Deep Rooting Clover at AgResearch

September 2012

AgResearch is developing forage legumes to improve nitrate and phosphate capture

AgResearch scientists are developing deep-rooted grasses for better interception of nitrate that might otherwise be lost to waterways and better access to subsoil moisture for improved drought tolerance.

To continue to enhance the value, productivity and profitability of New Zealand’s pastoral sector, scientists need to continue to improve our understanding of forages. Over the last 20 years, DairyNZ estimates that profits of dairy farms have increased $400/ha due to forage.

Traditionally forage plant breeders have concentrated on above ground production, but to develop more sustainable pasture systems, we need forages with improved root function.

Science team leader germplasm Dr Jim Crush says the history of forage plant breeding has been based on above ground research. Roots deliver water and nutrients to the plants and AgResearch scientists are interested in improving nutrient efficiency and drought tolerance. He says that improving root systems means less nitrogen and phosphorus is lost to waterways and that helps protect water quality. Deeper-rooted plants have better access to subsoil moisture and better drought tolerance.

Jim says that roots are vitally important, but often not considered. He says that if we are going to develop totally sustainable pasture systems he says we need improved root function.

AgResearch is screening plant germplasm for traits that contribute to improved root function and are starting to move into development of new forage breeding lines that express these traits. Some of this research will develop new plant germplasm so that pastures can become more productive, profitable and sustainable.

Key targets for forage plant root research include a reduction in nitrate leaching, more effective abstraction of subsoil moisture, higher rates of carbon sequestration in the subsoil, and improved phosphate efficiency, without penalising shoot growth for animal feed.

Jim says the first three goals can be met by having more roots at depth, either by changing the shape of root systems, or by simply having larger root systems. Root system size is heritable in perennial ryegrasses. That suggests the researchers will get a good response to selection. There are also significant differences in root system size among some contemporary breeding populations.

Jim says white clover has a weak root system that doesn’t compete with grasses that well for soil phosphate. Phosphate is a huge expense in the farm system and having to put on sufficient quantities to be available to clover as well as the grass, is inefficient.

So coming up with alternative clover hybrids that still do the job of clover in the sward but are more efficient in terms of their phosphate response is one of the aims of this part of the research.

Changing the shape of root systems towards deeper rooting is going to be more challenging because of the variation for this trait in ryegrass. The trick is selection of germplasm from appropriate environments.

Large numbers of grasses and clovers are grown in hydroponic culture, sand culture or soil. Roots are collected and scanned and the digital images are analysed for root length, thickness and branching patterns. Elite types are selected and bred, and their progeny retested.