John Roche: Pasture is an excellent feed, unless you mismanage it
DairyNZ principal scientist Dr John Roche revisits a century of myths about cow nutrition.
In reviewing old research for a history paper I am writing, I came across two quotes that I thought made for as interesting a discussion today as they did 70-80 years ago when they were first uttered.
The first, from the United States in 1936, states that "if a cow will eat enough immature grass to provide the required digestible nutrients and if this grass has a normal content of minerals, her ration is not likely to be deficient in any of the essential food constituents" – Woodward, 1936 .
In other words, 'immature' or vegetative pasture was known to be a well-balanced feed 80 years ago and the factor most limiting milk production on pasture was dry matter intake.
Despite this revelation, many did not believe that pasture, growing 'wild in the field', could be as good for a dairy cow as the intensively selected cereal grains that fattened pigs and chickens.
This leads me to what Dr Campbell McMeekan, as superintendent of Ruakura at the time, said, when questioning the rationale of feeding anything other than pasture: "From the qualitative angle, it is often assumed and frequently stated that pasture is not capable of supplying the needs of a high producing dairy cow and ideally needs to be supplemented with suitable concentrates" – McMeekan, 1947 .
It's 70 years since McMeekan's presentation at the New Zealand Society of Animal Production, but his commentary is as relevant today as it was then.
Many people still believe that pasture is not a wholly balanced feed ingredient for cows, highlighting its lack of sugars (or starch) relative to cereal grains, the excess of protein, relative to a cow's requirement, and the high milk yields of cows in dairy systems feeding grains relative to those achieved by cows 'forced' to live on pasture alone.
Is pasture a good feed for dairy cows?
In a word, YES!
In fact, our perennial ryegrass and white clover-based pasture in New Zealand is, arguably, the most balanced feed a dairy cow could eat.
· It contains the same energy density as wheat or maize, although the energy comes from the fermentation of fibre instead of grain.
· The composition of the protein is ideal to maximize milk production; there are no amino acids deficient, unless the cow is supplemented with a large amount of a low protein grain, palm kernel, or maize silage.
· The vitamin and mineral composition is also pretty well-balanced, although supplementation with calcium straight after calving, and magnesium and some trace elements is advised through calving and early lactation.
Decades of research in New Zealand confirm that cows consuming the same amount of metabolisable energy from pasture, or pasture plus a supplement (eg sugars, grains or silages) produce the same amount of milksolids (Carruthers et al., 1997; Roche et al., 2010) .
In fact, at a system level, the amount of milksolids produced from 100MJ pasture, 100MJ of maize grain, or 100MJ of maize silage is the same (Roche et al., 2010; Macdonald et al., 2017) . This is because in a grazing system, intake of metabolisable energy is the factor that limits milk production. So, it stands to reason that 100MJ from any feed would result in the same production response.
Considering the historical introduction to this article, it is important to point out that this is, also, not new.
Gustav Kuhn and his co-workers in Germany proved this in the 1880s - 130 years ago.
Yet, even in 2017, some people still refuse to accept that the microorganisms fermenting feed in the cow's rumen do not care if they eat Brussels sprouts or chocolate; they merely care about the metabolisable energy the feed contains.
It is this fact and nearly 13 decades of research work that leads us to point out that milk production, body condition score and reproduction responses to supplements are very small when cows are reasonably well fed on pasture.
As a result, supplementary feeds should only be used when cows do not have enough pasture (ie grazing residuals are less than 3.5cm - 7 clicks on the plate meter).
But, what about the excess of protein - doesn't this cost energy and reduce fertility?
As with most of the mythology I hear regarding feeding dairy cows, there is a degree of truth in this, and a whole pile of nonsense.
When cows consume protein in excess of their requirements, it is converted to urea in the liver and either returned to the rumen or excreted in urine. As the process of creating urea requires energy, it was once believed (and obviously still is by many) that there is a significant energy and protein cost to excreting excess protein.
