24 November 1995

Spring growth hit by late approach

EXTRA grass consumed by cows grazing late into the autumn fails to make up for lost spring growth.

Research at Moorepark, Co Cork, Eire, has shown that delaying pasture closing date from Oct 20 until Dec 2 reduced spring grass dry matter by about 550kg/ha to 1100kg/ha. That was due to the removal of 350kg DM/ha the previous autumn by grazing after Oct 20.

Loss of grass dry matter yield at turnout was not balanced by the grass consumed in the late autumn grazing, says researcher John Roche. The findings are similar to results obtained from previous work at the Irish research centre, Johnstown Castle.

Tiller numbers unaffected

Tiller numbers over the winter/ early spring period were not affected by pasture closing date. But delaying closing reduced the amount of green leaf cover on the pastures in early spring. Early closed paddocks had a leaf area index of 2.5 in late January, while the early December closing paddocks had a leaf area index of 1.0.

For a pasture in early spring to obtain the full benefit of the limited amount of sunshine and low temperature and make maximum use of early nitrogen a good cover of green leaf is desirable, say researchers.

Grazing in spring reduced the subsequent dry matter production on the paddocks closed in early December, to a far greater extent as compared with the early closed paddock (late October). This was difficult to avoid due to the much lower quantity of herbage available, which increased grazing pressure.

When the trial finished in late May, the quantity of herbage available in the paddocks closed early in the previous autumn (late October) was much greater.

Milk production and the quantities of feed consumed by the cows for the first 12 weeks of the grazing season are shown in table 1. Cows on the early autumn closed paddocks (Oct 20) produced 50 litres (11gal) a cow more milk, consumed more grass (150kg) and less silage (25kg) than the comparable cows on the late closed paddocks (early December).

The difference in milk yield increased as the season progressed and by late May the difference in milk yield was 1.36 litres (0.3gal) a cow a day.


Table 1: Effect of pasture closing date in autumn on milk production and on DM intake of spring-calving dairy cows the next spring

PeriodEarlyLate

(weeks)ClosingClosing

(Oct 20(Dec 2)

Yield

(kg/cow/day)

1-1223.422.8

Intake

(kg/DM/cow)

Grass1-12850700

Silage1-6135160

Meals1-12175175

Total

intake1-1211601035


&#8226 The aim of autumn grazing management must be to set up pastures for the next spring. Central to this is that pasture in early January should have 4cm (1.6in) cover of green leaf. This will allow the pasture to make maximum use of early nitrogen and the limited quantity of sunshine and temperature available.

&#8226 One consequence of late grazing was that there was less herbage available in spring. This led to the cows grazing more severely and recovery was slower in those pastures and so there was less grass again in the next rotation.

&#8226 While the experiment demonstrates the disadvantages of closing paddocks too late, there is ample evidence from work at Johnstown Castle of the detrimental effect of closing too early or carrying too much herbage into the winter on spring growth.

&#8226 All pastures in the autumn should be grazed out to 4-5cm (1.6-2in) once in the October to mid-November period. This will allow access of light near the base of the sward which will influence tillering over the winter.

&#8226 The precise optimum closing date will vary from year to year depending on grazing conditions and soil conditions. As a general guide, in an intensive spring calving situation, the last rotation should start in late October with a cessation of all grazing by mid to late November.

&#8226 This year, weather and ground conditions permitting, closing date might be delayed by a few days.

Source: Moorepark

Moorepark trials show the loss in yield of grass dry matter at turnout is not balanced by the grass consumed in late autumn grazing.