Although grass composition is undoubtedly important, the level of grass intake achieved by the cow is likely to be even more crucial in respect to overall levels of milk production achieved. It is known that a range of factors affect feed intake, but a useful and simple equation as described by Leaver (1981) describes intake as:

“ACTUAL intake = POTENTIAL intake – feed constraints – environmental constraints”

Potential intake is determined by the bodyweight of the cow as well as actual level of milk production whilst feed constraintsinclude herbage availability, quality and contamination, along with possible effects due to supplementary feeding. Environmental constraints include weather and day length, both of which affect the amount of time animals will or are able to graze freely. Grass dry matter intakes of between 16 and 18kg/cow/day, often equivalent to in excess of 100kg fresh grass/cow/day, can be achieved under well-managed rotational grazing and this is capable of supporting milk yields of 25 to 30litres/cow/day. However, it is accepted that in practice such levels of intakes and milk production will only be achieved in good grazing conditions and for a relatively short period during spring and early summer. In poorer weather, grass will typically be wetter, due to both increased levels of internal and surface water and levels of grass intake as well as milk production are likely to be considerably less than the accepted optimal. Grass intakes also typically decline as the season progresses, with increasing soil and dung contamination as well as reducing day length all contributing to reduce grazing times. There has been considerable detailed research carried out on the grazing behaviour of cows and the various factors affecting grass intakes. Grass intakes are known to depend on grazing time, grass biting rate and grass intake per bite, which all vary according to prevailing conditions.

This can be described as:

Grass DM intake = Grazing time x bite rate x bite size

With a grazing time of 600 minutes per day, an average bite rate of 60/min and an average bite size of 0.4gDM/bite, then a grass DM intake of 14.4kg DM/day is predicted according to Arnold, 1981 “60 typically cows graze for about 9 hours/day, but this can range from 7-12 hours/day”. Bite rate varies from about 45-65 bites/minute, whilst Philips & Leaver (1986) suggested that cows are constrained to a maximum of about 40,000 bites/day (11hrs grazing at 60 bites/minute). This effectively restricts the ability of the cow to compensate for any reductions in bite size which may be caused by limited herbage availability. Bite size varies considerably, typically from 0.3-0.7g DM/bite, with larger bites only possible when the cows are presented with longer, rotationally grazed grass.

Availability of forage is obviously a major factor affecting grass intake and overall levels of consumption are known to be compromised when grass stubble height is reduced. Generally, sward heights after grazing in a paddock system of 8-10cm are recommended for higher yielding cows. Continuous or set-stocked systems can often be more efficient as these tend to minimise grass wastage, but at the same time they can limit grass intakes because of the need to graze longer and/or faster to compensate for the smaller bite size typical of short, set-stocked grass.

For this reason, rational grazing is generally recommended for higher yielding cows (McGilloway & Mayne, 2002). From this, it is obvious that a number of factors can limit grass intake in practice. Very high levels of grass dry matter intake (>16kg/cow/day) can sometimes be achieved in spring/early summer, but only in ideal conditions. Typically, there are likely to be lower (12-14kg/cow/day) even though grass availability may not be a problem at this time. This reduction is considered to be due to lower grass quality which will need extra time to be processed by the cow. Much lower intakes (<6kgDM/cow/day) can occur when grass growth is reduced, typical of many areas in a hot dry summer, due primarily to declining forage availability. Finally, supplementary feeding will also affect grass intakes, the consequences of which are discussed below.

0mins x 60bites/min x 0.4gDM/bite =14,400gDM/day.

Typically, cows graze for about 9 hours/day, but this can range from 7-12 hours/day. Bite rate varies from about 45-65 bites/minute, but some suggest that cows are constrained to a maximum of about 40,000bites/day (11hrs grazing at 60 bites/minute). This effectively restricts the ability of the cow to compensate for any reductions in bite size which may be caused by limited herbage availability. Bite size varies considerably, typically from 0.3-0.7g DM/bite, with larger bites only possible when the cows are presented with longer, rotationally grazed grass.

Availability of forage is another major factor affecting grass intake and overall levels of consumption are known to be compromised when grass stubble height is reduced. Generally, sward heights after grazing in a paddock grazing system of 8-10cm are recommended for higher yielding cows. Continuous or set-stocked systems can often be more efficient as these tend to minimise grass wastage, but at the same time they can limit grass intakes because of the need to graze longer and/or faster to compensate for the smaller bite size yielding cows. From this it is obvious that a number of factors can limit grass intake in practice. Very high levels of grass dry matter intake (>16kg/cow/day) can sometimes be achieved in spring/early summer, but only in ideal conditions. Typically, they are likely to be lower (12-14kg/cow/day) even at this time of year. As the season advances, grass intakes are likely to further reduce (>10kg DM/cow/day) even though grass availability may not be problem at this time. This reduction is considered to be due to lower grass quality which will need extra time to be processed by the cow, as well as increased soil and dung contamination. Decreasing day length in the later summer and autumn will also have an effect. Much lower intakes (<6kgDM/cow/day) can occur when grass growth is reduced, typical of many areas in a hot dry summer, due primarily to declining forage availability.