Northland woe – grass quality in late autumn & winter assessed

Date: 2010-04-12 | Category: Dairy News

Low drymatter content in Northland pastures through late autumn and winter may be implicated in poor milk production, especially for autumn-calving herds. Rapidly growing ryegrass and kikuyu pastures can be lush but lacking in drymatter (DM), which means they could be intake restricting, leading to poor milk production.

The “mis-match” between pasture growth and milk production during spring this season is also concerning Northland dairy farmers and their advisers. Figures and observations are being collected to discern any reasons.

Long-time northern pasture assessor Graeme Piggot presented his suspicions to the Grassland Conference in Waitangi, drawing on 17 years of monthly pasture sampling from one Bay of Islands dairy farm, Te Ngaio, also visited by the conference-goers.

Piggot found that April, May and June samplings of regrowth had DM percentages equal to or below 13% in more than half the samplings. He chose 13% DM as being “low” arbitrarily but deliberately below the threshold for dairy cows of 15% set by Clark and Woodward in 2007, when low drymatter could affect milk production.

“Factors such as weather, pasture species and nitrogen applications may contribute to low DM in Northland; this could restrict drymatter intake of cows grazing these pastures and hence milk production,” he said. When searching for patterns, Piggot looked at long-term DM records for No 2 Dairy at Ruakura and found differences with his Northland records which he described as “compelling”.

Te Ngaio Farm owner Mike Guthrie, left, and sharemilkers Denise and Brett Larmer.

“Low values at Ruakura were fewer and unpredictable whereas in Northland low pasture DM% was common in late autumn and winter,” he said. “Why does feeding the cow low DM% pasture matter? From the drymatter intake viewpoint and looking at the topic theoretically, a cow eating 16kg/day DM of goodquality pasture with a 16% DM needs to consume 100kg, but this increases to 255kg if the DM% is only 6, as is occasionally recorded in northern NZ.

“The harvesting ability of the cow and its rumen volume are not limitless. Without supporting proof, it can only be a suspicion that low DM% seriously restricts intake of Northland pastures.

“But if low DM% can be proven to restrict cow intake on Northland farms, this might help explain both poor milk production and the difficulty of grazing pastures to acceptable residuals in the winter and spring.”

The latest (2008-09) DairyNZ industry statistics show Northland with 8.1% of the national dairy herds and the lowest stocking rate in the country – 2.24 cows/ha.

Production in Northland during the 2008-09 season was 644kg/ha MS and 287kg/cow, versus the national averages of 873 and 301 respectively. Piggot also advanced some possible causes of low pasture DM%, including lush growth, use of new ryegrass cultivars, and nitrogen fertiliser

Timing may also be a factor for autumn-calving herds, which are common in Northland, leading to an inability to reach high per-cow levels of milk production over winter.

This tendency for low DM% late in autumn could be considered a minor issue for spring-calving herds which are drying off when the pasture is still growing rapidly.

Piggot speculated that warmer and more overcast weather during autumn is the primary cause of low pasture DM%, and that the actual pasture DM% can be influenced by nitrogen fertiliser, regrassing and pasture species choices.

“It can be theorised that cow performance is being compromised and grazing and supplementary feeding regimes are affected, but no proof is available,” he concluded. Piggot’s paper “Low drymatter content of northern dairy pastures” was published in the 2009 Grassland Association proceedings (Volume 71).

When the Grassland conference goers visited Te Ngaio on Wakelins Road, Bay of Islands, post-grazing residuals proved the big talking point. Te Ngaio is 400ha effective, with two spring-calving milking herds of 560 Kiwi-cross cows each, milked through twin 52-bale rotaries. The primary aim is to get each cow up to 2kg/day MS and maintain as many as possible at that milking level until May 31.

Paddocks with covers of no more than 2400- 2500kg/ha DM are put in front of cows and taken down to a carefully monitored 1200kg, High stocking rates of 50-plus cows a hectare a day for short grazings and short rotations of 18 to 20 days in spring and summer are used.

By observing what has happened, plus or minus 1200kg/ha DM residuals, rather than trying to calculate intakes using pasture meters and computer programmes, cows are then topped-up to the required 17kg/day intake with maize silage. Adjustments to those supplementary intakes are made every few days, to be as responsive

Published courtesy Country-Wide January 2010

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