Feeding the cow and the rumen

How much and what nutrition does a cow need for superior milk production? Sue Macky finds out.

Feeding dairy cows is complicated as two entities with differing nutrient needs are involved; the cow and the rumen microbiome. It is the health and function of the rumen that determines what actually feeds the cow, in what form, and how efficiently.

Determining nutrient requirements for milk production is usually determined backwards, i.e., if we want a cow to produce 35 litres of milk or 2.5 kgs MS per day, what amount of energy, protein, minerals does she need to be fed?

Most ration software programmes attempt to balance those needs against feeds available. There is an underlying assumption that ‘this’ ration is what the cow consumes in every mouthful, i.e. a total mixed ration (TMR).

When grazed pasture is fed, true balance rarely occurs. Pasture is a living changing feed, not of consistent composition or supply. Any supplements fed to make a complementary balanced diet are only fed once or twice a day, in conjunction with milking. It is not possible to have true synergy between all nutrients in the rumen at all times, so balance may be on paper only.

If the diet has the required amount of energy/ kg DMI (MJME/kg DM), this is no guarantee of the outcome. ME is not a nutrient – it is a mathematical equation that allows us to determine whether a certain outcome is possible or not. It is how the rumen microbes deal with feed that determines what energy and nutrients the cow gets.

Maximising daily dry matter intake (DMI) is the first priority. For lactating cows, we should aim to at least offer the equivalent of 4% of DM on a liveweight basis (20kg DM for a Kiwicross, 500kg mature cow). Cows can eat much more than this, but most NZ cows don’t get this much. Cows cannot regularly collect even 18 kgs DMI from grazed pasture, and much less in many parts of the country, for much of the year. This only meets the intake needs of a small Jersey cow.

Feeds must meet the needs of an optimally functioning, healthy, high capacity rumen – a system developed to extract nutrients from the tough bits of plants that we humans cannot digest, and which performs best as designed if it gets enough physically effective fibre and has synergy between crude protein and fermentable carbohydrate.

A BALANCED DIET

An important component of ‘balance’ is ensuring that what is fed promotes the largest possible, hardest working, rumen microbial population, and delivers nutrients to the cow she can use. She will get most of her ‘fuel’ from glucose made in the liver from the volatile fatty acid byproducts (VFAs) of the rumen microbial fermentation of fibre, sugar, and starch, but

Sue Macky.

“We have enough feed in NZ to produce the same amount of milk from a lot less cows.”

if fermentation rate exceeds the ability of the rumen to manage pH, feed conversion efficiency will decline. She gets most of her usable protein – metabolisable protein (MP) – from the bugs washed out of the rumen, both dead and alive, but this will be compromised if complementary fermentable carbohydrate is lacking. As production per cow increases, the quantity and quality of MP becomes more important. Synergy of fermentable carbohydrate intake (fibre, starch, and sugar), and crude protein, and provision of limiting essential amino acids, to maximise production of metabolisable protein leaving the rumen will determine potential productivity (and urinary nitrogen).

Achieving best feed conversion efficiency with minimal environmental wastage of either methane or nitrogen, on a diet that maximises use of pasture requires more than a simple ration software programme based on the assumption that every mouthful of feed will contain all the nutrients needed by the cow every time. We need to ensure that both cow and rumen are fed appropriately. The rumen microbes feed the cow best with a consistent intake of a constant composition suited to their needs. At all levels of production, regardless of ration and intake, it is the nutrition and management of transition cows pre and post calving, cow comfort and rumen health that will determine feed conversion efficiency (FCE), i.e. what is actually produced. Feed conversion efficiency determines how much milk is produced per kg of DMI/year. and per kg of liveweight/year, and how much methane and nitrogen will be ‘wasted’ into the environment. FCE is not fixed, nor is it guaranteed by ration balancing or genetics.

No one would expect 100 people of the same age and gender, eating the same food, to be able to run 1000m in the same time. Conversion of energy (food) to output is not fixed. In cows, feed always goes somewhere – the where and how well it gets there varies a lot between cows and farms.

For most NZ dairy farms, having skeletally fully grown first calving heifers, cows that meet all condition score targets on time, all the time, optimising cow comfort and offering milkers a daily DMI of at least 4% of fully grown mature cow liveweight, to be processed in a healthy rumen operating as designed, would achieve more milk than prioritising ration balance. Both cow and pasture management must be excellent to achieve these targets.

For herds averaging around one kg MS per 1 kg of mature cow liveweight or more, the actual nutrient mix of the ration becomes more important, as well as achieving better intake. At greater than 1.1kg MS/1 kg LWT, all nutrients can become limiting, especially essential amino acids. So, what is a balanced ration for your dairy cow? It’s how you best use the feeds you have available to get best cow DM intake and best rumen function to provide your cows with the best energy and nutrients possible from those feeds, alongside optimum cow comfort and management, so as to maximise production, health and reproduction.

Starting point – do you have enough total feed and where might it go if not into milk? Why?

Sort those issues first. We have enough feed in NZ to produce the same amount of milk from a lot less cows.

• Sue Macky is a principal dairy consultant for

Dairy Production Systems Ltd.