Ingjerd Dønnem defended her thesis on December 1st 2010

Ane Gro Siri Skjelfjord

High quality grass silage to dairy goats - effect on energy balance, milk yield and quality

The general aim of this thesis has been to investigate the effect of high quality silage, harvested at an early stage of maturity, on energy intake, energy balance, milk production and milk quality of goats. It was also of interest to study the relationship between different levels of concentrate supplementation and the silage harvesting time on these parameters. Two studies were performed and formed the basis of three papers.

In the first study 18 goats were fed grass silage harvested at three different stages of maturity from the primary growth combined with two different levels of concentrate in a cyclic changeover design. The goats were grouped into three blocks according to their body condition just after kidding; poor, medium and high. The experiment was performed during the first 18 weeks of lactation. Paper I evaluated the effect of the nutritive characteristics and digestibility of the silages on the goat's performance. Increasing the digestibility by earlier harvesting time increased the intake of grass silage and the milk production. Increased energy intake due to improved silage digestibility resulted in higher milk production than seen with increased energy intake due to increased concentrate level. However, utilization of nutrients to milk production increased with postponed harvesting time, because less energy was used for body fat deposition. In the same study the energy balance and the milk quality of the goats were evaluated (Paper II). The calculated energy balance decreased and the serum non-esterified fatty acids (NEFA) concentration increased with decreasing energy content (postponed harvesting time and low level of concentrate) in the diet. The concentration of milk FFA was highest when the highest concentrate level was offered. During the first 18 weeks of lactation a high energy balance was correlated to a high milk FFA concentration and a poor sensory milk quality. Goats with initial poor body condition had higher milk FFA concentrations than goats in higher initial body condition during the whole experimental period.

In the second study, the aim was to study the changes of the goats' energy status during early and mid lactation when offered rations with various energy concentrations, and relate this to milk quality (Paper III). The experimental feeds were grass silage harvested in the primary growth at two stages of maturity supplemented with two different levels of concentrate. Energy status was estimated from lactation week 2 to 18 by studying changes in body composition (measured by computer tomography), calculated energy balance and blood parameters. After the indoor feeding experiment there were two measurements of milk parameters on mountain pasture. During the indoor experimental period only the goats fed the lowest energy diet (the latest harvesting time and lowest level of concentrate) mobilized from the adipose tissue. The rest of the goats had an energy intake high enough to deposit body fat throughout these 16 weeks. The concentration of FFA in milk increased with increased energy intake, which suggests that during the present condition, a high plane of nutrition during early and mid lactation increased lipolysis in milk.

The main conclusions from the present studies are that improving the silage digestibility by early harvesting time increases energy intake, energy balance and milk production, and it is possible to feed goats the first 18 weeks of lactation with sufficient energy to avoid fat mobilization. However, feeding to a positive energy balance worsened the milk quality. It appears that when a high nutrient supply supports both high milk production and deposition of adipose tissue, it increases the lipolytic activity in milk.

Updated: 06.12.10
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