A study of the effect of the digestibility of hay on its feeding value when given to lactating cows grazing on pasture : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science, Department of Animal Science, Massey University
Sixteen cows with an average milk yield of 20 litres per day, were randomly allocated
to two treatment groups. One group was supplemented with high digestibility hay
(57 .3% DMD) while the other group was supplemented with low digestibility hay
(52.0% DMD). Cows in both treatments grazed on pasture during the night time and
were stall-fed with either high or low digestibility hay during the day time. The
pastures were predominantly of perennial ryegrass (Lolium perene). The experiment
was carried out for 28 days in September 1988.
The two treatment groups were given a common pasture allowance of 11-12 kgDM per
cow per 12 hour period of grazing. Hay intake (fed ad libitum), pasture intake, milk
yield, milk composition, liveweight and condition score were measured.
Herbage intake was estimated by the sward cutting technique and was 3.85 and 4.30
kgDM per cow per day for the high and low digestibility hay groups respectively. The
difference between the groups in intake was significant (P<0.05). Daily intake of high
digestibility hay (8.65 kgDM per cow) was significantly (P<0.0001) greater than the
consumption of low digestibility hay (6.53 kgDM per cow). The estimated values for
daily metabolisable energy intake were 115 MJ per cow and 99 MJ per cow for the
high digestibility and low digestibility hay groups respectively. Residual herbage mass
was slightly higher (1130 v 1100 kgDM per hectare), but not significantly, when cows
were supplemented by high digestibility hay. Substitution rate for the increase in hay
intake was -0.45 kgDM pasture intake for an increase of 2.12 kgDM of hay intake or
0.21 kgDM pasture per one kgDM increase in hay intake.
Cows fed on the high digestibility hay produced slightly more milk than those on the
low digestibility hay. The difference was significant (P<0.01) in week 1 but not
significant thereafter. Yields of milk constituents were also slightly increased for cows
fed on the high digestibility hay.
Digestibility of hay had small and insignificant effects on the concentrations of milk
fat, milk protein and milk lactose. However the concentration of milk fat and milk
protein were slightly higher for cows fed low digestibility hay. Cows fed high
digestibility hay gained significantly more liveweight (P<0.05) and condition score
(P<O.Ol) than cows fed low digestibility hay.
The total intake in cows fed on high digestibility hay was significantly (P<O.OOl)
higher than in cows fed on low digestibility hay. It was estimated that hay intake
increased by 0.40 kgDM per unit rise in hay digestibility and milk production
increased by 0.23 kg milk per unit rise in hay digestibility. The increase in hay intake
and milk production per unit rise in digestibility is normally similar to other studies
with which the range response of -0.12 to 0.72 kgDM increase in intake per unit rise in
digestibility and 0.00 to 0.93 kg milk increase in milk yield per unit rise in
digestibility. However the present study was the only experiment for dairy cows
grazing on pasture.