The effects of supplementing alternative carbohydrate sources on production and fibre degradation of jersey cows grazing pasture.

[摘要] ENGLISH ABSTRACT: Problems identified in the pasture-based dairy systems of the southern Cape of South Africa include lowered milk production during summer months, low milk solids during winter months, unsynchronised timing of pasture and concentrate feeding, lowered pasture degradability and pasture substitution. To counter act these problems and despite them, supplemental feeding is provided in the form of an energy rich concentrate, usually fed in the milking parlour. Historically, cereal grains form the largest part of the concentrate supplement and play an important role in determining the profitability of a dairy farm. The high starch content of cereal grains could have a limiting effect on microbial activity in the rumen due to lactic acid production, possibly resulting in low ruminal pH, which then impacts fibre degradation and has various negative production implications. Despite the problems associated with feeding starches it is still practised widely due to the high energy content, which promotes milk production. Other non-fibre carbohydrates, such as sugar and pectin (prevalent in various fruit wastes), have been shown to have a more positive effect on the rumen environment and are able to maintain production when substituted in total mixed ration systems. This study aimed to determine how effectively and to what degree alternative carbohydrate sources such as dried citrus pulp and dried apple pomace could be fed to Jersey cows grazing kikuyu pasture over-sown with ryegrass and what the possible production implications would be. The effect of dried citrus pulp and dried apple pomace on rumen metabolism and bacterial community dynamics was also investigated. The study consisted of three trials focused on the quality and application of dried citrus pulp and dried apple pomace. The first trial looked at the use of dried citrus pulp for cows grazing ryegrass pasture (Lolium multiforum var. Italicum, cv. Jeanne) and used 68 lactating Jersey cows (μ ± SD; 84.5 ± 43.8 days in milk, 20.4 ± 3.09 kg milk/day) allocated to one of four treatments in a complete randomised block design. Treatments were: No dried citrus pulp (NDCP)-0% replacement of ground maize, Low dried citrus pulp (LDCP)-33% replacement of ground maize, Medium dried citrus pulp (MDCP)-66% replacement of ground maize and High dried citrus pulp (HDCP)-100% replacement of ground maize. An additional six ruminally cannulated, lactating Jersey cows were randomly allocated to the NDCP and HDCP treatments in a two period cross-over design. It was found that milk yield decreased between 2.1 and 3.2 kg/day when ground maize was substituted by dried citrus pulp. Milk fat content did not differ between treatments; however, treatment had a quadratic effect on milk protein and lactose content, with the LDCP and MDCP treatments having the highest content for both. No change in the diurnal ruminal pH curves and no differences in the rate and extent of pasture dry matter and neutral detergent fibre degradability between treatments were observed. It was concluded that replacing ground maize with dried citrus pulp was possible, but the large decrease in milk production was problematic. Furthermore, the lack of response of rumen metabolism and milk fat solids and the extremely low CP and high Ca content of DCP posed limitations on the use of dried citrus pulp as a replacer for ground maize. The composition of dried apple pomace is similar to dried citrus pulp, except that it possibly has a higher fibre, starch and protein content and is lower in Ca. Due to the unique composition of dried apple pomace and its proximity to the region, it was considered next. The second trial looked at the use of dried apple pomace for cows grazing kikuyu pasture. Seventy two lactating Jersey cows were blocked according to milk yield (mean ± SD; 16.1 ± 2.11 kg), days in milk (114 ± 46.2 d) and lactation number (3.8 ± 1.45) and randomly allocated to one of four treatments. Treatments were: 0% dried apple pomace inclusion (AP 0), 25% dried apple pomace inclusion (AP 25), 50% dried apple pomace inclusion (AP 50) and 75% dried apple pomace inclusion (AP 75). An additional eight ruminally cannulated, lactating cows were used and were subjected to a four period crossover design with a 14 day adaptation period between treatments. Although milk yield was not affected by the inclusion level of DAP, there was a linear decrease in 4% fat corrected milk (FCM) and fat yield as the level of dried apple pomace inclusion in the diet increased. Cows receiving the AP 0 concentrate supplement yielded 0.9 and 1.2 kg more 4% FCM than cows on both the AP 50 and AP 75 concentrate supplements (P <0.001), respectively. Treatment had no effect on milk composition, except for the lactose content, which was lower for cows receiving the AP 0 concentrate supplement (P <0.001). Mean rumen pH was lower for cows receiving the AP 75 concentrate supplement (P <0.001); however, treatment did not affect the volatile fatty acids (VFA) profile or pasture DM and NDF degradability. Here the use of dried apple pomace seemed viable; however, the lack of milk solids response and no improvement of the rumen environment were unfortunate. Due to the high fibre nature of kikuyu pasture the rumen environment is naturally under less stress when cows are grazing these summer pastures, as compared to winter pastures such as ryegrass that are more easily digestible and have lower physically effective NDF (peNDF) or rumen buffering capacity. This trial was then essentially repeated on ryegrass pasture to determine whether the high fibre content of the dried apple pomace would be more effective in maintaining and possibly improving the rumen environment under more stressed conditions. In this third trial, 76 lactating Jersey cows were blocked according to milk yield (mean ± SE; 18.4 ± 0.01 kg), days in milk (97.2 ± 0.27 d) and lactation number (3.79 ± 0.04) and randomly allocated to one of four treatments. Treatments were: NDAP-0% dried apple pomace and 75% ground maize; LDAP-25% dried apple pomace and 50% ground maize; MDAP-50% dried apple pomace and 25% ground maize; HDAP-75% dried apple pomace and 0% ground maize. Additionally, four ruminally cannulated cows were used to monitor treatment effect on rumen activity and health. Milk yield and 4% FCM yield were lower for cows in treatment HDAP than for cows in treatments NDAP and LDAP, differences ranging between 1.7 and 2.3 kg 4% FCM/day. The milk protein yield remained unchanged between treatments, whereas milk protein content was lowest for cows in treatments NDAP and MDAP, showing a cubic trend (P = 0.005). Treatment had no effect on rumen metabolism parameters. In this trial it was determined that dried apple pomace could sustain milk production on ryegrass pasture; however, milk solids could possibly be negatively impacted.In addition to the production and rumen metabolism studies, a ruminal bacterial community dynamics study was also undertaken. Rumen fluid samples were collected for further study from cannulated cows in the second and third trials. It was interesting to note that the composition of the bacterial community was affected by a change in diet, even though that was not always reflected in the rumen metabolism (pH, VFA concentration and pasture degradation). The detailed description of the ruminal bacterial community will be of great value for future research regarding the nutrition of dairy cows grazing pasture and was the first of its kind.In conclusion, this research has provided insight into the use of fruit waste as a feed for dairy cows in pasture-based systems in a South African context. There are various limitations regarding the application thereof, but both dried citrus pulp and dried apple pomace are feed sources with potential as a ruminant feed and should not be over-looked by farmers and feed processors alike.

[发布机构] Stellenbosch University