Evaluation of Sweet Potato Vines Silage as a Protein Supplement for Lactating Dairy Cows

Abstract

Dairy production in East Africa is dominated by the smallholder production system, characterized by suboptimal milk production attributed to poor nutrition. Grain-based concentrates can be used to cover the energy and protein deficits arising from forage feeding in rain-fed systems, but this strategy is constrained by grain competition with human and monogastric animals such as poultry, making these feeds very expensive. Sweet potato can serve both as a source of human food (tuber) and animal feed (vines). The objectives of this study were; 1) to determine the effect of harvesting regime on DM yield and nutrient content of sweet potato variety SPK 013, Kenspot 1 and SPK 117, 2) to determine the effect of amount of molasses and storage period on silage quality of sweet potato vines from the three sweet potato varieties, 3) to assess effect of supplementing lactating dairy cows on Napier grass basal diet with sweet potato vine silage on dry matter intake, milk production, milk composition, 4) assessing the economic benefit of feeding sweet potato vines silage as an alternative protein supplement. The biomass yield of three new orange-fleshed dual-purpose sweet potato varieties were assessed under different harvesting regimes. The three varieties were chosen because they were newly introduced into the market as dual-purpose varieties. The vines were harvested either at intermediate stage (75 days) and again at final stage (150 days) (INT) or at final stage only (150 days) (FIN). Of the three varieties tested, Cultivar SPK013 yielded the most vine in both treatments (7.3 and 5.6 t DM/ha for INT (75 and150 days) and FIN respectively) and tuber at FIN (2 t DM/ha), but lowest tuber yield at INT (0.8 t DM/ha. The intermediate harvesting increased vine yield for all varieties (3.6 vs 2.3, 7.3 vs 5.6 and 5.0 vs 4.2 for INT vs FIN for variety Kenspot1, SPK013 and SOK117 respectively). For cultivar SPK013, intermediate harvesting resulted in a 58% decline in tuber yield (2.0 t DM/ha to 0.8t DM/ha) (p<0.05). xiv Vines from the three varieties were mixed with three amounts of molasses (0, 20 and 40g of molasses/ Kg of vines), and ensiled for 7, 14, 21, 28 and 56 days. The pH and CP content of the silage were monitored. The pH of 0% molasses amount was the highest (4.56) but had the lowest CP (16.25%). Ensilage period significantly affected the quality of silage with CP decreasing from 19.54 to 15.62% at 56d of storage. From the study, molasses amount of 20g per Kg of vines was recommended for both short term and long-term storage. The efficacy of sweet potato vine silage plus wheat bran (SPVSWB) as a low-cost supplement compared to grain-based commercial dairy concentrate (CDC) was assessed. Multiparous Holstein–Friesian cattle (n = 12) were fed on a basal diet of Napier grass (Pennisetum purpureum cv. South Africa) ad libitum, plus a fixed amount of either SPVSWB or CDC, (the ration formulated to be both iso-nitrogenous and iso-caloric) during late (LL) and early (EL) lactation. Daily milk yield was not significantly different (p>0.05) between SPVSWB and CDC groups during both LL (6.2 vs 7.5 L/day) and EL (14.2 vs 16.0 L/day). However, the lower cost of production for SPVSWB (23.2 vs 48.7 KES/Kg DM) resulted in margins on milk income over feed (per cow per day) being greater for SPVSWB in both periods (LL: 71 vs 14.5; and EL: 426 vs 400 KES/day). Despite SPVSWB eliciting lower milk production (LL 6.2 and EL14.2 L/day) than CDC (LL7.5 and EL 16.0 L/day), SPVSWB is a cost-effective, accessible alternative to grain-based supplementation in small-holder dairy-farming systems of Kenya. The study recommended 2% inclusion of molasses when making sweet potato vine silage. The silage can be fed in combination with other by-products, for moderate milk production which will result to a lower cost of supplementation compared conventional grain-based supplementation.