Four experiments were conducted with lactating dairy cows offered a hay and concentrate diet (0·5:0·5) to investigate the effects of (1) the frequency of feeding a completely mixed diet (experiment 1) compared with feeding the concentrate fraction and the roughage fraction separately (experiment 2), and (2) the addition of sodium chloride to a completely mixed diet (experiment 3), on the fractional rate of outflow (FRO) of chromium (Cr)-treated fish meal from the rumen, and on milk yield and composition. The cows were offered the diet at either twice the maintenance requirement (experiments 1 and 2), or 2-5 x maintenance (experiment 3) in a 4 x 4 Latin-square design. The effect of the particle size of the Cr-treated soya bean meal was investigated in experiment 4.
The frequency of feeding of the completely mixed diet had no significant effect on the rate of outflow of Cr-treated fish meal from the rumen, or on milk yield or composition. FRO per h were 0·070, 0·085, 0·079 and 0·086 when the diet was offered once, twice, four times or 12 times per day respectively. Increasing the frequency of feeding of the concentrate fraction of the diet had no significant effect on FRO. FRO per h were 0·073, 0·078, 0·081 and 0·081 when the concentrate fraction was offered once, twice, four times or 12 times per day respectively.
The addition of NaCl to the diet significantly increased water intake (P < 0·001), but had no significant effects on FRO or milk yield. FRO per h were 0·074, 0·075, 0·076 and 0080 when 50, 265, 529 or 794 g of NaCl were added into the diet respectively. The respective intakes of water were 66·6, 74·1, 88·4 and 101·6 kg/day.
The FRO per h of fine particles of Cr-treated soya bean meal was 0·085 and for coarse particles, 0·096. The difference was not significant.