Minskande andelar kraftfoder i foderstaten under betesperioden
effekt på mjölkavkastning och betesbeteende hos mjölkkor
Feed costs constitute a large part of the expenses of dairy farmers. Pasture is a high quality feed with a low cost. However, feeding with concentrates has been shown to enhance milk yield (Leaver, 1985; Bargo et al., 2003; Stockdale, 2004). The purpose with this study was to investigate how milk yield and milk composition were affected as the concentrate proportion in the diet decreased. Pasture behaviour was examined to see if the different concentrate levels affected the time cows spent grazing and ruminating. Since ruminating animals give rise to methane emissions, methane emissions from the different feed ratios were estimated using equations obtained from the literature. In the trial 27 lactating dairy cows (Swedish Red Breed) were blocked by parity, lactation number, days in milk and current milk yield. The cows were randomly assigned to five treatments consisting of different concentrate levels in the diet corresponding to: 20, 30, 40, 50 and 60% of the cow?s energy requirement at the start of the experiment. The trial was carried out during the summer of 2008. All cows were milked twice daily and milk samples were collected once a week. During a two week adaptation period (AP) the cows were adapted to their intended concentrate levels in each treatment group. These levels were fed throughout the six week experimental period (FP), assuming a standardized decrease in milk yield of 0.6 and 0.4 kg ECM for older cows and heifers, respectively. During AP and FP the cows grazed together on 10 ha in a rotational grazing system with 6 paddocks. Sward height was measured before and after the cows grazed each paddock and pasture samples were collected and analyzed. The concentrates were supplied in the barn at milking and a supplement of 4 kg dry matter silage was also given daily. At the end of the experimental period all animals were fed according to their requirements during a four week after-period (AP) to estimate residual effects of the experimental feeding regimes. During FP pasture had an average energy content of 10.8 MJ/kg DM. Pasture allowance was lower than expected during the trial and towards the end of the period there was a shortage of pasture (< 25 kg DM/cow and day). The behaviour study showed that a 10% increase in the proportion of concentrates in the diet reduced grazing and ruminating time with 4.1% (P < 0.001) and 2.1% (P < 0.01), respectively. With a 10% increase of the proportion of concentrate in the diet, milk yield increased with 1.5 kg (P < 0.001). An increase in concentrate supplementation with 1 kg gave an average response in milk yield with 0.8 kg milk. There were no differences between high- and low yielding cows in the size of the response to increasing the proportion or the amount of concentrates in the diet. Fat content decreased with 0.06% when the concentrate level increased with 10% while no effects were observed on protein content in milk. Estimated methane emissions varied between 250-350 g/day depending on the empirical models used.