This study, which was co-funded by DARD and AgriSearch, involved 96 cows.
As the milk-production potential of dairy cows in Northern Ireland has increased, concentrate feeding has also increased in an attempt to meet the greater energy requirements of these higher-yielding cows.
However, as concentrates are more expensive than forages, it is important to ensure that concentrates are used efficiently. Despite this, there is evidence that higher levels of concentrates than needed are being offered on some farms.
In these scenarios, the milk-yield response to concentrates offered are often poor and uneconomic, and in many cases a proportion of the concentrate offered could be replaced by lower-cost silage.
A range of concentrate feeding strategies are used on farms, but these strategies usually involve offering a 'basal diet' of grass silage plus concentrates, with this normally designed to support the milk yields of the lowest-yielding cows in the herd (sometimes termed the 'maintenance-plus' yield). Additional concentrates are then offered to individual cows on a 'feed-to-yield' basis, with concentrate allocations based on milk yields above this maintenance-plus yield.
Additional concentrates are normally offered using either 'out-of-parlour' or 'in-parlour' concentrate-feeding systems. Recent technological developments have allowed feeding systems to be linked to milking-parlour software, making it possible to automatically adjust concentrate feed levels based on milk yields. Within these feed-to-yield systems, additional concentrates are normally offered at a fixed 'feed rate' for each litre of milk produced above the maintenance-plus yield.
The feed-rate that is most often used is 0.45 kg of concentrate per litre of milk. This value is based on the assumption that the production of one litre of milk requires approximately 5.2 megajoules (MJ) of metabolisable energy (ME), and that one kilogram of concentrate contains approximately 11.5 MJ of ME. The 0.45 kg feed rate is then determined by dividing 5.2 by 11.5. However, alternative feed rates are also used by nutritionists. In addition, this feed rate does not take account of 'substitution', the process by which forage intakes decrease when additional concentrates are offered.
In view of the latter, the milk yield supported by the basal diet (i.e. the maintenance-plus yield) may decrease at higher concentrate feed levels, meaning that alternative maintenance-plus yields may need to be adopted for higher-yielding cows.
The aim of the current study was to provide a better understanding of the impact of adopting a range of concentrate feed rates on cow performance, and to improve our understanding of dietary substitution within a feed-to-yield system.
This study, which was co-funded by DARD and AgriSearch, involved 96 cows (including 24 heifers). These cows calved between 28 August and 10 December (average calving date of 13 October).
Following calving, cows were given ad libitum access to a 'basal' mixed ration consisting of silage (grass silage plus maize silage in a 70:30 ratio on a dry matter (DM) basis) and concentrates. Concentrates were included in the ration to achieve a target concentrate intake of 5.0 kg per cow each day. This mixed ration was designed to meet the maintenance-energy requirements of the cows, plus the energy required to produce 24 (for heifers) or 27 (for cows) kg of milk per cow each day (the maintenance-plus yields).
In addition to this basal ration, cows were also offered concentrates at one of four feed rates, namely 0.35, 0.40, 0.45, or 0.55 kg of concentrates for each kg of milk produced above the maintenance-plus yields.
The amount of concentrate offered to each individual cow was determined weekly based on her average daily milk yield during the previous seven days.
So, for example, if a cow on the 0.45 kg feed rate produced an average daily milk yield of 41 kg over the previous week, her daily concentrate allocation for the next week was 6.3 kg (in addition to the concentrate offered via the basal diet). This was determined based on 14 kg of milk being produced above the maintenance-plus yield (i.e. 41 minus 27 kg) multiplied by the 0.45 feed rate. Cows remained on these four feed rate treatments until week 21 post-calving.
As the concentrate feed rate increased, there was a general decrease in silage DM intakes (Table 1). However, increasing concentrate feed rate from 0.35 to 0.45 kg had no effect on concentrate DM intake, suggesting that a feed rate within this range will have relatively little impact on total concentrate intakes when supplementing a basal diet.
Considering these results, the finding that increasing the feed rate from 0.35 to 0.45 kg had no effect on milk yield was as expected. However, concentrate intakes increased substantially at the 0.55 kg feed rate, and there was a small numerical increase in milk yield. This was accompanied by a reduction in milk fat content at the 0.55 kg feed rate due to the higher concentrate intakes. The overall effect was that neither milk fat-plus-protein yield nor milk value differed between any of the feed-rate treatments. The body condition score data highlights that concentrate feed rate had little effect on changes in body tissue reserves.
Combining data from all four treatments allows us to examine the relationship between intakes (silage, concentrate, and total) and average daily milk yields of individual cows (Figure 1). As expected based on the feed-to-yield system operated in this study, both concentrate DM intake (blue line) and total DM intake (red line) increased as the milk yields of the cows increased. However, silage DM intake (green line) did not change substantially as milk yields increased.
Increasing concentrate feed rate from 0.35 to 0.45 kg per kg of milk had no effect on either concentrate or total DM intake, or on milk yields. However, the adoption of a feed rate of 0.55 kg depressed milk fat due to increased levels of concentrates being offered and lower silage intakes. This study highlights the need to be cautious when adopting feed rates in excess of 0.45 kg per litre in feed-to-yield systems, and suggests that levels of concentrate offered should be based on a 'corrected milk volume' that takes account of milk composition.