However, more recent research has highlighted that this depends on the type of protein and is largely untrue for the type of protein in pasture. There isn't a large cost to excreting surplus protein that is degraded in the rumen and this is the main type of protein in pasture.
Therefore, in most day to day situations, dealing with excess protein is an insignificant detail to the grazing dairy cow.
When milk companies began presenting milk urea results on the milk docket, many farmers began asking what this meant. There was plenty of advice from 'nutritionists' that the levels in grazing cows were very high. Pasture was labelled as 'poison' and needed to be supplemented with a low protein feed, like cereal grains or maize silage. If you didn't do this, your cows wouldn't get pregnant.
This is utter nonsense. Although there was limited evidence in the US that high protein diets reduce the chances that a cow will get pregnant (Canfield et al., 1990; Butler, 1998) , the only information available for grazing dairy cows suggests that fertility improves with increasing blood and milk urea nitrogen (Roche et al., 2011) . This evidence comes both from experiments and from testing bulk milk from real farms.
If pasture is such a good feed, why do New Zealand cows produce less milk than cows in the US?
The difference in milk yield between cows grazing pasture or fed a ration in a barn is primarily due to a difference in dry matter intake (Kolver and Muller, 1998) . Simply put, grazing cows have to work for their feed, while cows fed indoors can consume their daily feed requirement with little or no effort.
However, not many cows are fed rations in barns in New Zealand. Instead, when they are offered supplementary feeds, they still have to graze pasture and 'work for their supper'.
In this situation, when cows are fed a supplement they refuse to work hard. They reduce both their time spent grazing and their bite rate, so their intake of pasture declines. This is referred to as substitution – cows refuse pasture when offered a supplementary feed.
Substitution is as much a biological fact as the apple falling from the tree was a physical fact in Isaac Newton's story about gravity.
As humans, we're the same; if someone fed you cake before dinner, you are less likely to eat all your vegetables. This isn't because your stomach is full. It is because your stomach and intestines produce hormones that tell your brain that you have eaten a calorie-loaded meal. We have measured these hormones in dairy cows and have found that cows are less hungry after eating supplementary feeds (Roche et al., 2007, 2008) . It doesn't matter if they're fed in the shed, the feed-pad or in the paddock, or if they're fed grains, silage or complex rations.
It's quite simple, if cows are fed a supplement they refuse pasture.
The main factor that influences how much pasture a cow refuses when she eats a supplement is her pasture intake before she was offered the supplement (Stockdale, 2000) ; the more pasture a cow is eating, the more pasture she will waste when she is fed a supplementary feed. Because of this, we recommend that cows should not be supplemented unless the post-grazing residual is less than 3.5 cm (7 clicks on the rising plate meter).
By following this simple rule, the cow will be well-fed, substitution rate will be low (i.e., very little pasture is wasted), and the milk production response to the supplementary feed will be greater.
But can't we just get that pasture next time?
No you can't. As a farmer mentor of mine says, "use it or lose it". In fact, the effect is much worse than just wasting pasture. If supplements are offered when cows are not hungry, substitution will be high and post-grazing residuals will rise. This will result in poor quality pasture in the following grazing and a drop in milk production because of this.
People sometimes associate this drop in production with removing supplements when, in fact, it is because supplements were offered earlier and when they weren't needed.
Conclusions and implications
Pasture is a very high quality, balanced feed, unless you mismanage it by leaving high post-grazing residuals. It is energy dense and has very high protein quality; but, because cows have to work to eat their fill, they are limited by dry matter intake. This limitation is overcome by making sure that the stocking rate for the farm is correct and that the dry matter intake/ha optimises pasture utilisation.
If someone tells you that pasture is not a good feed and needs another feed to complement it or 'balance it' for some deficiency, my suggestion is that you should probably stop listening at that point; the next piece of information is likely to cost you money.
