He told a DairyNZ FeedRight workshop in Hawera crude protein levels in good quality pasture normally sit between 24% and 29%. Dietary protein requirements for cows vary throughout lactation, starting at about 18% in early lactation, dropping to about 16% in mid-lactation and 14% towards the end.
“So we’re generally not looking for anything other than an energy source that can be eaten and purchased as cheaply as possible,” he said.
“Most of the data out there suggests that unless you are getting up to 2.8-3kg milksolids per cow [per day] and running that fairly regularly for a period of time, energy is going to be your first limiting constraint to increasing production.”
Hughes said that effectively the search came down to determining the least expensive form of metabolisable energy (ME) that would fit into a specific farming system, given available feeding out and storage facilities.
Palm kernel was competitively priced but unless fed through an in-dairy feeding system had a high wastage potential. Starch-based products such as cereal grains were more expensive but fed through in-bail feeding systems offered a low level of wastage. Silages are a less expensive energy source but the quality of the forage used was critical in terms of the impacts on milk production and liveweight gain.
Monitoring post-grazing pasture residuals is critical in the decision-making process around the feeding of supplements. A low residual would indicate cows were being underfed while a higher residual pointed towards a possible reduction in the amount of supplements being fed.
Lincoln University senior lecturer, Dr Jim Gibbs, said grazing pastures to a 1500kg DM/ha residual – “the Kiwi miracle system” – meant cows were consuming about 90% of their potential intake. Getting them to eat the last 10% meant having to accept a post-grazing residual so high it would impact on pasture production, making the system uneconomic.
Fellow presenter and DairyNZ scientist, Dr Jane Kay recommended basing supplement purchase decisions on a cents per megajoule of ME (MJ ME).
“You might be told to buy a starch-based feed that is going to push up your [milk] protein to fat ratio, that feeding something like palm kernel which is high in fibre pushes up the fat percentage in the milk,” she said.
“But in 99.9% of the cases, if you buy on that cents/MJ ME basis then the return that you get on your milksolids will be the best. If you pay more for that maize grain, you are not going to recapture that cost of getting that extra protein in feed.”
The protein to fat ratio was another indicator sometimes suggested as proxy measure of the cows’ energy balance. It has been suggested a low protein to fat ratio could indicate a negative energy balance, something Kay said was not accurate.
“If you are feeding something like palm kernel you are going to push up that milkfat, and you will have a lower protein to fat ratio. If you have got cows that you have been feeding 14kg DM to, and then you come in and feed them an extra 3-4kg of palm kernel, they will have a lower protein to fat ratio but they are actually in a better energetic state.”
Putting on cow condition
Palm kernel is more efficient than maize and pasture silage, and autumn pasture, at putting condition on to dry cows.
Dr Jane Kay said trial work conducted by DairyNZ showed that to put one condition score on a 500kg cow would require 125kg DM of palm kernel over and above animal maintenance requirements, 160-180kg DM of maize or spring pasture silage, or (query with Erin) 205kg DM of autumn pasture.
“Palm kernel is quite a high fibre feed and when you feed something like that it actually produces acetate or acetic acid in the rumen – that is the building block for fat synthesis.”
In contrast, feeds high in starch like maize silage favoured the production of propionate in the rumen, the precursor to glucose. Kay offered a possible explanation – the fatty acids, such as acetic acid, could not cross the foetal membrane while the glucose could be siphoned off by the foetus.
One of the questions at the FeedRight workshop focused on whether lactating cows were more efficient at putting on body condition than dry cows. Kay said while that was true at a cellular level, the reality was different.
“If you look at the whole animal, with a lactating cow she has got increased maintenance because the liver has to produce glucose for milk production – all the organs are working harder so she has got increased maintenance requirements. She has got increased activity – she is walking to the dairy. Also, we have bred this cow that is going to partition most of this energy that you are giving her towards milk production.”
To supplement or not?
A substitution effect was inevitable when feeding supplements and capturing the benefits required the adaption of grazing strategies.
Dr Terry Hughes said if supplement was sourced at the right price, profitability was improved largely by extending lactation – growing days in milk.
“The people who were really making a good job of supplementation systems had previously been very good managers of pasture, and when they introduced supplement they made the necessary [system] changes, and they got 270-plus days of lactation out of their whole herd.”
