadione calcium (PHC for short to save me typing it !), the data from the U.S claims 300 GDD longevity using a 0°C base temperature in the calculation which roughly translates to 170GDD using a 6°C base. Looking at the GDD chart above, an application of PHC made on the 1st of May would have reached 300GDD (0°C base) / 170GDD (6°C base) on the 22nd of May, so that's just over 3 weeks and in line with the label.


Now just as we are looking to adopt GDD / PGR models, there's already an argument in the U.S to move away from them because of the interaction between PGR's and DMI fungicides like Propiconazole (which also have a PGR effect on the plant).


It is an interesting debate and you can read about it here


Does the argument hold much sway over here ?


Well I think less so because over there they have a lot of different DMI fungicides and a lot of different PGR types as well. We don't, we only have 2 DMI fungicides A.I's, Tebuconazole and Difenoconazole and I don't think their effect on growth at the rates we apply is significant.


So currently I think GDD models have their place but one thing I'm wondering is if they really relate to what we see on our surfaces from a growth suppression perspective ?


For those of you measuring clipping yield, do you see a big increase in clip if you go beyond the application dates of your PGR and a big reduction in clipping after you apply your PGR ?


Do PGR's apply a selection pressure on your surfaces ?


I ask this because I'm convinced that over time a PGR will have a lesser effect on your grass sward due to a process known as natural selection. It is logical. You apply a PGR to a mixed sward of grasses, some are more regulated than others. The less regulated grass species will grow better than the more regulated. The less regulated will therefore out-compete the more regulated. Over time the sward dynamic / mix will change to the species that is less regulated becoming more dominant. For this reason, the same PGR application at a given rate will be less effective. I think we have been seeing this in the field for a good while now and it's also why I think it is a good thing that we have more than one type of PGR A.I available here in the U.K & Ireland. There's certainly a place for both.


Ok that's enough chin scratching for the week :)


All the best.


Mark Hunt

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