Maintaining Milk Quality at Grass

With increased pressure to reduce total feed costs per litre, many more lactating cows will be or have been turned out to grass this grazing season.

However, maintaining milk quality through the grazing season is crucial to achieving the best price per litre possible on constituent based milk contracts. With most milk processors now demanding higher milk components, it is important to maintain litres but also produce the solids to increase income.

With one of the major processors in the UK switching from litre to kilograms of milk, the prospect of being paid on kg of milk solid rather litres could soon be reality Is this the way the remainder of the industry will go?

Dairy Cow

Increasing dry matter intakes (DMI) is the key to achieving higher milk solids. Low DMI can affect both milk fat and milk protein. Forage intakes should contribute a minimum of 60% of DMI. To achieve this conserved forage and grazing has to be good quality in order to maintain litres as well as the yield of milk solids. Fibre measured as NDF has a huge part to play within the diet.

Many high yielding herds run a total NDF percentage of the diet at grass of around 35%. NDF from forage at grass needs to be in the region of >26%. Fibre level and digestion will affect milk fat, however if DMI’s are increased on good quality forage there will be a milk protein response from the extra energy consumed.

If the average price paid for milk fat is 2.5p per percent, an increase of 0.2% milk fat on 25 litres would be worth 12.5p/cow/day. Individual farm calculations would have to be done to evaluate if reducing yield slightly but maintaining total milk fat (increasing the percentage per litre) would leave you in a better situation, however it would be suggested that maintaining litres and affecting the health and management of the cow could increase milk solids with no extra cost.

Milk fat depression at grass can be caused by many factors, from fibre intake to the type of oil being fed. Fifty percent of milk fat is derived from volatile fatty acid production in the rumen, predominantly from fibre digestion, the other 50% comes from oil within the diet and fat processed in the liver or from body reserves.

The easiest and cheapest way to affect the fat is to manage fibre intakes.

Maintaining rumen pH at grass is crucial to having a healthy rumen that promotes the fermentation of fibre by cellulolytic bacteria. The fermentation of fibre results in the production of acetate and butyrate for fat production.

Maintaining a good rumen matt, which isn’t always easy at grass will help maintain rumen pH and slow down rumen outflow meaning less undigested material passes through. SARA (Sub Acute Rumen Acidosis) is not often associated with grazing cows, however grazing highly digestible perennial rye grass which contains high concentrations of rapidly fermentable carbohydrate and low concentrations of physical effective fibre can put the rumen at risk.

Rapid fermentation and the absence of an effective rumen mat can cause an increased acid load compromising milk yield as well as milk solids. O’Grady et al. (2008) showed the incidence of SARA associated with grazing cows in Ireland by measuring ruminal pH (Figure 1).

Figure 1- Ruminal pH in grazing cows on perennial ryegrass pasture fed 2kg of concentrates

DMI Intake
L. O’Grady et al., 2008

 

 

Figure 1 shows the range of ruminal pH associated with grazing cows. Eleven percent of cows were classified as affected with SARA (pH ⩽ 5.5), 42% were marginal and affected by SARA (pH 5.6–5.8) and 47% were normal (pH > 5.8). SARA would have a direct effect on milk yield, milk quality and have an influence on fertility.

Grazing management may need to be improved on some dairy units to achieve greater milk solids, grass measuring and correct allocation of grass ensures grass utilisation efficiency is optimal, and DMI maximised.

When paddock grazing it is important to get grazing residuals down to at least 1500kg DM/ha, however this is not easy to achieve when grazing Holstein type cows but is not impossible. Taking the grass residual down to at least 1600kg DM/ha is important to allow the cow to eat the lower part of the plant which is higher in NDF.

Leaving residuals of >1700kg DM/ha would mean the cows have only eaten the leaves and very little of the stem which would mean lower NDF intakes, and higher intakes of rapidly fermentable carbohydrates that could cause an increase in acid load.

DMI Intake Picture
Figure 2- Grazing pattern of cows

As you can see from figure 2, cows would graze a typical field in 3 bites, each bite is lower in depth and lower in quality than the previous one. Ensuring the cows take the 3rd bite is integral to maintaining fat, but grass allocation has to be accurate so that DMI is not compromised.

