Gradual turnout - turning cows out to grass creates a substantial change in diet for the cow. It takes around three weeks for the bugs in the rumen to adapt to significant diet changes such as this, so it is important to manage the transition to grazing gradually. Even a few hours of on/off grazing by day, when weather conditions allow, will mean that the rumen bugs can adapt to fresh grass. Cows should be able to consume 5kg DM in approximately 3 hours in suitable swards and weather conditions.
Dry matter intakes – The moisture content of grass can vary significantly in spring, typically from 80-90% moisture depending on the weather, and this can have a major impact on dry matter intakes. For every 1kg of fresh grass a cow eats at 15% DM she will only be consuming 150 grams of dry matter so this means that a cow estimated to consume 15kg of grass dry matter needs to eat 100kg of fresh grass. Remember, there is no energy in the water/moisture content of grass, therefore, it is important that you don’t overestimate what dry matter intake a cow can take from grazing, or performance and fertility will be compromised.
Excess crude protein – Lush, leafy spring grass can often have a crude protein content in excess of 250g/kg DM, particularly after fertiliser application, and this is mainly rumen degradable protein (RDP). Rumen microbes are unable to utilise this much protein from high grass intakes, particularly if there is a shortage of fermentable energy available to them, and so excess RDP is broken down into ammonia in the rumen, and then absorbed into the blood stream and converted to urea in the liver. Elevated blood urea nitrogen levels (BUN) from excessive crude protein in the diet can increase body condition score loss, reduce fertility and impact on hoof health.
Buffer feeding – During the transition to grazing when grass is gradually being built up in the diet, high D-value forages such as maize and/or grass silage should be fed while cows are initially in by night and out by day, to ensure dry matter intake is maximised. Only forages with a high energy content and digestibility should be fed, so that cows are supplemented to their required energy requirements and grazed grass is not substituted. Starch-based forages such as maize silage are a great combination with grass, as the use of nitrogen in the rumen is enhanced and microbial protein synthesis is increased due to the fermentable energy being supplied by the maize starch. This will help to maintain milk constituents and protein, in particular. Buffer feeding should also be strategically implemented during periods of wet weather throughout the grazing season when required grass dry matter intakes cannot be maintained due to low grass dry matters. In order to maximise the use of grass during wet weather, buffer feed should be fed after cows have grazed and ideally a few hours before milking.
Highly digestible grass swards can challenge rumen function - Lush spring grass tends to have a high proportion of leaf to stem, resulting in low structural fibre levels in the overall diet. This lack of ‘scratch factor’ can impact on cudding rates and saliva production, further compromising rumen function. While the nutrient analysis of grass can vary wildly, this lack of structural fibre can be accompanied by high sugar levels – often more than 18% during sunny, dry weather. High sugar levels are great for rumen fermentation, promoting good milk proteins and strong yield, but when supplied in excess in combination with low structural fibre they can challenge rumen function, leading to sub-acute rumen acidosis (SARA). Consequently, butterfat % and milk protein % can be compromised as a result of the change in rumen fermentation, whilst prolonged challenges can impact on fertility. High levels of unsaturated fatty acids in spring grass can also cause butterfat % to be reduced, so lower butterfat doesn’t always mean SARA is a problem, however.
Compound feeding – It is important that compound feed is fed to top up the dry matter that grass and forages do not supply in order to match the energy requirements for a given yield. It is also important that the nutrient content of the compound feed balances that of the grass to optimise rumen fermentation and maximise performance. Aim for a feed that has around 14-16% crude protein, a high digestible fibre content (such as sugar beet pulp and soya hulls), a balanced source of cereals including maize and barley and a source of bypass protein. It should also contain minerals that grass is deficient in, for example magnesium, which as well as having a vital role from a grass tetany prevention point of view, can lead to poor rumen muscular function, and hence sub-optimal rumen function, if deficient.
Monitor what the cows are telling you – assess rumen fill 2-3 hours after milking to determine whether adequate grass has been allocated; monitor cudding rate – you are looking for more than 65% of the herd to be lying down chewing the cud 2-3 hours after milking; check dung consistency – loose, bubbly dung with undigested fibre in it is indicative of poor rumen function, as is the presence of cud balls in collecting yards or cubicles; monitor condition – cows losing body condition can point to insuficient feed intake, a possible metabolic disorder, health issue or sub-optimal rumen function.
Milk quality – Monitor bulk tank milk collections for average yields and constituents. A fall in butterfat or protein of 0.3% or greater in one week is a warning sign for poor rumen function and the occurrence of SARA. It is also useful to keep an eye on the butterfat to protein ratio to ensure this falls within the optimum range of circa 1.2:1. Look back at last year’s individual milk records for cows at different time points during the spring grazing season to see what percentage of the herd had butterfat to protein ratios outside of the optimum range to identify bottlenecks ahead of this season. This should be an ongoing process this spring after each milk recording, as it is more accurate than looking at bulk tank records, which are a more general initial indicator.
Feed Actisaf live yeast – Adding Actisaf live yeast to your cows’ ration will reduce setbacks in performance at turnout by helping the rumen bugs adjust to grazed grass and improving rumen function. Actisaf also reduces the risk of SARA, both at turnout and throughout the grazing period. Actisaf helps to stabilise rumen function and promotes milk solids and milk yield and can be included in compound feed or added to blends or on-farm mixes as a farmpack.
Summary:
- Turning cows out to grazing is a substantial change in diet and it takes around 3 weeks for the rumen bugs to adapt. Gradual turnout will help.
- Spring grass can challenge the cow as it is highly digestible, with low structural fibre and can have excess crude protein. Dry matter intakes can also be variable.
- Supplementation with Actisaf live yeast helps stabilise rumen function, reducing setbacks.