Effects of hydric stress on the growth, blood chemistry and meat quality characteristics of indigenous chickens.

Abstract

The broad objective of the study was to determine the effects of restricted water intake on the growth, blood chemistry and meat quality characteristics of indigenous chickens in semi arid environments. A total of 15 flocks in communal villages and 12 flocks in resettlement schemes of Msinga District in South Africa were monitored for 30 months to determine the effects of production system and season on flock size, dynamics and constraints faced by indigenous chicken producers. As a follow up, 281 and 233 chicks hatched in November 2011 from 18 and 9 households in communal villages and resettlement schemes, respectively were monitored using a structured checklist to determine survival and causes of mortality from hatching up to 12 weeks of age. Kaplan-Meier survival distributions and the odds ratios for effects of potential risk factors were determined using survival analysis and logistic regression models, respectively.

Flock composition and structure were significantly affected by production system and month. Farmers in resettlement schemes had approximately one cock and three hens more (P < 0.05) than those in communal villages. The number of growers and chicks were similar (P > 0.05) between the two production systems but chicks were the predominant age group (38 %) of each flock. The cock to hen ratio in households in communal villages was 1:3.5, while that in resettlement schemes was 1:3.7, suggesting that inbreeding might have been reducing flock productivity. Flock sizes in communal villages peaked in March (45.1 ± 3.02) during the post

rainy season and declined steadily to a low of 34.7 ± 3.63 in September during the hot-dry season. The largest flock sizes in resettlement schemes were observed in January (52.4 ± 5.09) during the hot-wet season and the smallest in August (36.1 ± 5.98) during the cold- dry season. Households in communal villages had more chicks (P < 0.05) than cocks, hens and growers throughout the year except in June when the number of growers was equal to that of chicks. In resettlement schemes, the number of chicks was lower (P < 0.05) than the number of hens and growers, except in June and July when it was equal to that of growers. Total entries were not affected (P > 0.05) by production system. Hatched chicks were the major mode of entry, accounting for more than 97 % of entries into flocks. The contribution of purchases, gifts and exchanges was negligible. Mortality was the main cause of exits from flocks, accounting for 70 and 63 % of total exits among households in communal villages and resettlement schemes, respectively. The major causes of mortality were aerial predators, sub-optimalnutrition and inclement weather conditions, which were similar between production systems. The number of birds that exited flocks through slaughter for household consumption was higher (P < 0.05) among households in resettlement (34% of total exits) than communal villages (21 % of total exits). The proportion of chickens exiting flocks through sales was higher in communal (9 %) than resettlement (4.3 %) households.

Chick survival from hatching to 12 weeks was higher (P < 0.05) in communal villages (55 ± 3.14 %) than in resettlement schemes (41 ± 4.19 %). Mean chick survival time did not differ (P > 0.05) between communal (56 ± 3.30 days) and resettlement (49 ± 3.23 days) flocks. Provision of water ad libitum and treatment of sick birds were important covariates in prolonging the survival time of chicks.

