tmpar <- par( mfrow = c(1,2), mar = c(3,3,0,0) )
on.exit( { par( tmpar ); rm( tmpar ) } )

lsd.plot( drink2.fit, factors = c("period","trt"),
   xpos = 1.25, xlab = "", ylab = "" )
mtext( "(a) cows in first two periods", 1, 2)
mtext( "7-day milk yield (lb)", 2, 2)

lsd.plot( drink4.fit, factors = c("period","trt"),
   xpos = 1.75, xlab = "", ylab = "" )
mtext( "(b) cows in all four periods", 1, 2)