Deferred Grazing

Legumes in pasture provide high-quality food for animals and nitrogen fixation. However, on New Zealand’s 6 million hectares of hill country, legume content is often maintained through regular over-sowing (broadcasting seed onto paddocks accompanied by fertiliser). 

An AgResearch field study underway in Waikato and the Bay of Plenty is asking - could legumes be encouraged to persist in hill-country pasture by implementing strategic grazing techniques, such as deferring grazing for a short period over summer and grazing intensively in spring?  

Building on research from the 1950s, the field study is working out what’s happening with the pasture production and roots in under grazed pastures and gathering all that information the implications for up into a farm system. With new technologies now available, scientists can get a much more accurate picture of the impact of this decades-old technique. 

Deferred grazing is a management tool to help maintain pasture quality from mid-spring onwards. It involves dropping some paddocks out of rotation to optimise grazing pressure on the remaining paddocks, so pasture quality is maintained.  

On a summer dry beef and sheep hill country farm northwest of Hamilton, it was estimated that deferred grazing on 15% of the farm increased total farm and per-hectare gross margins by 8% (FARMAX modelling). Deferred grazing is also a low risk and cheap tool to rejuvenate pastures. 

It is about letting plants rest, replenish and re-seed, AgResearch Senior Scientist Katherine Tozer says. “It’s plugging gaps where we've been able to come to a greater understanding of what's happening with the resident plant population, what's from receding plants reseeding growth and what's from new tillers from the existing plants. And how does it affect their energy reserves and the plants and root growth, which is so important for pasture persistence?” 

It’s also about coming up with some decision rules for farmers about when to start deferring so they can get the best for their paddock and for their farm. Tozer says farmers are saying, “we’re lacking tools with a variable climate, variable feed flow variable market. We're lacking tools to manage the feed supply”.  

Some farmers throughout New Zealand have been using deferred grazing successfully, “but the way that the industry has been, understandably, there's a focus on high quality feed and therefore maintaining all paddocks that are green leafy vegetable state to maintain the feed quality.” 

Deferring grazing allows plant material to go to seed so you get a reduction in quality. That’s a challenging idea from a farmer and industry perspective: why would you let that paddock go to seed and to lose quality?  

Tozer says this overlooks the fact that you actually get a lot more good growth in that paddock after it's been deferred because you're storing up energy which gets converted into growth in the year after it’s deferred.  

“So instead of the plant putting the energy into regrowing leaf material, it's just accumulating in all parts of the plant. It's going into the roots so that in the autumn, after the crappy deferred feed has been grazed off. You get a lot of regrowth and in many cases get benefits for legumes as well. So, in the long run, at the farm systems level, what you're doing is locking up your surplus.” 

For the rest of the farm, you're matching your feed demand and your feed supply. And at the farm level you're improving the quality of the farm scalefeed and livestock performance, she says. You also created a buffer of feed if there is a drought. It means that, in this case, going to seed isn’t a bad thing. 

“Seeding can have a huge impact. In a paddock just recently, for measuring the number of seedlings coming up, we were able to modify the amount there. We found that seeding resulted in the equivalent of 60 kg of oversown ryegrass seed, assuming every single seed that had been over-sown had germinated and produced a seedling. And of course, if you're over-sowing hill country, you’d be really, really lucky to get 10% of the over-sown seed germinating and establishing.” 

Through glasshouse and field work, researchers found out how important deferred grazing is for accumulating water-soluble carbohydrate reserves in plants and restoring root growth. 

“It’s such a simple, easy tool to apply and we can adapt it for the pastures in a particular paddock or a particular farm. So we're learning based on old research and using that as a platform to learn more, that you can change the timing of when you start the deferred period and when you end it, to weaken the species that you don't want there as much – and to rest and replenish the reserves, the plants that you do want there.” 

Tozer says part of a plant’s persistence, as a survival mechanism, is to allow those reserves of water-soluble carbohydrate which are used for growth to accumulate in the plant, to allow the roots to grow down and grow bigger. 

“Sure, they might be going to seed and not growing much, but the subsequent autumn, after the deferred pasture is removed, it's like a plant on steroids,” she says. 

The plants might look ugly for a while, but they’ll really kick on. Bill Garland near Maungatautari did one of these trials in his front paddocks and a neighbour gave him a little bit of a flack.  

“It looks shocking, but it’s come away really well. You will be promoting the growth of the competitive pasture species like ryegrass, that will dominate over others. If you had a paddock that was all Kikuyu or all yellow bristle grass and you lock that up, you're not going to suddenly get ryegrass and white clover, so you need to kind of use some common sense in terms of is this paddock appropriate to defer, or is it really poor in terms of what’s there and it’s not going to magically recover.” 

Having said that, it can be low and quite weedy, but if the fertility’s good and you have the right class of stock to graze it off, you can get a heck of a lot of recovery, Tozer says. 

“If anything, the impact of the deferred grazing will last longer in dairy pastures than in beef and sheep hill country, because the flatter land is so much more conducive for ryegrass persistence - and the benefit also persists longer.” 

To measure this growth in the root and plant, researchers use rhizotrons, clear plastic tubes inserted below ground. You put a camera down them and take repeated measures of the root so you can see how they grow over time. 

Tozer says the deferred grazing study is backed by good, sound ecological science and principles. “And therefore, it fits into a conventional framework because people might be using herbicides, fertilisers - standard grazing practice. But this is saying, for this season of the year when it's so critical to refuel the plant, to replenish the reserves, allow those roots to get down, you need to rest it. It's still consistent with a conventional framework, and it's also consistent with a regenerative framework as well.” 

There will always be regional differences in field testing of this sort, Tozer says, but the same principles apply to pastures based on Cocksfoot, Phalaris, and Microlaena, which is indigenous to both New Zealand and Australia. 

“It’s this whole principle of just resting to replenish the reserves, store the roots and refill refuel the regrowth. It’s something that's been around for years, but we're just understanding how it works a little bit more. And we’re working with farmers and rural professionals in the pastoral industry, pooling that knowledge so that we can come up with recommendations that are as helpful for farmers as possible.”