The effects of restricted water intake on growth performance, blood chemistry, physicochemical properties, and sensory characteristics of meat from Naked Neck (NNK) and Ovambo (OVB) chickens were also assessed. In the experiment, 54 pullets of each strain with an average weight of 641± 10g per bird were randomly assigned to three water restriction levels for 60 days in a completely randomized design. The treatments were ad libitum, 70% of ad libitum and 40% of ad libitum water intake. Each treatment group was replicated three times. The pectoralis (breast) muscle was sampled for meat quality, fatty acid composition and sensory quality analyses. Ovambo chickens had superior body-weight at 16 weeks of age, average daily gains (ADG) and average daily water intake (ADWI) than NNK chickens. Body weight of birds at 16 weeks of age, ADG, average daily feed intake (ADFI), ADWI and water to feed ratio (WFR) declined progressively (P < 0.05) with increasing severity of water restriction, while food conversion ratio (FCR) values increased (P < 0.05) as the severity of water restriction increased. Naked Neck chickens had better FCR at the 40 % of ad libitum water intake level than OVB chickens. The dressing percentage per bird was higher (P<0.05) in water-restricted birds than those on ad libitum water consumption, irrespective of strain. Heart weight was significantly lower in birds on 40% of ad libitum water intake than those on ad libitum and 70% of ad libitum water intake, respectively. Packed cell volume was higher (P < 0.05) in NNK than OVB chickens offered waterad libitum, but similar in birds offered 70 and 40 % of ad libitum. There were no differences in erythrocyte count (RBC) and mean corpuscular volume (MCV) values between strains, but MCV was higher in birds on 40 than 70 % of ad libitum water intake, irrespective of strain. Naked neck chickens had higher (P < 0.05) white blood cell count (WBC) values than OVB chickens at 40 % restriction level, but lower WBC than OVB at 70 % water restriction level. Uric acid, creatinine, triacylglycerides, total cholesterol, low density lipid cholesterol, total protein and globulin increased (P < 0.05) with each increment in water restriction, but the increase in creatinine and total cholesterol was more pronounced in OVB than NNK chickens. The opposite was observed for uric acid. Alanine transaminase, alkaline phosphatase and aspartate transaminase activities were not influenced by strain and water restriction. It was concluded that the two strains could withstand up to 40 % of ad libitum water restriction, but NNK chickens tolerated water stress better than OVB chickens.

Water intake levels of 40% of ad libitum produced meat with significantly lower (P < 0.05) cooking loss, and higher (P < 0.05) redness (a*) values in NNK chickens compared with OVB chickens. Water intake level had no effect (P > 0.05) on lightness (L*) and yellowness (b*) values, shear force, moisture and protein contents in both strains. The fat content of NNK meat was 41 % lower (P < 0.05) than that of OVB meat at 70 % of ad libitum, but 31 % higher at 40 % of ad libitum water intake. The ash content was significantly elevated (P < 0.05) in birds on 70 % of ad libitum compared to those on ad libitum and 40 % of ad libitum water intake, which had similar (P > 0.05) ash contents. Birds on 40 % of ad libitum water intake had significantly higher (P < 0.05) proportions of octadecanoic acid (C18:0), cis, cis-9,12-octadecadienoic acid (C18:2 ῳ-6), cis-8,11,14,17-eicosatetraenoic acid (C20:4 ῳ-6), cis-7,10,13,16-docosatetraenoic acid (C22:4 ῳ-6), cis-4,7,10,13,16,19-docosahexaenoic acid (C22:6 ῳ-3), total polyunsaturated fatty acid (PUFA), total omega-3 PUFA and total omega-6 PUFA proportions, but lower (P < 0.05) cis-7-hexadecenoic (C16:1c7), cis-9-octadecenoic (C18:1c9), cis-11-octadecenoic acid (C18:1c11), cis-13-docosenoic acid (C22:1c13), total monounsaturated fatty acids than those on the 70% of ad libitum and ad libitum water intake, respectively. The proportion of trans-9-octadenoic acid (C18:1t9) was higher (P < 0.05) in NNK chickens on 40 % of ad libitum water intake than OVB chickens. It was concluded that water restriction at 40 % of ad libitum water intake resulted in favourable cooking loss values and meat redness (a*) values, omega-3 and 6 PUFA proportions and a high ῳ-6/ῳ-3 ratio. The high fat content of NNK chickens at 40 % of ad libitum water intake compared to OVB chickens suggests a superior adaptation to hydric stress.

Naked Neck breast meat had higher initial impression of juiciness scores than that from OVB chickens, but only in birds on ad libitum and 70 % of ad libitum water intake. Sensory scores for first bite, connective tissue and tenderness decreased with increasing severity of water restriction (P < 0.05). Aroma, flavour and atypical flavour were not affected by strain or water restriction level (P > 0.05). There were significant strain differences for sustained impression of juiciness and tenderness, with the highest scores occurring in NNK chickens (P < 0.05). Aroma had a significant influence on the flavour of breast meat (P < 0.05). Fat content was significantly correlated with initial impression of juiciness, first bite and sustained impression of juiciness of breast meat. It was concluded that water restriction up to 40 % of ad libitum had a significant and adverse impact on juiciness and first bite scores of meat.