In these phrases Grass means Spring.

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1598.  

        Three milky feet, a feather on his breast;
Whom seven-years-old at the next grass he ghest.
Sylvester’s ‘Du Bartas, The Handy-Crafts,’ IV. I. ii. 414. (N.E.D.)    

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1685.  A Black brown gelding,… six years old last Grass.—London Gazette, No. 2061. (N.E.D.)

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1778.  Strayed, a black Colt, three years old next grass.—Maryland Journal, March 24.

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1790.  Nebuchadnezzar will be six years old next grass.—Id., April 6.

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1804.  The noted horse, Prince Herod, is three years old this grass.—Lancaster (Pa.) Journal, April 14.

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1805.  Ethiopian is a jet black, nine years old this grass, full sixteen hands high.—Id., June 21.

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1843.  ‘Next grass,’ replied Hans, meaning thereby that at some period during the next spring he expected to boast the prerogatives of a freeman.—Knick. Mag., xxi. 524 (June).

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