Take the myths out of feeding cows. The marginal cost and benefit from offering supplements in a feed deficit situation and under different milk prices can be predicted using DairyNZ's Supplement Price Calculator: https://www.dairynz.co.nz/feed/supplements/supplement-price-calculator/).
Butler, W.R. 1998. Review: effect of protein nutrition on ovarian and uterine physiology in dairy cattle. J. Dairy Sci. 81:2533–2539. doi:10.3168/jds.S0022-0302(98)70146-8.
Canfield, R.W., C.J. Sniffen, and W.R. Butler. 1990. Effects of Excess Degradable Protein on Postpartum Reproduction and Energy Balance in Dairy Cattle. J. Dairy Sci. 73:2342–2349. doi:10.3168/jds.S0022-0302(90)78916-3.
Carruthers, V.R., P.G. Neil, and D.E. Dalley. 1997. Effect of altering the non-structural:structural carbohydrate ratio in a pasture diet on milk production and ruminal metabolites in cows in early and late lactation. Anim. Sci. 64:393–402.
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Macdonald, K.A., J.W. Penno, J.A.S. Lancaster, A.M. Bryant, J.M. Kidd, and J.R. Roche. 2017. Production and economic responses to intensification of pasture-based dairy production systems. J. Dairy Sci. 100:In press.
McMeekan, C.P. 1947. Pasture and Animal Production. Proc. New Zeal. Soc. Anim. Prod. 7:13–26.
Roche, J.R., D. Blache, J.K. Kay, D.R. Miller, A.J. Sheahan, and D.W. Miller. 2008. Neuroendocrine and physiological regulation of intake with particular reference to domesticated ruminant animals. Nutr. Res. Rev. 21:207–234. doi:10.1017/S0954422408138744.
Roche, J.R., C.R. Burke, S. Meier, and C.G. Walker. 2011. Nutrition reproduction interaction in pasture-based systems: Is nutrition a factor in reproductive failure? Anim. Prod. Sci. 51:1045–1066. doi:10.1071/AN10162.
Roche, J.R., J.K. Kay, C.V.C. Phyn, S. Meier, J.M. Lee, and C.R. Burke. 2010. Dietary structural to nonfiber carbohydrate concentration during the transition period in grazing dairy cows. J. Dairy Sci. 93:3671–83. doi:10.3168/jds.2009-2868.
Roche, J.R., a J. Sheahan, L.M. Chagas, and D.P. Berry. 2007. Concentrate supplementation reduces postprandial plasma ghrelin in grazing dairy cows: a possible neuroendocrine basis for reduced pasture intake in supplemented cows. J. Dairy Sci. 90:1354–1363. doi:S0022-0302(07)71622-3 [pii]\r10.3168/jds.S0022-0302(07)71622-3.
Stockdale, C.R. 2000. Levels of pasture substitution when concentrates are fed to grazing dairy cows in Northern Victoria. Aust. J. Exp. Agric. 40:913–921.
Woodward, T.E. 1936. The Quantities of Grass that Dairy Cows Will Graze. J. Dairy Sci. 19:347–357. doi:10.3168/jds.S0022-0302(36)93075-8.
Dr John Roche is principal scientist for animal science at DairyNZ.
He has held science appointments with the National Centre for Dairy Production Research at Moorepark in Ireland, the Department of Primary Industries in Australia, and the University of Tasmania, and is senior section editor for the management and economics section of the Journal of Dairy Science.
At DairyNZ he leads a team of 12 scientists and post-graduate students. He has published about 150 peer-reviewed, science journal articles and book chapters in the last 10 years. He is a regular contributor at both science and farming conferences domestically and internationally.
Dr Roche's research programme focuses heavily on the role of nutrition in dairy cow metabolic health, with particular reference to managing the transition dairy cows undergo between calving and early lactation, the role of nutrition in immune function, and cow body condition score.
He was raised on a dairy farm in the South-West of Ireland and moved to Australia in 1995, followed by New Zealand in 2000. As well as owning a farm in Ireland, he has had both passive and active roles in dairy farm development in different countries.