Nutritional benefits for dairy cows
Feed value relative to other concentrates
High producing cows have a high energy requirement and they must be fed high amounts (kgDM) of high energy density feeds.
A concentrate is a feed with a high concentration of energy. It contains highly digestible components like starches, sugars, other readily available carbohydrates, and fats or oils. Concentrates are mostly processed, i.e. ground, kibbled or pelleted and have very little physically effective fibre. Concentrates create less pasture substitution than forages (such as fresh crops, silages or hay) and they can add additional energy and other nutrients to fill deficits in the diet.
Pasture quality varies throughout the season and pasture-based diets can be deficient in energy and/or protein at times of the year. The composition of the base diet (which in New Zealand is usually pasture) will determine the best concentrate option to feed.
Concentrates which are commonly available in New Zealand include dairy meals, maize or cereal grains (wheat and barley), molasses and palm kernel expeller (PKE). Maize grain has higher energy content than other grains and many other commonly available concentrates.
|Figure 1: Typical energy values of New Zealand concentrates 1,2|
Concentrates have differing sources of energy. Some feeds deliver energy via sugar (e.g. molasses) or fibre and fat (e.g. PKE). Maize grain energy comes mainly from starch. The nutrient composition of feeds influences how they are digested and the end products of digestion affect milk component levels.
|Figure 2. Typical nutrient composition of concentrates3|
The nutrient composition of PKE and molasses are taken from DairyNZ Farmfact 1- 71 and from the Agrifeeds website respectively.
Rate of starch digestion & other nutritional benefits
Although starch and sugar deliver high density energy, over feeding can cause a negative animal health effect known as acidosis. Acidosis occurs when high levels of sugar or starch are converted to large quantities of acid by the rumen microbes. This results in a fast reduction in rumen pH with negative effects on rumen digestion. While the amount of starch or sugar fed is critical, the speed and extent of starch digestion also influence the risk of acidosis. Maize starch is less risky than many other starch types because it is digested more slowly and to a lesser extent in the rumen.
|Figure 3. Rumen degradability of maize grain compared with wheat grain4|
An extended research project in Germany demonstrated that maize starch is digested in the rumen to a lower extent than wheat starch. This resulted in a healthier rumen pH reflected also by a higher rumen digestion of fibre which is a typical indicator of rumen health.
|Table 1. Effects of different concentrates (containing 87% maize grain or 87% wheat) |
on rumen digestion5
|Starch digested in rumen (%)||76a||95b|
|Crude fibre digestion of diet in rumen||76a||69b|
a,b Values with different superscripts are significantly different (P<0.05).
Impact of maize grain on milk protein
Feeding starch, fibre or sugar based feeds has an effect not only on rumen digestion, but also on milk yield and milk composition.
A recent New Zealand study6 investigated the effects of feeding different types of concentrates in addition to pasture:
- Starch - maize grain.
- Fibre – broll (wheat middlings).
- Sugar – molasses.
While energy intakes were similar between starch and fibre feeds, there were significant differences in milk yield and composition.
|Table 2. Effect of carbohydrate type on milk and milk component yields6|
|Drymatter intake, kg/day||14.9||17.6||18.1||16.1|
|ME intake MJ/day||164.5||201.3||197.9||179.9|
|Milk yield kg/day||23.14||27.7||26.21||23.56|
|Fat yield kg/day||1.03||1.07||1.16||1.06|
|Crude Protein Yield||0.8||0.8||0.94||0.79|
Maize grain feeding resulted in higher milk and milk protein yield. The authors concluded: “Early lactation cows offered energetically similar supplements differing in carbohydrate type responded differently in milk and milk component yields. Compared with an un-supplemented pasture diet, starch-based supplements increased milk and milk protein yield, but did not affect fat yield, whereas fibre based supplements primarily increased milk and milk fat yield. Bolus doses of sugar (molasses) did not affect milk or milk component yields in this study6.”
|Figure 4. Impact of feeding different supplement energy types on milk yield and composition6|
Impact of feeding maize grain on cow condition score and reproductive performance
The benefits of having cows in better condition (the target is condition score 5.0 for mature cows and 5.5 for two and three year old cows) are substantial. A cow calving at condition score 5.0 will produce on average 12 kg more milksolids than a cow that calves at condition score 4.0. Cows that are fatter at calving cycle earlier, have higher in -calf rates and are more likely to give birth to a heifer calf the following year.
|Figure 5. Relationship between BCS at calving and annual milksolids production7|
Following the post-calving period of body condition score (BCS) loss, cows begin to gain BCS. The rate of gain is affected by both genetics and nutrition. In general high milk production cows gain less BCS than low yielding cows while milking cows fed supplement that contains non-structural carbohydrates (i.e. starch and sugar) gain more BCS than cows fed pasture alone. With the decline in mid-season pasture quality (and possibly insufficient quantity), BCS gain slows down or cows lose BCS once more. This loss of BCS is different to the loss in BCS post-calving and can be minimised by ensuring pasture quality is high or by providing the cow with high quality supplementary feeds when there is insufficient pasture7.
Cows selected for high milk production preferentially partition nutrients to milk production and not to BCS gain. BCS increases much more quickly when cows are offered supplements to pasture after they have been dried off. However different feeds are used with different efficiencies for BCS gain. Energy in autumn pasture is used inefficiently for gaining BCS. Energy from feeds like pasture silage, palm kernel extract (PKE) and maize silage are used 50% more efficiently7.
Substitution rates (concentrate vs. forage)
When grazing cows are fed supplements, pasture drymatter intake usually decreases. The rate at which pasture is replaced by supplements is known as the substitution rate.
Forage supplementation decreases pasture drymatter intake more than concentrate. Trials have shown that the substitution rate ranged from 0.84 to 1.02 kg/kg for grass silage supplementation and from 0.11 to 0.50 kg/kg for concentrate supplementation8.
In practical terms this means that feeding concentrates to grazing cows will reduce pasture intake, on average 0.3 kg DM pasture/day for each kgDM concentrate eaten. It will lift total energy intake and enable cows to produce more because metabolisable energy intake is the first limiting factor for milk production for most New Zealand cows.
Feeding forage (e.g. pasture, cereal or maize silage) results in higher pasture substitution rates than feeding concentrates. This reduces grazing pressure and can be used to manipulate farm pasture cover levels, reducing overgrazing.
2 Kolver, E. 2006. PKE - Economically priced supplement. Dexcelink Autumn 2006.
3 Kolver, E. 2000. Nutrition Guidelines for the high producing cow. Ruakura Farmers Conference pg 17-28.
4 Rodehutscord 2003. Proceedings of the 2nd Pioneer Silage Conference, Brehna, Germany.
5 Matthé, A. 2001. Nutrient conversion in the digestive tract of ruminants after corn or wheat starch application in different amounts. p. 58 & 160. Ph. D. thesis, Univ. Giessen and Federal Agric. Research Center Braunschweig, Germany.
6 Higgs et al., 2011. The effect of carbohydrate type on milk and milk component yields in early lactation dairy cows. Proceedings of the New Zealand Society of Animal Production 71:23-27.
7 DairyNZ. 2012. DairyNZ body condition scoring. The reference guide for New Zealand dairy farmers http://resources.dairynz.co.nz/DownloadResource.aspx?id=502.
8 Bargo, F., L. D. Muller, E. S. Kolver, and J. E. Delahoy. 2003. Invited Review: Production and digestion of supplemented dairy cows on pasture. J. Dairy Sci. 86:1-42.