Friday, March 29, 2024

To top or not to top

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In my last two articles I have discussed the major influence that pasture quality has in ensuring the highest achievable production levels in pasture-fed dairy herds in the latter part of the milking season. Having firmly established this precept the logical follow-up is to look at the principles and practices for maximising summer and autumn pasture quality on an ongoing basis. The first of these is topping. Many will disagree with this, as I’ve not infrequently heard it stated that the practice of topping is an indication of failed pasture management. In my view the opposite is the case – the failure to top when it should be done is definitely an indication of failed pasture management. Given the normal seasonal fluctuations in pasture growth, it‘s unrealistic to expect that on-farm pasture growth will constantly be evenly matched to cow requirements throughout the year. There will be times when growth rates will get ahead of feed requirements.
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Given the whole thrust of summer/autumn pasture growth will be on its reproduction (stalk and seed head formation), when pastures have bolted away it will be unlikely they can be brought back under control by grazing alone. In attempting to do so it’s likely that while one paddock is being put back under control, another two are getting away.

It’s important to remember that the pastures are there for the benefit of the stock, and not vice versa. It’s also important to have a clear idea of the objectives behind the topping operations. These are quite likely to differ. In the late spring/early summer the prime objective will be the prevention and elimination of stalk and seed head formation. But in late summer/autumn it is much more likely to be weed control, in which case it will be much more critical to have the mower blades set as low as possible. 

California nightmare

While on the subject of weed control, if I was asked what was the worst weed on New Zealand dairy farms I would have no hesitation in nominating California thistle, which, like many others, will be at its worst in summer and autumn. California thistle is at its most destructive when its invasion is such that the population density prevents stock from moving among it to graze the surrounding grass. This has the effect of not only removing large areas from grazing, but also allowing the grasses between the plants to become long, rank and unpalatable. This creates a much more open pasture when the thistles eventually die back in late autumn /winter, creating a favourable situation for the invasion and survival of other weeds. While topping calis may be of only limited success in terms of long-term control it can be very important in short-term pasture management and utilisation.

The common criticism of summer/autumn pasture topping is the fear that it may destroy feed reserves that would be critical if drought conditions appear later There‘s not much justification for this. If the topping is really needed then the bulk of the herbage being cut will be rubbish of little feed value. In fact, if the topping has been done with a mower (as distinct from a rotary slasher or topper) some of the toppings may well be rendered more palatable after a period of wilting and so could be cleaned up by stock. If left standing they would be completely wasted. 

While there is the risk that hot drying winds will dessicate a short pasture more severely than a longer one, a pasture shortened by overgrazing will be much more at risk than one shortened by topping. 

Rolling regrowth

The best way of reducing the risk is to adopt a rolling programme of topping over the whole summer and autumn. When the herd has finished grazing a paddock it’s assessed as to whether or not it needs to be topped. If it does then it should be done in the next day or so, before there is any significant regrowth, as a second hit of defoliation on this would certainly inhibit recovery and regrowth. 

Under this regime there will be a continual supply of fresh leafy pastures coming back into the grazing round. A variation on the theme is to top half the paddock and leave the other half until the next round, a practice that has both advantages and disadvantages. The key point though is to adopt the graduated softly, softly, approach rather than waiting until much of the farm is totally out of control and then having a rush of blood to the head and tearing out and topping half the farm.

Now let’s look at pasture control by grazing, which superficially looks like an easier and cheaper option. But it won’t be if it’s actually costing a dollop of foregone production. Of course, many farms will have paddocks where topping is not possible due to slope, stones or other restrictions. 

The secret in these cases comes back to the old sayings of prevention is better than cure, or a stitch in time saves nine – take your pick. The aim is to not let these paddocks get away in the first place, by continually applying the appropriate grazing pressure. 

If this means that pasture control on the best (toppable) paddocks has to be sacrificed, no problem. They can easily be brought back under control with the topper.

The important thing is that if the cows are being grazed harder to tidy up an out of control pasture they need a very good feed of quality pasture at the next grazing. Cows will tolerate the occasional one off underfeeding as the gut volume allows a fair bit of carry-over capacity. However continuous harder grazing, giving ongoing unrelenting underfeeding, will very soon result in significant and ongoing production losses.

How do you know when overgrazing is occurring?

Watch the cows when they go into the paddock. Their first preference will be the fresh regrowth on the shorter pasture. After largely cleaning this out they will, as second choice, start nibbling on the more clumpy bits that were left after the last grazing. Well and good – this is what you want them to do.

The danger point comes when they stop grazing the clumps and start having a second bite at the short stuff. This residual will be characterised by a good proportion of clovers, which, being more prostrate, will have to an extent escaped the first, more lenient grazing, Clovers are more palatable than grasses, so the stage will be reached when the cows will prefer to nibble at the short sweeter stuff, in preference to the easier grazed, but coarser, rank stuff.

Unlike grasses the clover population survives and prospers by use of over ground stolons. When the cows are grazing the short stuff so hard that these are being eaten then the alarm bells should be really ringing. This level of grazing will be doing no good at all to either stock or pasture. It will retard pasture regrowth, reduce the essential clover population, open out the pasture thus increasing susceptibility to drying out and weed invasion, and fail in not actually cleaning out the rank clumps to a satisfactory extent.

When the cows are having a second bite at the short stuff beware. And when it’s a third or subsequent graze, and the clovers, particularly their stolons, are taking a hammering, it’s definitely time to get the cows out of there and do any further tidying up with the topper.

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