If grass is getting ahead of the cows (<3000kg DM/ha), and if weather conditions allow, it is advantageous to pre mow in front of the cows. Pre mown grass has a higher DM which would result in improved DMI. In addition the cows would be eating the whole plant, leaving a clean paddock for the next rotation.

Buffer feeding can be used as a tool to get longer more fibrous forage into the diet promoting the formation of a rumen matt. Buffer feeding should also be introduced when grass growth on the grazing platform falls below the demand for the herd. Buffer feeding can but not always be a way of increasing DMI and to achieve higher yields.

However, buffer feeding does make cows lazy at grazing and does stop the cows taking the 3rd bite as seen in figure 2. To have the desired effect on milk fat, depending on grass quality, 3kg DM of conserved forage should be offered, this should maintain fats without compromising the cows appetite for grazing.

The ideal time for buffer feeding is prior to afternoon milking. Grass quality peaks in the afternoon which is why it is important to allow cows back to a fresh paddock after evening milking.

No buffer should be offered after milkings as this will reduce the cows willingness to graze.

Weather conditions can play a huge roll in the grazing season and also affect milk quality dramatically.

Heat stress, although not thought of as a major issue in the UK, can reduce intakes especially forage and energy intakes resulting in lower milk fat and  milk protein respectively. When the daily temperature is over 18.5°C and 65% relative humidity, the cow will be affected by heat stress.

Behaviour changes such as cows crowding in shaded areas or crowding around a water trough are indicative of heat stress.

What are the options?

  • Ensure there is enough water access in each paddock, drinking will cool the cows.
  • If cows are only grazing at day and in at night, how well are the sheds ventilated? With many sheds now fully ventilated is there an option to graze cows at night, when it is cooler on better quality grass, and keep them inside during the day?
  • Cool cows in the collecting yard, cows can get heat stressed in collecting yards and this will have an effect on the cow’s appetite for at least 3 hours post milking.
  • Heat stressed cows will choose to eat concentrates over forage. The digestion of forage in the rumen produces more heat than the digestion of concentrates.

Wet weather reduces the grazing efficiency of the cow. When the DM of the grass drops achieving the desired DMI becomes difficult. When this happens grazing residuals begin to increase and sward quality declines.

Many rely on supplemental feeding of concentrates and fat to adjust milk quality. Milk protein can easily be manipulated by increasing starch intake. Feeding starch sources along with grass can be done if the NDF within the diet is adequate.

Using starch sources with a reduced acid load is advisable if milk fat depression is to be avoided e.g. wholecrop cereals, maize silage, caustic wheat or cracked maize could all be options. Grass crude protein can be in excess of 20%, feeding supplemental protein shouldn’t be required at high levels of grass intake, feeding good quality protein with a high proportion of DUP is recommended.

Over feeding starch to grazed cows can cause milk fat depression. heifers

On good quality pastures feeding compounds high in digestible fibre (HDF) with high levels of Sugar Beet Pulp or Soya Hulls, which include pectin’s, will add energy to the diet without adding an acid load. These raw materials won’t increase the fat but they will help to maintain  rumen stability. NIS (Nutritionally Improved Straw) can also be included within compounds to balance diets out at grass. NIS has an alkaline affect that, along with the fibre, will reduce the acid load on the rumen. This leads to an increase in the production of desirable volatile fatty acids to increase milk solids.

Milk fats can also be influenced by levels and type of oils within raw materials. Reducing raw materials with high levels of saturated fat especially polyunsaturated fats, such as maize by products and confectionary by products, can have a positive effect on milk fat.

Feeding C16 fat to manipulate milk fat would look to be economically unviable at present time, feeding 300g of C16 fat (£800/t) would cost 24p/day, from the calculations made earlier an increase of 0.2% BF on 25 litres would be 12.5p/day. This clearly demonstrates management techniques and other forms of manipulating the rumen would have far greater economic returns.

For further information on improving milk solids at grass and managing swards for improved efficiency please contact one of the Dairy Specialist Team at Wynnstay.

Written by Iwan Vaughan – Dairy Technical Specialist

 

 

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