The inability of high producing cows on pasture to harvest enough forage during the time available for grazing is an important factor limiting their milk production. This is especially true if the dry matter (DM) content of herbage is low, such as for fresh wet spring growth which may contain only 12-16% DM. For example, a cow with a forage intake requirement of 30lb DM/day has to eat 200 lb of wet pasture if the DM content is 15%! The demands that are placed on the dairy cow to eat this quantity of forage can be more fully appreciated if they are expressed in terms of grazing behaviour. The amount of pasture consumed per day (herbage dry matter intake DMI; lb DM/d) is determined by the amount of time spent grazing (grazing time (GT); minutes/day), the rate at which pasture is taken into the mouth (biting rate (BR); bites/minute) and the amount of herbage taken with each bite (intake per bite (or bite size) BS; oz/bite). This can be written more simply as:
Intake = grazing time x biting rate x bite size
These variables, which have been quantified by grazing behaviour studies (mainly in the British Isles and France) show that grazing time for dairy cows ranges from 420-700 minutes/day (7.0 – 11.6 hours) with a median of around 510 minutes/day (8.5 hours). The amount of time spent grazing increases as the amount of pasture decreases, which is why high producing cows need to be provided with pastures at least 5 inches tall. Grazing time is also influenced by sward structure (e.g. how dense and leafy the pasture is), daylight hours and the level of milk production and cow genotype. The rate of biting for dairy cows is in the range 55-65/minutes. An Australian researcher suggested in the 1960’s that the maximum number of bites a Jersey cow could take per day was 36,000 – a cow grazing at 60 bites/minute for 510 minutes would take 30,600 bites in a day. Pasture bite sizes for European dairy cows vary from 0.022 to 0.007 oz DM/bite, and probably average 0.012 oz DM/bite over a grazing season. Variation in bite size is the most important factor determining how much pasture a cow can eat per day.
The application of these grazing behaviour principles to the DM intake requirements of a “typical” 1300 lb Holstein dairy cow producing at different levels in mid- to late-lactation are shown in Table 4.
Table 4 Aspects of daily pasture feed intake requirements for a 1300 lb dairy cow at different levels of milk production.
|(lb 4% cm)
a Adapted from 1989 NRC – Nutrient Requirements of Dairy Cattle.
b Probable DMI may be up to 18% less in early lactation
c Fresh weight of herbage consumed at 15% DM content
d Assuming GT=510 min/d and BR=60/min (i.e. 30,600 bites/d)
e Assuming GT=510 min/d and BS=0.022 oz DM (i.e. 13.2 min/lb DM)
Under pasture grazing only, dairy cows usually consume less than 3.0% of their bodyweight as DM, but in high producing cows this may be increased to the equivalent of 3.4% bodyweight. This suggests that high producing Holsteins have a potential milk production of about 60 lb 4% fcm/day if high quality pasture (44lb/day) was the sole source of energy (i.e. no body reserves are used). This corresponds to the consumption of 295 lb/day of fresh herbage with a DM density of 15% or an average bite size of 0.023 oz DM. The latter value is slightly greater than the maximum reported by researchers, but is probably physically attainable in a large Holstein at a biting rate of 55/minute and on high quality, leaf pasture. At a bite size of 0.022 oz DM the cow would need to spend 584 minutes per day grazing to achieve a 44 lb DM intake. Grazing time will increase in high producing cows. A French study, for example, found that cows rotationally grazed on ryegrass and producing 45-77 lb milk/day increased grazing time by 12 minutes for each 2.2 lb increase in milk production.
The grazing behaviour calculations have important practical applications. First, they broadly define the physical constraints to DM intake of pasture by dairy cows and clearly show that some supplementation with high energy concentrates is required in high producing herds (> 60 lb milk/d) on pastures even if ‘ideal’ herbage could be offered throughout the grazing season. Second, they emphasize the importance of grazing management in consistently providing high quality pastures to the lactating dairy cow so that she is able to take large bites of leafy herbage. This requires the intervals between grazings to be adjusted so that the pasture is at the optimum in terms of quality. It may be necessary to either make hay/silage or introduce extra supplements to the daily ration to achieve the quality of pasture required.